Hi all,
First of all, sorry for the delay!
These patches continue the code review for the following patches:
https://lore.kernel.org/xen-devel/[email protected]/t/#u
The original description of the patch set is as follows:
"The changes in this patch series intend to fix the Xen grant device
driver, so that grant mapping leaks caused by partially failed grant
mapping operations are avoided with the first patch, and so that the
splitting of VMAs does not result in incorrectly unmapped grant pages
with the second patch. The second patch also prevents a similar issue
in a double-mapping scenario, where mmap() is used with MAP_FIXED to
map grants over an existing mapping created with the same grants, and
where grant pages are unmapped incorrectly as well."
A summary of the changes from v1 is as follows:
- Addressed Juergen's code review comment regarding the first patch.
- Amended the description of the second patch to note that the described
issues are encountered with PV domains.
Verification notes:
- I have tested these commits on top of Linux v5.15.70 and v5.15.71, and
I verified that they compile successfully on top of the tag
"next-20220930", which corresponds to the base commit ID included at
the bottom of this e-mail.
- My tests consist of using a kernel with Qubes OS v4.1's patches and
these patches on my main computer for day-to-day tasks, in conjunction
with Qubes OS's version of the Xen hypervisor v4.14.5, with the latter
custom-compiled with CONFIG_DEBUG.
- I used a test program that verifies the following scenarios with an
unprivileged paravirtualized (PV) Xen domain:
- A program mmap()s two pages from another Xen domain and munmap()s
the pages one by one. This used to result in implicit unmap errors
to be reported by Xen and a general protection fault to be triggered
by Xen in the affected domain, but now works as expected.
- A program mmap()s two pages from another Xen domain and then
attempts to remap (via MAP_FIXED) the same mapping again over the
same virtual address. This used to result in similar issues
(implicit unmap errors and general protection fault), but now is
rejected by the kernel.
- A program mmap()s two pages from another Xen domain and then
attempts to mmap() the same mapping again to a different virtual
address, by passing NULL as mmap()'s first argument. This used to be
rejected by the kernel, and it continues to be rejected by the
kernel.
- Unprivileged PVH Xen domains were also sanity tested with the same
test program. I should note that PVH domains worked as expected
without these patches too.
- Finally, I have verified that the original "g.e. 0x1234 still pending"
issue does not appear after rapidly resizing GUI windows in Qubes OS
v4.1.
Thank you,
Vefa
M. Vefa Bicakci (2):
xen/gntdev: Prevent leaking grants
xen/gntdev: Accommodate VMA splitting
drivers/xen/gntdev-common.h | 3 +-
drivers/xen/gntdev.c | 80 +++++++++++++++++++------------------
2 files changed, 44 insertions(+), 39 deletions(-)
base-commit: 274d7803837da78dfc911bcda0d593412676fc20
--
2.37.3
Prior to this commit, if a grant mapping operation failed partially,
some of the entries in the map_ops array would be invalid, whereas all
of the entries in the kmap_ops array would be valid. This in turn would
cause the following logic in gntdev_map_grant_pages to become invalid:
for (i = 0; i < map->count; i++) {
if (map->map_ops[i].status == GNTST_okay) {
map->unmap_ops[i].handle = map->map_ops[i].handle;
if (!use_ptemod)
alloced++;
}
if (use_ptemod) {
if (map->kmap_ops[i].status == GNTST_okay) {
if (map->map_ops[i].status == GNTST_okay)
alloced++;
map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
}
}
}
...
atomic_add(alloced, &map->live_grants);
Assume that use_ptemod is true (i.e., the domain mapping the granted
pages is a paravirtualized domain). In the code excerpt above, note that
the "alloced" variable is only incremented when both kmap_ops[i].status
and map_ops[i].status are set to GNTST_okay (i.e., both mapping
operations are successful). However, as also noted above, there are
cases where a grant mapping operation fails partially, breaking the
assumption of the code excerpt above.
The aforementioned causes map->live_grants to be incorrectly set. In
some cases, all of the map_ops mappings fail, but all of the kmap_ops
mappings succeed, meaning that live_grants may remain zero. This in turn
makes it impossible to unmap the successfully grant-mapped pages pointed
to by kmap_ops, because unmap_grant_pages has the following snippet of
code at its beginning:
if (atomic_read(&map->live_grants) == 0)
return; /* Nothing to do */
In other cases where only some of the map_ops mappings fail but all
kmap_ops mappings succeed, live_grants is made positive, but when the
user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
will then make map->live_grants negative, because the latter function
does not check if all of the pages that were requested to be unmapped
were actually unmapped, and the same function unconditionally subtracts
"data->count" (i.e., a value that can be greater than map->live_grants)
from map->live_grants. The side effects of a negative live_grants value
have not been studied.
The net effect of all of this is that grant references are leaked in one
of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
mechanism extensively for X11 GUI isolation), this issue manifests
itself with warning messages like the following to be printed out by the
Linux kernel in the VM that had granted pages (that contain X11 GUI
window data) to dom0: "g.e. 0x1234 still pending", especially after the
user rapidly resizes GUI VM windows (causing some grant-mapping
operations to partially or completely fail, due to the fact that the VM
unshares some of the pages as part of the window resizing, making the
pages impossible to grant-map from dom0).
The fix for this issue involves counting all successful map_ops and
kmap_ops mappings separately, and then adding the sum to live_grants.
During unmapping, only the number of successfully unmapped grants is
subtracted from live_grants. The code is also modified to check for
negative live_grants values after the subtraction and warn the user.
Link: https://github.com/QubesOS/qubes-issues/issues/7631
Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
Cc: [email protected]
Signed-off-by: M. Vefa Bicakci <[email protected]>
---
Changes since v1:
- To determine which unmap operations were successful, the previous
version of this patch set the "unmap_ops[i].status" and
"kunmap_ops[i].status" fields to the value "1" prior to passing these
data structures to the hypervisor. Instead of doing that, the code now
checks whether the "handle" fields in the same data structures were
*not* set to "INVALID_GRANT_HANDLE". (Suggested by Juergen Gross.)
---
drivers/xen/gntdev.c | 22 +++++++++++++++++-----
1 file changed, 17 insertions(+), 5 deletions(-)
diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
index 84b143eef395..eb0586b9767d 100644
--- a/drivers/xen/gntdev.c
+++ b/drivers/xen/gntdev.c
@@ -367,8 +367,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
for (i = 0; i < map->count; i++) {
if (map->map_ops[i].status == GNTST_okay) {
map->unmap_ops[i].handle = map->map_ops[i].handle;
- if (!use_ptemod)
- alloced++;
+ alloced++;
} else if (!err)
err = -EINVAL;
@@ -377,8 +376,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
if (use_ptemod) {
if (map->kmap_ops[i].status == GNTST_okay) {
- if (map->map_ops[i].status == GNTST_okay)
- alloced++;
+ alloced++;
map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
} else if (!err)
err = -EINVAL;
@@ -394,8 +392,14 @@ static void __unmap_grant_pages_done(int result,
unsigned int i;
struct gntdev_grant_map *map = data->data;
unsigned int offset = data->unmap_ops - map->unmap_ops;
+ int successful_unmaps = 0;
+ int live_grants;
for (i = 0; i < data->count; i++) {
+ if (map->unmap_ops[offset + i].status == GNTST_okay &&
+ map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
+ successful_unmaps++;
+
WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
pr_debug("unmap handle=%d st=%d\n",
@@ -403,6 +407,10 @@ static void __unmap_grant_pages_done(int result,
map->unmap_ops[offset+i].status);
map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
if (use_ptemod) {
+ if (map->kunmap_ops[offset + i].status == GNTST_okay &&
+ map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
+ successful_unmaps++;
+
WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
pr_debug("kunmap handle=%u st=%d\n",
@@ -411,11 +419,15 @@ static void __unmap_grant_pages_done(int result,
map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
}
}
+
/*
* Decrease the live-grant counter. This must happen after the loop to
* prevent premature reuse of the grants by gnttab_mmap().
