Hi Dave,
Following our previous conversation I have updated each
patch with a highlight of potential problematic places
with regards to the memory ordering guarantees, as well
as link to the document I am working on to clarify the
matter.
While I didn't see any issues with these 5 particular
conversions, of course I don't understand code anywhere
good enough to make the final claim that "everything is
100% ok".
Please let me know what kind of other clarification or
anything is needed that can help you to make the needed
verification.
Best Regards,
Elena.
Changes in v2:
- each patch now has a clarification on potential
problematic places with regards to the memory ordering
guarantees as well as reference to a hopefully soon to be
merged more extensive documentation on the
differences between atomic_t and refcount_t on ordering.
- Rebase on linux-next master.
This series, for xfs, replaces atomic_t reference
counters with the new refcount_t type and API (see include/linux/refcount.h).
By doing this we prevent intentional or accidental
underflows or overflows that can lead to use-after-free vulnerabilities.
The patches are fully independent and can be cherry-picked separately.
If there are no objections to the patches, please merge them via respective trees.
Elena Reshetova (5):
fs, xfs: convert xfs_bui_log_item.bui_refcount from atomic_t to
refcount_t
fs, xfs: convert xfs_efi_log_item.efi_refcount from atomic_t to
refcount_t
fs, xfs: convert xlog_ticket.t_ref from atomic_t to refcount_t
fs, xfs: convert xfs_cui_log_item.cui_refcount from atomic_t to
refcount_t
fs, xfs: convert xfs_rui_log_item.rui_refcount from atomic_t to
refcount_t
fs/xfs/xfs_bmap_item.c | 6 +++---
fs/xfs/xfs_bmap_item.h | 2 +-
fs/xfs/xfs_extfree_item.c | 6 +++---
fs/xfs/xfs_extfree_item.h | 2 +-
fs/xfs/xfs_linux.h | 1 +
fs/xfs/xfs_log.c | 10 +++++-----
fs/xfs/xfs_log_priv.h | 2 +-
fs/xfs/xfs_refcount_item.c | 6 +++---
fs/xfs/xfs_refcount_item.h | 2 +-
fs/xfs/xfs_rmap_item.c | 6 +++---
fs/xfs/xfs_rmap_item.h | 2 +-
11 files changed, 23 insertions(+), 22 deletions(-)
--
2.7.4
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atomic_t variables are currently used to implement reference
counters with the following properties:
- counter is initialized to 1 using atomic_set()
- a resource is freed upon counter reaching zero
- once counter reaches zero, its further
increments aren't allowed
- counter schema uses basic atomic operations
(set, inc, inc_not_zero, dec_and_test, etc.)
Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.
The variable xfs_bui_log_item.bui_refcount is used as pure reference counter.
Convert it to refcount_t and fix up the operations.
**Important note for maintainers:
Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.
The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.
Please double check that you don't have some undocumented
memory guarantees for this variable usage.
For the xfs_bui_log_item.bui_refcount it might make a difference
in following places:
- xfs_bui_release(): decrement in refcount_dec_and_test() only
provides RELEASE ordering and control dependency on success
vs. fully ordered atomic counterpart
Suggested-by: Kees Cook <[email protected]>
Reviewed-by: David Windsor <[email protected]>
Reviewed-by: Hans Liljestrand <[email protected]>
Signed-off-by: Elena Reshetova <[email protected]>
---
fs/xfs/xfs_bmap_item.c | 6 +++---
fs/xfs/xfs_bmap_item.h | 2 +-
fs/xfs/xfs_linux.h | 1 +
3 files changed, 5 insertions(+), 4 deletions(-)
diff --git a/fs/xfs/xfs_bmap_item.c b/fs/xfs/xfs_bmap_item.c
index dd136f7..0f2e3d8 100644
--- a/fs/xfs/xfs_bmap_item.c
+++ b/fs/xfs/xfs_bmap_item.c
@@ -201,7 +201,7 @@ xfs_bui_init(
buip->bui_format.bui_nextents = XFS_BUI_MAX_FAST_EXTENTS;
buip->bui_format.bui_id = (uintptr_t)(void *)buip;
atomic_set(&buip->bui_next_extent, 0);
- atomic_set(&buip->bui_refcount, 2);
+ refcount_set(&buip->bui_refcount, 2);
return buip;
}
@@ -217,8 +217,8 @@ void
xfs_bui_release(
struct xfs_bui_log_item *buip)
{
- ASSERT(atomic_read(&buip->bui_refcount) > 0);
- if (atomic_dec_and_test(&buip->bui_refcount)) {
+ ASSERT(refcount_read(&buip->bui_refcount) > 0);
+ if (refcount_dec_and_test(&buip->bui_refcount)) {
xfs_trans_ail_remove(&buip->bui_item, SHUTDOWN_LOG_IO_ERROR);