*/
- atomic_sub(data->count, &map->live_grants);
+ live_grants = atomic_sub_return(successful_unmaps, &map->live_grants);
+ if (WARN_ON(live_grants < 0))
+ pr_err("%s: live_grants became negative (%d) after unmapping %d pages!\n",
+ __func__, live_grants, successful_unmaps);
/* Release reference taken by __unmap_grant_pages */
gntdev_put_map(NULL, map);
--
2.37.3
On Sun, Oct 02, 2022 at 06:20:05PM -0400, M. Vefa Bicakci wrote:
> Prior to this commit, if a grant mapping operation failed partially,
> some of the entries in the map_ops array would be invalid, whereas all
> of the entries in the kmap_ops array would be valid. This in turn would
> cause the following logic in gntdev_map_grant_pages to become invalid:
>
> for (i = 0; i < map->count; i++) {
> if (map->map_ops[i].status == GNTST_okay) {
> map->unmap_ops[i].handle = map->map_ops[i].handle;
> if (!use_ptemod)
> alloced++;
> }
> if (use_ptemod) {
> if (map->kmap_ops[i].status == GNTST_okay) {
> if (map->map_ops[i].status == GNTST_okay)
> alloced++;
> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
> }
> }
> }
> ...
> atomic_add(alloced, &map->live_grants);
>
> Assume that use_ptemod is true (i.e., the domain mapping the granted
> pages is a paravirtualized domain). In the code excerpt above, note that
> the "alloced" variable is only incremented when both kmap_ops[i].status
> and map_ops[i].status are set to GNTST_okay (i.e., both mapping
> operations are successful). However, as also noted above, there are
> cases where a grant mapping operation fails partially, breaking the
> assumption of the code excerpt above.
>
> The aforementioned causes map->live_grants to be incorrectly set. In
> some cases, all of the map_ops mappings fail, but all of the kmap_ops
> mappings succeed, meaning that live_grants may remain zero. This in turn
> makes it impossible to unmap the successfully grant-mapped pages pointed
> to by kmap_ops, because unmap_grant_pages has the following snippet of
> code at its beginning:
>
> if (atomic_read(&map->live_grants) == 0)
> return; /* Nothing to do */
>
> In other cases where only some of the map_ops mappings fail but all
> kmap_ops mappings succeed, live_grants is made positive, but when the
> user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
> will then make map->live_grants negative, because the latter function
> does not check if all of the pages that were requested to be unmapped
> were actually unmapped, and the same function unconditionally subtracts
> "data->count" (i.e., a value that can be greater than map->live_grants)
> from map->live_grants. The side effects of a negative live_grants value
> have not been studied.
>
> The net effect of all of this is that grant references are leaked in one
> of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
> mechanism extensively for X11 GUI isolation), this issue manifests
> itself with warning messages like the following to be printed out by the
> Linux kernel in the VM that had granted pages (that contain X11 GUI
> window data) to dom0: "g.e. 0x1234 still pending", especially after the
> user rapidly resizes GUI VM windows (causing some grant-mapping
> operations to partially or completely fail, due to the fact that the VM
> unshares some of the pages as part of the window resizing, making the
> pages impossible to grant-map from dom0).
>
> The fix for this issue involves counting all successful map_ops and
> kmap_ops mappings separately, and then adding the sum to live_grants.
> During unmapping, only the number of successfully unmapped grants is
> subtracted from live_grants. The code is also modified to check for
> negative live_grants values after the subtraction and warn the user.
>
> Link: https://github.com/QubesOS/qubes-issues/issues/7631
> Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
Looks like this patch has been pretty buggy, sorry. This is the second
time there has been a problem with it. Thanks for the fix.
> Cc: [email protected]
> Signed-off-by: M. Vefa Bicakci <[email protected]>
> ---
>
> Changes since v1:
> - To determine which unmap operations were successful, the previous
> version of this patch set the "unmap_ops[i].status" and
> "kunmap_ops[i].status" fields to the value "1" prior to passing these
> data structures to the hypervisor. Instead of doing that, the code now
> checks whether the "handle" fields in the same data structures were
> *not* set to "INVALID_GRANT_HANDLE". (Suggested by Juergen Gross.)
> ---
> drivers/xen/gntdev.c | 22 +++++++++++++++++-----
> 1 file changed, 17 insertions(+), 5 deletions(-)
>
> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> index 84b143eef395..eb0586b9767d 100644
> --- a/drivers/xen/gntdev.c
> +++ b/drivers/xen/gntdev.c
> @@ -367,8 +367,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
> for (i = 0; i < map->count; i++) {
> if (map->map_ops[i].status == GNTST_okay) {
> map->unmap_ops[i].handle = map->map_ops[i].handle;
> - if (!use_ptemod)
> - alloced++;
> + alloced++;
> } else if (!err)
> err = -EINVAL;
>
> @@ -377,8 +376,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>
> if (use_ptemod) {
> if (map->kmap_ops[i].status == GNTST_okay) {
> - if (map->map_ops[i].status == GNTST_okay)
> - alloced++;
> + alloced++;
> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
> } else if (!err)
> err = -EINVAL;
> @@ -394,8 +392,14 @@ static void __unmap_grant_pages_done(int result,
> unsigned int i;
> struct gntdev_grant_map *map = data->data;
> unsigned int offset = data->unmap_ops - map->unmap_ops;
> + int successful_unmaps = 0;
> + int live_grants;
>
> for (i = 0; i < data->count; i++) {
> + if (map->unmap_ops[offset + i].status == GNTST_okay &&
> + map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
> + successful_unmaps++;
> +
> WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
> map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
> pr_debug("unmap handle=%d st=%d\n",
> @@ -403,6 +407,10 @@ static void __unmap_grant_pages_done(int result,
> map->unmap_ops[offset+i].status);
> map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
> if (use_ptemod) {
> + if (map->kunmap_ops[offset + i].status == GNTST_okay &&
> + map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
> + successful_unmaps++;
> +
> WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
> map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
> pr_debug("kunmap handle=%u st=%d\n",
> @@ -411,11 +419,15 @@ static void __unmap_grant_pages_done(int result,
> map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
> }
> }
> +
> /*
> * Decrease the live-grant counter. This must happen after the loop to
> * prevent premature reuse of the grants by gnttab_mmap().
> */
> - atomic_sub(data->count, &map->live_grants);
> + live_grants = atomic_sub_return(successful_unmaps, &map->live_grants);
> + if (WARN_ON(live_grants < 0))
> + pr_err("%s: live_grants became negative (%d) after unmapping %d pages!\n",
> + __func__, live_grants, successful_unmaps);
>
> /* Release reference taken by __unmap_grant_pages */
> gntdev_put_map(NULL, map);
> --
> 2.37.3
Is there a possibility that live_grants could overflow, as it is now
set to a value twice as large as what it had been previously?
If not, you can add:
Acked-by: Demi Marie Obenour <[email protected]>
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
On 2022-10-02 20:29, Demi Marie Obenour wrote:
> On Sun, Oct 02, 2022 at 06:20:05PM -0400, M. Vefa Bicakci wrote:
>> Prior to this commit, if a grant mapping operation failed partially,
>> some of the entries in the map_ops array would be invalid, whereas all
>> of the entries in the kmap_ops array would be valid. This in turn would
>> cause the following logic in gntdev_map_grant_pages to become invalid:
>>
>> for (i = 0; i < map->count; i++) {
>> if (map->map_ops[i].status == GNTST_okay) {
>> map->unmap_ops[i].handle = map->map_ops[i].handle;
>> if (!use_ptemod)
>> alloced++;
>> }
>> if (use_ptemod) {
>> if (map->kmap_ops[i].status == GNTST_okay) {
>> if (map->map_ops[i].status == GNTST_okay)
>> alloced++;
>> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>> }
>> }
>> }
>> ...
>> atomic_add(alloced, &map->live_grants);
>>
>> Assume that use_ptemod is true (i.e., the domain mapping the granted
>> pages is a paravirtualized domain). In the code excerpt above, note that
>> the "alloced" variable is only incremented when both kmap_ops[i].status
>> and map_ops[i].status are set to GNTST_okay (i.e., both mapping
>> operations are successful). However, as also noted above, there are
>> cases where a grant mapping operation fails partially, breaking the
>> assumption of the code excerpt above.
>>
>> The aforementioned causes map->live_grants to be incorrectly set. In
>> some cases, all of the map_ops mappings fail, but all of the kmap_ops
>> mappings succeed, meaning that live_grants may remain zero. This in turn
>> makes it impossible to unmap the successfully grant-mapped pages pointed
>> to by kmap_ops, because unmap_grant_pages has the following snippet of
>> code at its beginning:
>>
>> if (atomic_read(&map->live_grants) == 0)
>> return; /* Nothing to do */
>>
>> In other cases where only some of the map_ops mappings fail but all
>> kmap_ops mappings succeed, live_grants is made positive, but when the
>> user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
>> will then make map->live_grants negative, because the latter function
>> does not check if all of the pages that were requested to be unmapped
>> were actually unmapped, and the same function unconditionally subtracts
>> "data->count" (i.e., a value that can be greater than map->live_grants)
>> from map->live_grants. The side effects of a negative live_grants value
>> have not been studied.
>>
>> The net effect of all of this is that grant references are leaked in one
>> of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
>> mechanism extensively for X11 GUI isolation), this issue manifests
>> itself with warning messages like the following to be printed out by the
>> Linux kernel in the VM that had granted pages (that contain X11 GUI
>> window data) to dom0: "g.e. 0x1234 still pending", especially after the
>> user rapidly resizes GUI VM windows (causing some grant-mapping
>> operations to partially or completely fail, due to the fact that the VM
>> unshares some of the pages as part of the window resizing, making the
>> pages impossible to grant-map from dom0).
>>
>> The fix for this issue involves counting all successful map_ops and
>> kmap_ops mappings separately, and then adding the sum to live_grants.
>> During unmapping, only the number of successfully unmapped grants is
>> subtracted from live_grants. The code is also modified to check for
>> negative live_grants values after the subtraction and warn the user.
>>
>> Link: https://github.com/QubesOS/qubes-issues/issues/7631
>> Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
>
> Looks like this patch has been pretty buggy, sorry. This is the second
> time there has been a problem with it. Thanks for the fix.
Hi,
No problem! :-) Debugging this issue and coming up with a fix was a
nice challenge for me.
>
>> Cc: [email protected]
>> Signed-off-by: M. Vefa Bicakci <[email protected]>
>> ---
>>
>> Changes since v1:
>> - To determine which unmap operations were successful, the previous
>> version of this patch set the "unmap_ops[i].status" and
>> "kunmap_ops[i].status" fields to the value "1" prior to passing these
>> data structures to the hypervisor. Instead of doing that, the code now
>> checks whether the "handle" fields in the same data structures were
>> *not* set to "INVALID_GRANT_HANDLE". (Suggested by Juergen Gross.)
>> ---
>> drivers/xen/gntdev.c | 22 +++++++++++++++++-----
>> 1 file changed, 17 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>> index 84b143eef395..eb0586b9767d 100644
>> --- a/drivers/xen/gntdev.c
>> +++ b/drivers/xen/gntdev.c
>> @@ -367,8 +367,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>> for (i = 0; i < map->count; i++) {
>> if (map->map_ops[i].status == GNTST_okay) {
>> map->unmap_ops[i].handle = map->map_ops[i].handle;
>> - if (!use_ptemod)
>> - alloced++;
>> + alloced++;
>> } else if (!err)
>> err = -EINVAL;
>>
>> @@ -377,8 +376,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>>
>> if (use_ptemod) {
>> if (map->kmap_ops[i].status == GNTST_okay) {
>> - if (map->map_ops[i].status == GNTST_okay)
>> - alloced++;
>> + alloced++;
>> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>> } else if (!err)
>> err = -EINVAL;
>> @@ -394,8 +392,14 @@ static void __unmap_grant_pages_done(int result,
>> unsigned int i;
>> struct gntdev_grant_map *map = data->data;
>> unsigned int offset = data->unmap_ops - map->unmap_ops;
>> + int successful_unmaps = 0;
>> + int live_grants;
>>
>> for (i = 0; i < data->count; i++) {
>> + if (map->unmap_ops[offset + i].status == GNTST_okay &&
>> + map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
>> + successful_unmaps++;
>> +
>> WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
>> map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>> pr_debug("unmap handle=%d st=%d\n",
>> @@ -403,6 +407,10 @@ static void __unmap_grant_pages_done(int result,
>> map->unmap_ops[offset+i].status);
>> map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>> if (use_ptemod) {
>> + if (map->kunmap_ops[offset + i].status == GNTST_okay &&
>> + map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
>> + successful_unmaps++;
>> +
>> WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
>> map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>> pr_debug("kunmap handle=%u st=%d\n",
>> @@ -411,11 +419,15 @@ static void __unmap_grant_pages_done(int result,
>> map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>> }
>> }
>> +
>> /*
>> * Decrease the live-grant counter. This must happen after the loop to
>> * prevent premature reuse of the grants by gnttab_mmap().
>> */
>> - atomic_sub(data->count, &map->live_grants);
>> + live_grants = atomic_sub_return(successful_unmaps, &map->live_grants);
>> + if (WARN_ON(live_grants < 0))
>> + pr_err("%s: live_grants became negative (%d) after unmapping %d pages!\n",
>> + __func__, live_grants, successful_unmaps);
>>
>> /* Release reference taken by __unmap_grant_pages */
>> gntdev_put_map(NULL, map);
>> --
>> 2.37.3
>
> Is there a possibility that live_grants could overflow, as it is now
> set to a value twice as large as what it had been previously?
Good point! My answer in summary: I think that the code could be improved,
but with reasonable values for the "limit" module parameter, there should
not be issues.