xfs_bui_item_free(buip);
}
diff --git a/fs/xfs/xfs_bmap_item.h b/fs/xfs/xfs_bmap_item.h
index c867daa..7048b14 100644
--- a/fs/xfs/xfs_bmap_item.h
+++ b/fs/xfs/xfs_bmap_item.h
@@ -61,7 +61,7 @@ struct kmem_zone;
*/
struct xfs_bui_log_item {
struct xfs_log_item bui_item;
- atomic_t bui_refcount;
+ refcount_t bui_refcount;
atomic_t bui_next_extent;
unsigned long bui_flags; /* misc flags */
struct xfs_bui_log_format bui_format;
diff --git a/fs/xfs/xfs_linux.h b/fs/xfs/xfs_linux.h
index 99562ec..31658ac 100644
--- a/fs/xfs/xfs_linux.h
+++ b/fs/xfs/xfs_linux.h
@@ -20,6 +20,7 @@
#include <linux/types.h>
#include <linux/uuid.h>
+#include <linux/refcount.h>
/*
* Kernel specific type declarations for XFS
--
2.7.4
From 1585307229518484375@xxx Tue Nov 28 10:59:48 +0000 2017
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atomic_t variables are currently used to implement reference
counters with the following properties:
- counter is initialized to 1 using atomic_set()
- a resource is freed upon counter reaching zero
- once counter reaches zero, its further
increments aren't allowed
- counter schema uses basic atomic operations
(set, inc, inc_not_zero, dec_and_test, etc.)
Such atomic variables should be converted to a newly provided
refcount_t type and API that prevents accidental counter overflows
and underflows. This is important since overflows and underflows
can lead to use-after-free situation and be exploitable.
The variable xlog_ticket.t_ref is used as pure reference counter.
Convert it to refcount_t and fix up the operations.
**Important note for maintainers:
Some functions from refcount_t API defined in lib/refcount.c
have different memory ordering guarantees than their atomic
counterparts.
The full comparison can be seen in
https://lkml.org/lkml/2017/11/15/57 and it is hopefully soon
in state to be merged to the documentation tree.
Normally the differences should not matter since refcount_t provides
enough guarantees to satisfy the refcounting use cases, but in
some rare cases it might matter.
Please double check that you don't have some undocumented
memory guarantees for this variable usage.
For the xlog_ticket.t_ref it might make a difference
in following places:
- xfs_log_ticket_put(): decrement in refcount_dec_and_test() only
provides RELEASE ordering and control dependency on success
vs. fully ordered atomic counterpart
Suggested-by: Kees Cook <[email protected]>
Reviewed-by: David Windsor <[email protected]>
Reviewed-by: Hans Liljestrand <[email protected]>
Signed-off-by: Elena Reshetova <[email protected]>
---
fs/xfs/xfs_log.c | 10 +++++-----
fs/xfs/xfs_log_priv.h | 2 +-
2 files changed, 6 insertions(+), 6 deletions(-)
diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index a503af9..6e1528f 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -3600,8 +3600,8 @@ void
xfs_log_ticket_put(
xlog_ticket_t *ticket)
{
- ASSERT(atomic_read(&ticket->t_ref) > 0);
- if (atomic_dec_and_test(&ticket->t_ref))
+ ASSERT(refcount_read(&ticket->t_ref) > 0);
+ if (refcount_dec_and_test(&ticket->t_ref))
kmem_zone_free(xfs_log_ticket_zone, ticket);
}
@@ -3609,8 +3609,8 @@ xlog_ticket_t *
xfs_log_ticket_get(
xlog_ticket_t *ticket)
{
- ASSERT(atomic_read(&ticket->t_ref) > 0);
- atomic_inc(&ticket->t_ref);
+ ASSERT(refcount_read(&ticket->t_ref) > 0);
+ refcount_inc(&ticket->t_ref);
return ticket;
}
@@ -3732,7 +3732,7 @@ xlog_ticket_alloc(
unit_res = xfs_log_calc_unit_res(log->l_mp, unit_bytes);
- atomic_set(&tic->t_ref, 1);
+ refcount_set(&tic->t_ref, 1);
tic->t_task = current;
INIT_LIST_HEAD(&tic->t_queue);
tic->t_unit_res = unit_res;
diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 1299759..7482145 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -168,7 +168,7 @@ typedef struct xlog_ticket {
struct list_head t_queue; /* reserve/write queue */
struct task_struct *t_task; /* task that owns this ticket */
xlog_tid_t t_tid; /* transaction identifier : 4 */
- atomic_t t_ref; /* ticket reference count : 4 */
+ refcount_t t_ref; /* ticket reference count : 4 */
int t_curr_res; /* current reservation in bytes : 4 */
int t_unit_res; /* unit reservation in bytes : 4 */
char t_ocnt; /* original count : 1 */
--
2.7.4
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