Grant mappings are set up via ioctl calls, and the structure field that
holds the number of grant references has u32 type:
(Quoting from kernel v5.15.71 for convenience)
include/uapi/xen/gntdev.h
=== 8< ===
struct ioctl_gntdev_map_grant_ref {
/* IN parameters */
/* The number of grants to be mapped. */
__u32 count;
=== >8 ===
However, the number of grant references is further limited in the actual
ioctl handler function gntdev_ioctl_map_grant_ref(), which calls
gntdev_test_page_count() to ensure that the number of granted pages
requested to be mapped does not exceed "limit". "limit" defaults to 64K,
which should be okay to use with an atomic_t type (i.e., a 32-bit signed
integer type) like "live_grants", assuming that the system administrator
does not go overboard and set "limit" to a very large value:
drivers/xen/gntdev.c
=== 8< ===
static unsigned int limit = 64*1024;
module_param(limit, uint, 0644);
MODULE_PARM_DESC(limit,
"Maximum number of grants that may be mapped by one mapping request");
/* trimmed */
bool gntdev_test_page_count(unsigned int count)
{
return !count || count > limit;
}
/* trimmed */
static long gntdev_ioctl_map_grant_ref(struct gntdev_priv *priv,
struct ioctl_gntdev_map_grant_ref __user *u)
{
/* trimmed */
pr_debug("priv %p, add %d\n", priv, op.count);
if (unlikely(gntdev_test_page_count(op.count)))
return -EINVAL;
/* trimmed */
}
=== >8 ===
To be fair, the "count" field of the gndev_grant_map structure is a signed
integer, so very large values of count could overflow live_grants, as
live_grants needs to accommodate values up to and including 2*count.
drivers/xen/gntdev-common.h
=== 8< ===
struct gntdev_grant_map {
atomic_t in_use;
struct mmu_interval_notifier notifier;
bool notifier_init;
struct list_head next;
int index;
int count;
/* trimmed */
}
=== >8 ===
> If not, you can add:
>
> Acked-by: Demi Marie Obenour <[email protected]>
Thank you! I hope that the explanation and rationale above are satisfactory.
Please let me know what you think.
Vefa
On Mon, Oct 03, 2022 at 09:31:25PM -0400, M. Vefa Bicakci wrote:
> On 2022-10-02 20:29, Demi Marie Obenour wrote:
> > On Sun, Oct 02, 2022 at 06:20:05PM -0400, M. Vefa Bicakci wrote:
> > > Prior to this commit, if a grant mapping operation failed partially,
> > > some of the entries in the map_ops array would be invalid, whereas all
> > > of the entries in the kmap_ops array would be valid. This in turn would
> > > cause the following logic in gntdev_map_grant_pages to become invalid:
> > >
> > > for (i = 0; i < map->count; i++) {
> > > if (map->map_ops[i].status == GNTST_okay) {
> > > map->unmap_ops[i].handle = map->map_ops[i].handle;
> > > if (!use_ptemod)
> > > alloced++;
> > > }
> > > if (use_ptemod) {
> > > if (map->kmap_ops[i].status == GNTST_okay) {
> > > if (map->map_ops[i].status == GNTST_okay)
> > > alloced++;
> > > map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
> > > }
> > > }
> > > }
> > > ...
> > > atomic_add(alloced, &map->live_grants);
> > >
> > > Assume that use_ptemod is true (i.e., the domain mapping the granted
> > > pages is a paravirtualized domain). In the code excerpt above, note that
> > > the "alloced" variable is only incremented when both kmap_ops[i].status
> > > and map_ops[i].status are set to GNTST_okay (i.e., both mapping
> > > operations are successful). However, as also noted above, there are
> > > cases where a grant mapping operation fails partially, breaking the
> > > assumption of the code excerpt above.
> > >
> > > The aforementioned causes map->live_grants to be incorrectly set. In
> > > some cases, all of the map_ops mappings fail, but all of the kmap_ops
> > > mappings succeed, meaning that live_grants may remain zero. This in turn
> > > makes it impossible to unmap the successfully grant-mapped pages pointed
> > > to by kmap_ops, because unmap_grant_pages has the following snippet of
> > > code at its beginning:
> > >
> > > if (atomic_read(&map->live_grants) == 0)
> > > return; /* Nothing to do */
> > >
> > > In other cases where only some of the map_ops mappings fail but all
> > > kmap_ops mappings succeed, live_grants is made positive, but when the
> > > user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
> > > will then make map->live_grants negative, because the latter function
> > > does not check if all of the pages that were requested to be unmapped
> > > were actually unmapped, and the same function unconditionally subtracts
> > > "data->count" (i.e., a value that can be greater than map->live_grants)
> > > from map->live_grants. The side effects of a negative live_grants value
> > > have not been studied.
> > >
> > > The net effect of all of this is that grant references are leaked in one
> > > of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
> > > mechanism extensively for X11 GUI isolation), this issue manifests
> > > itself with warning messages like the following to be printed out by the
> > > Linux kernel in the VM that had granted pages (that contain X11 GUI
> > > window data) to dom0: "g.e. 0x1234 still pending", especially after the
> > > user rapidly resizes GUI VM windows (causing some grant-mapping
> > > operations to partially or completely fail, due to the fact that the VM
> > > unshares some of the pages as part of the window resizing, making the
> > > pages impossible to grant-map from dom0).
> > >
> > > The fix for this issue involves counting all successful map_ops and
> > > kmap_ops mappings separately, and then adding the sum to live_grants.
> > > During unmapping, only the number of successfully unmapped grants is
> > > subtracted from live_grants. The code is also modified to check for
> > > negative live_grants values after the subtraction and warn the user.
> > >
> > > Link: https://github.com/QubesOS/qubes-issues/issues/7631
> > > Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
> >
> > Looks like this patch has been pretty buggy, sorry. This is the second
> > time there has been a problem with it. Thanks for the fix.
>
> Hi,
>
> No problem! :-) Debugging this issue and coming up with a fix was a
> nice challenge for me.
You’re welcome! I’m glad you were able to do this.
> > > Cc: [email protected]
> > > Signed-off-by: M. Vefa Bicakci <[email protected]>
> > > ---
> > >
> > > Changes since v1:
> > > - To determine which unmap operations were successful, the previous
> > > version of this patch set the "unmap_ops[i].status" and
> > > "kunmap_ops[i].status" fields to the value "1" prior to passing these
> > > data structures to the hypervisor. Instead of doing that, the code now
> > > checks whether the "handle" fields in the same data structures were
> > > *not* set to "INVALID_GRANT_HANDLE". (Suggested by Juergen Gross.)
> > > ---
> > > drivers/xen/gntdev.c | 22 +++++++++++++++++-----
> > > 1 file changed, 17 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
> > > index 84b143eef395..eb0586b9767d 100644
> > > --- a/drivers/xen/gntdev.c
> > > +++ b/drivers/xen/gntdev.c
> > > @@ -367,8 +367,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
> > > for (i = 0; i < map->count; i++) {
> > > if (map->map_ops[i].status == GNTST_okay) {
> > > map->unmap_ops[i].handle = map->map_ops[i].handle;
> > > - if (!use_ptemod)
> > > - alloced++;
> > > + alloced++;
> > > } else if (!err)
> > > err = -EINVAL;
> > > @@ -377,8 +376,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
> > > if (use_ptemod) {
> > > if (map->kmap_ops[i].status == GNTST_okay) {
> > > - if (map->map_ops[i].status == GNTST_okay)
> > > - alloced++;
> > > + alloced++;
> > > map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
> > > } else if (!err)
> > > err = -EINVAL;
> > > @@ -394,8 +392,14 @@ static void __unmap_grant_pages_done(int result,
> > > unsigned int i;
> > > struct gntdev_grant_map *map = data->data;
> > > unsigned int offset = data->unmap_ops - map->unmap_ops;
> > > + int successful_unmaps = 0;
> > > + int live_grants;
> > > for (i = 0; i < data->count; i++) {
> > > + if (map->unmap_ops[offset + i].status == GNTST_okay &&
> > > + map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
> > > + successful_unmaps++;
> > > +
> > > WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
> > > map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
> > > pr_debug("unmap handle=%d st=%d\n",
> > > @@ -403,6 +407,10 @@ static void __unmap_grant_pages_done(int result,
> > > map->unmap_ops[offset+i].status);
> > > map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
> > > if (use_ptemod) {
> > > + if (map->kunmap_ops[offset + i].status == GNTST_okay &&
> > > + map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
> > > + successful_unmaps++;
> > > +
> > > WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
> > > map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
> > > pr_debug("kunmap handle=%u st=%d\n",
> > > @@ -411,11 +419,15 @@ static void __unmap_grant_pages_done(int result,
> > > map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
> > > }
> > > }
> > > +
> > > /*
> > > * Decrease the live-grant counter. This must happen after the loop to
> > > * prevent premature reuse of the grants by gnttab_mmap().
> > > */
> > > - atomic_sub(data->count, &map->live_grants);
> > > + live_grants = atomic_sub_return(successful_unmaps, &map->live_grants);
> > > + if (WARN_ON(live_grants < 0))
> > > + pr_err("%s: live_grants became negative (%d) after unmapping %d pages!\n",
> > > + __func__, live_grants, successful_unmaps);
> > > /* Release reference taken by __unmap_grant_pages */
> > > gntdev_put_map(NULL, map);
> > > --
> > > 2.37.3
> >
> > Is there a possibility that live_grants could overflow, as it is now
> > set to a value twice as large as what it had been previously?
>
> Good point! My answer in summary: I think that the code could be improved,
> but with reasonable values for the "limit" module parameter, there should
> not be issues.
>
> Grant mappings are set up via ioctl calls, and the structure field that
> holds the number of grant references has u32 type:
>
> (Quoting from kernel v5.15.71 for convenience)
> include/uapi/xen/gntdev.h
> === 8< ===
> struct ioctl_gntdev_map_grant_ref {
> /* IN parameters */
> /* The number of grants to be mapped. */
> __u32 count;
> === >8 ===
>
> However, the number of grant references is further limited in the actual
> ioctl handler function gntdev_ioctl_map_grant_ref(), which calls
> gntdev_test_page_count() to ensure that the number of granted pages
> requested to be mapped does not exceed "limit". "limit" defaults to 64K,
> which should be okay to use with an atomic_t type (i.e., a 32-bit signed
> integer type) like "live_grants", assuming that the system administrator
> does not go overboard and set "limit" to a very large value:
>
> drivers/xen/gntdev.c
> === 8< ===
> static unsigned int limit = 64*1024;
> module_param(limit, uint, 0644);
> MODULE_PARM_DESC(limit,
> "Maximum number of grants that may be mapped by one mapping request");
>
> /* trimmed */
>
> bool gntdev_test_page_count(unsigned int count)
> {
> return !count || count > limit;
> }
>
> /* trimmed */
>
> static long gntdev_ioctl_map_grant_ref(struct gntdev_priv *priv,
> struct ioctl_gntdev_map_grant_ref __user *u)
> {
> /* trimmed */
>
> pr_debug("priv %p, add %d\n", priv, op.count);
> if (unlikely(gntdev_test_page_count(op.count)))
> return -EINVAL;
>
> /* trimmed */
> }
> === >8 ===
>
> To be fair, the "count" field of the gndev_grant_map structure is a signed
> integer, so very large values of count could overflow live_grants, as
> live_grants needs to accommodate values up to and including 2*count.
Could this be replaced by an unsigned and/or 64-bit integer?
Alternatively, one could use module_param_cb and param_set_uint_minmax
to enforce that the limit is something reasonable. That said, one needs
almost 8TiB to trigger this problem, so while it ought to be fixed it
isn’t a huge deal. Certainly should not block getting this merged.
> drivers/xen/gntdev-common.h
> === 8< ===
> struct gntdev_grant_map {
> atomic_t in_use;
> struct mmu_interval_notifier notifier;
> bool notifier_init;
> struct list_head next;
> int index;
> int count;
> /* trimmed */
> }
> === >8 ===
>
> > If not, you can add:
> >
> > Acked-by: Demi Marie Obenour <[email protected]>
>
> Thank you! I hope that the explanation and rationale above are satisfactory.
> Please let me know what you think.
They are indeed.
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
On 2022-10-03 21:51, Demi Marie Obenour wrote:
> On Mon, Oct 03, 2022 at 09:31:25PM -0400, M. Vefa Bicakci wrote:
>> On 2022-10-02 20:29, Demi Marie Obenour wrote:
>>> On Sun, Oct 02, 2022 at 06:20:05PM -0400, M. Vefa Bicakci wrote:
>>>> Prior to this commit, if a grant mapping operation failed partially,
>>>> some of the entries in the map_ops array would be invalid, whereas all
>>>> of the entries in the kmap_ops array would be valid. This in turn would
>>>> cause the following logic in gntdev_map_grant_pages to become invalid:
>>>>
>>>> for (i = 0; i < map->count; i++) {
>>>> if (map->map_ops[i].status == GNTST_okay) {
>>>> map->unmap_ops[i].handle = map->map_ops[i].handle;
>>>> if (!use_ptemod)
>>>> alloced++;
>>>> }
>>>> if (use_ptemod) {
>>>> if (map->kmap_ops[i].status == GNTST_okay) {
>>>> if (map->map_ops[i].status == GNTST_okay)
>>>> alloced++;
>>>> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>>>> }
>>>> }
>>>> }
>>>> ...
>>>> atomic_add(alloced, &map->live_grants);
>>>>
>>>> Assume that use_ptemod is true (i.e., the domain mapping the granted
>>>> pages is a paravirtualized domain). In the code excerpt above, note that
>>>> the "alloced" variable is only incremented when both kmap_ops[i].status
>>>> and map_ops[i].status are set to GNTST_okay (i.e., both mapping
>>>> operations are successful). However, as also noted above, there are
>>>> cases where a grant mapping operation fails partially, breaking the
>>>> assumption of the code excerpt above.
>>>>
>>>> The aforementioned causes map->live_grants to be incorrectly set. In
>>>> some cases, all of the map_ops mappings fail, but all of the kmap_ops
>>>> mappings succeed, meaning that live_grants may remain zero. This in turn
>>>> makes it impossible to unmap the successfully grant-mapped pages pointed
>>>> to by kmap_ops, because unmap_grant_pages has the following snippet of
>>>> code at its beginning:
>>>>
>>>> if (atomic_read(&map->live_grants) == 0)
>>>> return; /* Nothing to do */
>>>>
>>>> In other cases where only some of the map_ops mappings fail but all
>>>> kmap_ops mappings succeed, live_grants is made positive, but when the
>>>> user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
>>>> will then make map->live_grants negative, because the latter function
>>>> does not check if all of the pages that were requested to be unmapped
>>>> were actually unmapped, and the same function unconditionally subtracts
>>>> "data->count" (i.e., a value that can be greater than map->live_grants)
>>>> from map->live_grants. The side effects of a negative live_grants value
>>>> have not been studied.
>>>>
>>>> The net effect of all of this is that grant references are leaked in one
>>>> of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
>>>> mechanism extensively for X11 GUI isolation), this issue manifests
>>>> itself with warning messages like the following to be printed out by the
>>>> Linux kernel in the VM that had granted pages (that contain X11 GUI
>>>> window data) to dom0: "g.e. 0x1234 still pending", especially after the
>>>> user rapidly resizes GUI VM windows (causing some grant-mapping
>>>> operations to partially or completely fail, due to the fact that the VM
>>>> unshares some of the pages as part of the window resizing, making the
>>>> pages impossible to grant-map from dom0).
>>>>
>>>> The fix for this issue involves counting all successful map_ops and
>>>> kmap_ops mappings separately, and then adding the sum to live_grants.
>>>> During unmapping, only the number of successfully unmapped grants is
>>>> subtracted from live_grants. The code is also modified to check for
>>>> negative live_grants values after the subtraction and warn the user.
>>>>
>>>> Link: https://github.com/QubesOS/qubes-issues/issues/7631
>>>> Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
>>>
>>> Looks like this patch has been pretty buggy, sorry. This is the second
>>> time there has been a problem with it. Thanks for the fix.
>>
>> Hi,
>>
>> No problem! :-) Debugging this issue and coming up with a fix was a
>> nice challenge for me.
>
> You’re welcome! I’m glad you were able to do this.
>
>>>> Cc: [email protected]
>>>> Signed-off-by: M. Vefa Bicakci <[email protected]>
>>>> ---
>>>>
>>>> Changes since v1:
>>>> - To determine which unmap operations were successful, the previous
>>>> version of this patch set the "unmap_ops[i].status" and
>>>> "kunmap_ops[i].status" fields to the value "1" prior to passing these
>>>> data structures to the hypervisor. Instead of doing that, the code now
>>>> checks whether the "handle" fields in the same data structures were
>>>> *not* set to "INVALID_GRANT_HANDLE". (Suggested by Juergen Gross.)
>>>> ---
>>>> drivers/xen/gntdev.c | 22 +++++++++++++++++-----
>>>> 1 file changed, 17 insertions(+), 5 deletions(-)
>>>>
>>>> diff --git a/drivers/xen/gntdev.c b/drivers/xen/gntdev.c
>>>> index 84b143eef395..eb0586b9767d 100644
>>>> --- a/drivers/xen/gntdev.c
>>>> +++ b/drivers/xen/gntdev.c
>>>> @@ -367,8 +367,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>>>> for (i = 0; i < map->count; i++) {
>>>> if (map->map_ops[i].status == GNTST_okay) {
>>>> map->unmap_ops[i].handle = map->map_ops[i].handle;
>>>> - if (!use_ptemod)
>>>> - alloced++;
>>>> + alloced++;
>>>> } else if (!err)
>>>> err = -EINVAL;
>>>> @@ -377,8 +376,7 @@ int gntdev_map_grant_pages(struct gntdev_grant_map *map)
>>>> if (use_ptemod) {
>>>> if (map->kmap_ops[i].status == GNTST_okay) {
>>>> - if (map->map_ops[i].status == GNTST_okay)
>>>> - alloced++;
>>>> + alloced++;
>>>> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
>>>> } else if (!err)
>>>> err = -EINVAL;
>>>> @@ -394,8 +392,14 @@ static void __unmap_grant_pages_done(int result,
>>>> unsigned int i;
>>>> struct gntdev_grant_map *map = data->data;
>>>> unsigned int offset = data->unmap_ops - map->unmap_ops;
>>>> + int successful_unmaps = 0;
>>>> + int live_grants;
>>>> for (i = 0; i < data->count; i++) {
>>>> + if (map->unmap_ops[offset + i].status == GNTST_okay &&
>>>> + map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
>>>> + successful_unmaps++;
>>>> +
>>>> WARN_ON(map->unmap_ops[offset + i].status != GNTST_okay &&
>>>> map->unmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>>>> pr_debug("unmap handle=%d st=%d\n",
>>>> @@ -403,6 +407,10 @@ static void __unmap_grant_pages_done(int result,
>>>> map->unmap_ops[offset+i].status);
>>>> map->unmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>>>> if (use_ptemod) {
>>>> + if (map->kunmap_ops[offset + i].status == GNTST_okay &&
>>>> + map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE)
>>>> + successful_unmaps++;
>>>> +
>>>> WARN_ON(map->kunmap_ops[offset + i].status != GNTST_okay &&
>>>> map->kunmap_ops[offset + i].handle != INVALID_GRANT_HANDLE);
>>>> pr_debug("kunmap handle=%u st=%d\n",
>>>> @@ -411,11 +419,15 @@ static void __unmap_grant_pages_done(int result,
>>>> map->kunmap_ops[offset+i].handle = INVALID_GRANT_HANDLE;
>>>> }
>>>> }
>>>> +
>>>> /*
>>>> * Decrease the live-grant counter. This must happen after the loop to
>>>> * prevent premature reuse of the grants by gnttab_mmap().
>>>> */
>>>> - atomic_sub(data->count, &map->live_grants);
>>>> + live_grants = atomic_sub_return(successful_unmaps, &map->live_grants);
>>>> + if (WARN_ON(live_grants < 0))
>>>> + pr_err("%s: live_grants became negative (%d) after unmapping %d pages!\n",
>>>> + __func__, live_grants, successful_unmaps);
>>>> /* Release reference taken by __unmap_grant_pages */
>>>> gntdev_put_map(NULL, map);
>>>> --
>>>> 2.37.3
>>>
>>> Is there a possibility that live_grants could overflow, as it is now
>>> set to a value twice as large as what it had been previously?
>>
>> Good point! My answer in summary: I think that the code could be improved,
>> but with reasonable values for the "limit" module parameter, there should
>> not be issues.
>>
>> Grant mappings are set up via ioctl calls, and the structure field that
>> holds the number of grant references has u32 type:
>>
>> (Quoting from kernel v5.15.71 for convenience)
>> include/uapi/xen/gntdev.h
>> === 8< ===
>> struct ioctl_gntdev_map_grant_ref {
>> /* IN parameters */
>> /* The number of grants to be mapped. */
>> __u32 count;
>> === >8 ===
>>
>> However, the number of grant references is further limited in the actual
>> ioctl handler function gntdev_ioctl_map_grant_ref(), which calls
>> gntdev_test_page_count() to ensure that the number of granted pages
>> requested to be mapped does not exceed "limit". "limit" defaults to 64K,
>> which should be okay to use with an atomic_t type (i.e., a 32-bit signed
>> integer type) like "live_grants", assuming that the system administrator
>> does not go overboard and set "limit" to a very large value:
>>
>> drivers/xen/gntdev.c
>> === 8< ===
>> static unsigned int limit = 64*1024;
>> module_param(limit, uint, 0644);
>> MODULE_PARM_DESC(limit,
>> "Maximum number of grants that may be mapped by one mapping request");
>>
>> /* trimmed */
>>
>> bool gntdev_test_page_count(unsigned int count)
>> {
>> return !count || count > limit;
>> }
>>
>> /* trimmed */
>>
>> static long gntdev_ioctl_map_grant_ref(struct gntdev_priv *priv,
>> struct ioctl_gntdev_map_grant_ref __user *u)
>> {
>> /* trimmed */
>>
>> pr_debug("priv %p, add %d\n", priv, op.count);
>> if (unlikely(gntdev_test_page_count(op.count)))
>> return -EINVAL;
>>
>> /* trimmed */
>> }
>> === >8 ===
>>
>> To be fair, the "count" field of the gndev_grant_map structure is a signed
>> integer, so very large values of count could overflow live_grants, as
>> live_grants needs to accommodate values up to and including 2*count.
>
> Could this be replaced by an unsigned and/or 64-bit integer?
> Alternatively, one could use module_param_cb and param_set_uint_minmax
> to enforce that the limit is something reasonable. That said, one needs
> almost 8TiB to trigger this problem, so while it ought to be fixed it
> isn’t a huge deal. Certainly should not block getting this merged.
Thank you for the continued feedback.
I agree that these can be implemented to prevent overflowing "live_grants".
"live_grants" could be made an atomic64_t, and/or a to-be-chosen maximum
value less than or equal to INT_MAX/2 can be imposed on "limit" using the
approach you suggested.
I think that the latter option could be better, as the driver uses signed
integers in a number of places (including the gntdev_grant_map structure),
but the requested number of mappings (i.e., "count" in
ioctl_gntdev_map_grant_ref, provided by user-space) and "limit" are
unsigned integers.
>
>> drivers/xen/gntdev-common.h
>> === 8< ===
>> struct gntdev_grant_map {
>> atomic_t in_use;
>> struct mmu_interval_notifier notifier;
>> bool notifier_init;
>> struct list_head next;
>> int index;
>> int count;
>> /* trimmed */
>> }
>> === >8 ===
>>
>>> If not, you can add:
>>>
>>> Acked-by: Demi Marie Obenour <[email protected]>
>>
>> Thank you! I hope that the explanation and rationale above are satisfactory.
>> Please let me know what you think.
>
> They are indeed.
Thanks!
Vefa
On 03.10.22 00:20, M. Vefa Bicakci wrote:
> Prior to this commit, if a grant mapping operation failed partially,
> some of the entries in the map_ops array would be invalid, whereas all
> of the entries in the kmap_ops array would be valid. This in turn would
> cause the following logic in gntdev_map_grant_pages to become invalid:
>
> for (i = 0; i < map->count; i++) {
> if (map->map_ops[i].status == GNTST_okay) {
> map->unmap_ops[i].handle = map->map_ops[i].handle;
> if (!use_ptemod)
> alloced++;
> }
> if (use_ptemod) {
> if (map->kmap_ops[i].status == GNTST_okay) {
> if (map->map_ops[i].status == GNTST_okay)
> alloced++;
> map->kunmap_ops[i].handle = map->kmap_ops[i].handle;
> }
> }
> }
> ...
> atomic_add(alloced, &map->live_grants);
>
> Assume that use_ptemod is true (i.e., the domain mapping the granted
> pages is a paravirtualized domain). In the code excerpt above, note that
> the "alloced" variable is only incremented when both kmap_ops[i].status
> and map_ops[i].status are set to GNTST_okay (i.e., both mapping
> operations are successful). However, as also noted above, there are
> cases where a grant mapping operation fails partially, breaking the
> assumption of the code excerpt above.
>
> The aforementioned causes map->live_grants to be incorrectly set. In
> some cases, all of the map_ops mappings fail, but all of the kmap_ops
> mappings succeed, meaning that live_grants may remain zero. This in turn
> makes it impossible to unmap the successfully grant-mapped pages pointed
> to by kmap_ops, because unmap_grant_pages has the following snippet of
> code at its beginning:
>
> if (atomic_read(&map->live_grants) == 0)
> return; /* Nothing to do */
>
> In other cases where only some of the map_ops mappings fail but all
> kmap_ops mappings succeed, live_grants is made positive, but when the
> user requests unmapping the grant-mapped pages, __unmap_grant_pages_done
> will then make map->live_grants negative, because the latter function
> does not check if all of the pages that were requested to be unmapped
> were actually unmapped, and the same function unconditionally subtracts
> "data->count" (i.e., a value that can be greater than map->live_grants)
> from map->live_grants. The side effects of a negative live_grants value
> have not been studied.
>
> The net effect of all of this is that grant references are leaked in one
> of the above conditions. In Qubes OS v4.1 (which uses Xen's grant
> mechanism extensively for X11 GUI isolation), this issue manifests
> itself with warning messages like the following to be printed out by the
> Linux kernel in the VM that had granted pages (that contain X11 GUI
> window data) to dom0: "g.e. 0x1234 still pending", especially after the
> user rapidly resizes GUI VM windows (causing some grant-mapping
> operations to partially or completely fail, due to the fact that the VM
> unshares some of the pages as part of the window resizing, making the
> pages impossible to grant-map from dom0).
>
> The fix for this issue involves counting all successful map_ops and
> kmap_ops mappings separately, and then adding the sum to live_grants.
> During unmapping, only the number of successfully unmapped grants is
> subtracted from live_grants. The code is also modified to check for
> negative live_grants values after the subtraction and warn the user.
>
> Link: https://github.com/QubesOS/qubes-issues/issues/7631
> Fixes: dbe97cff7dd9 ("xen/gntdev: Avoid blocking in unmap_grant_pages()")
> Cc: [email protected]
> Signed-off-by: M. Vefa Bicakci <[email protected]>
Reviewed-by: Juergen Gross <[email protected]>
Juergen
On 03.10.22 00:20, M. Vefa Bicakci wrote:
> Hi all,
>
> First of all, sorry for the delay!
>
> These patches continue the code review for the following patches:
> https://lore.kernel.org/xen-devel/[email protected]/t/#u
>
> The original description of the patch set is as follows:
>
> "The changes in this patch series intend to fix the Xen grant device
> driver, so that grant mapping leaks caused by partially failed grant
> mapping operations are avoided with the first patch, and so that the
> splitting of VMAs does not result in incorrectly unmapped grant pages
> with the second patch. The second patch also prevents a similar issue
> in a double-mapping scenario, where mmap() is used with MAP_FIXED to
> map grants over an existing mapping created with the same grants, and
> where grant pages are unmapped incorrectly as well."
>
> A summary of the changes from v1 is as follows:
> - Addressed Juergen's code review comment regarding the first patch.
> - Amended the description of the second patch to note that the described
> issues are encountered with PV domains.
>
> Verification notes:
>
> - I have tested these commits on top of Linux v5.15.70 and v5.15.71, and
> I verified that they compile successfully on top of the tag
> "next-20220930", which corresponds to the base commit ID included at
> the bottom of this e-mail.
>
> - My tests consist of using a kernel with Qubes OS v4.1's patches and
> these patches on my main computer for day-to-day tasks, in conjunction
> with Qubes OS's version of the Xen hypervisor v4.14.5, with the latter
> custom-compiled with CONFIG_DEBUG.
>
> - I used a test program that verifies the following scenarios with an
> unprivileged paravirtualized (PV) Xen domain:
>
> - A program mmap()s two pages from another Xen domain and munmap()s
> the pages one by one. This used to result in implicit unmap errors
> to be reported by Xen and a general protection fault to be triggered
> by Xen in the affected domain, but now works as expected.
> - A program mmap()s two pages from another Xen domain and then
> attempts to remap (via MAP_FIXED) the same mapping again over the
> same virtual address. This used to result in similar issues
> (implicit unmap errors and general protection fault), but now is
> rejected by the kernel.
> - A program mmap()s two pages from another Xen domain and then
> attempts to mmap() the same mapping again to a different virtual
> address, by passing NULL as mmap()'s first argument. This used to be
> rejected by the kernel, and it continues to be rejected by the
> kernel.
>
> - Unprivileged PVH Xen domains were also sanity tested with the same
> test program. I should note that PVH domains worked as expected
> without these patches too.
>
> - Finally, I have verified that the original "g.e. 0x1234 still pending"
> issue does not appear after rapidly resizing GUI windows in Qubes OS
> v4.1.
Series pushed to xen/tip.git for-linus-6.1
Juergen
On Fri, Oct 07, 2022 at 07:17:41AM +0200, Juergen Gross wrote:
> On 03.10.22 00:20, M. Vefa Bicakci wrote:
> > Hi all,
> >
> > First of all, sorry for the delay!
> >
> > These patches continue the code review for the following patches:
> > https://lore.kernel.org/xen-devel/[email protected]/t/#u
> >
> > The original description of the patch set is as follows:
> >
> > "The changes in this patch series intend to fix the Xen grant device
> > driver, so that grant mapping leaks caused by partially failed grant
> > mapping operations are avoided with the first patch, and so that the
> > splitting of VMAs does not result in incorrectly unmapped grant pages
> > with the second patch. The second patch also prevents a similar issue
> > in a double-mapping scenario, where mmap() is used with MAP_FIXED to
> > map grants over an existing mapping created with the same grants, and
> > where grant pages are unmapped incorrectly as well."
> >
> > A summary of the changes from v1 is as follows:
> > - Addressed Juergen's code review comment regarding the first patch.
> > - Amended the description of the second patch to note that the described
> > issues are encountered with PV domains.
> >
> > Verification notes:
> >
> > - I have tested these commits on top of Linux v5.15.70 and v5.15.71, and
> > I verified that they compile successfully on top of the tag
> > "next-20220930", which corresponds to the base commit ID included at
> > the bottom of this e-mail.
> >
> > - My tests consist of using a kernel with Qubes OS v4.1's patches and
> > these patches on my main computer for day-to-day tasks, in conjunction
> > with Qubes OS's version of the Xen hypervisor v4.14.5, with the latter
> > custom-compiled with CONFIG_DEBUG.
> >
> > - I used a test program that verifies the following scenarios with an
> > unprivileged paravirtualized (PV) Xen domain:
> >
> > - A program mmap()s two pages from another Xen domain and munmap()s
> > the pages one by one. This used to result in implicit unmap errors
> > to be reported by Xen and a general protection fault to be triggered
> > by Xen in the affected domain, but now works as expected.
> > - A program mmap()s two pages from another Xen domain and then
> > attempts to remap (via MAP_FIXED) the same mapping again over the
> > same virtual address. This used to result in similar issues
> > (implicit unmap errors and general protection fault), but now is
> > rejected by the kernel.
> > - A program mmap()s two pages from another Xen domain and then
> > attempts to mmap() the same mapping again to a different virtual
> > address, by passing NULL as mmap()'s first argument. This used to be
> > rejected by the kernel, and it continues to be rejected by the
> > kernel.
> >
> > - Unprivileged PVH Xen domains were also sanity tested with the same
> > test program. I should note that PVH domains worked as expected
> > without these patches too.
> >
> > - Finally, I have verified that the original "g.e. 0x1234 still pending"
> > issue does not appear after rapidly resizing GUI windows in Qubes OS
> > v4.1.
>
> Series pushed to xen/tip.git for-linus-6.1
Thanks!
--
Sincerely,
Demi Marie Obenour (she/her/hers)
Invisible Things Lab
On 2022-10-07 01:17, Juergen Gross wrote:
> On 03.10.22 00:20, M. Vefa Bicakci wrote:
>> Hi all,
>>
>> First of all, sorry for the delay!
>>
>> These patches continue the code review for the following patches:
>> https://lore.kernel.org/xen-devel/[email protected]/t/#u
>>
>> The original description of the patch set is as follows:
>>
>> "The changes in this patch series intend to fix the Xen grant device
>> driver, so that grant mapping leaks caused by partially failed grant
>> mapping operations are avoided with the first patch, and so that the
>> splitting of VMAs does not result in incorrectly unmapped grant pages
>> with the second patch. The second patch also prevents a similar issue
>> in a double-mapping scenario, where mmap() is used with MAP_FIXED to
>> map grants over an existing mapping created with the same grants, and
>> where grant pages are unmapped incorrectly as well."
>>
>> A summary of the changes from v1 is as follows:
>> - Addressed Juergen's code review comment regarding the first patch.
>> - Amended the description of the second patch to note that the described
>> issues are encountered with PV domains.
>>
>> Verification notes:
>>
>> - I have tested these commits on top of Linux v5.15.70 and v5.15.71, and
>> I verified that they compile successfully on top of the tag
>> "next-20220930", which corresponds to the base commit ID included at
>> the bottom of this e-mail.
>>
>> - My tests consist of using a kernel with Qubes OS v4.1's patches and
>> these patches on my main computer for day-to-day tasks, in conjunction
>> with Qubes OS's version of the Xen hypervisor v4.14.5, with the latter
>> custom-compiled with CONFIG_DEBUG.
>>
>> - I used a test program that verifies the following scenarios with an
>> unprivileged paravirtualized (PV) Xen domain:
>>
>> - A program mmap()s two pages from another Xen domain and munmap()s
>> the pages one by one. This used to result in implicit unmap errors
>> to be reported by Xen and a general protection fault to be triggered
>> by Xen in the affected domain, but now works as expected.
>> - A program mmap()s two pages from another Xen domain and then
>> attempts to remap (via MAP_FIXED) the same mapping again over the
>> same virtual address. This used to result in similar issues
>> (implicit unmap errors and general protection fault), but now is
>> rejected by the kernel.
>> - A program mmap()s two pages from another Xen domain and then
>> attempts to mmap() the same mapping again to a different virtual
>> address, by passing NULL as mmap()'s first argument. This used to be
>> rejected by the kernel, and it continues to be rejected by the
>> kernel.
>>
>> - Unprivileged PVH Xen domains were also sanity tested with the same
>> test program. I should note that PVH domains worked as expected
>> without these patches too.
>>
>> - Finally, I have verified that the original "g.e. 0x1234 still pending"
>> issue does not appear after rapidly resizing GUI windows in Qubes OS
>> v4.1.
>
> Series pushed to xen/tip.git for-linus-6.1
>
>
> Juergen
I am a bit late, but thank you for reviewing the changes and merging them!
Vefa