From: Stefano Stabellini <[email protected]>
If the xenstore page hasn't been allocated properly, reading the value
of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
error. Instead, it will succeed and return zero. Instead of attempting
to xen_remap a bad guest physical address, detect this condition and
return early.
Note that although a guest physical address of zero for
HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
and zero has never been validly used in that capacity.
Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
above ULONG_MAX should never be passed by the Xen tools to HVM guests
anyway, so check for this condition and return early.
Cc: [email protected]
Signed-off-by: Stefano Stabellini <[email protected]>
---
Changes in v2:
- add check for ULLONG_MAX (unitialized)
- add check for ULONG_MAX #if BITS_PER_LONG == 32 (actual error)
- add pr_err error message
drivers/xen/xenbus/xenbus_probe.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)
diff --git a/drivers/xen/xenbus/xenbus_probe.c b/drivers/xen/xenbus/xenbus_probe.c
index 94405bb3829e..c7472ff27a93 100644
--- a/drivers/xen/xenbus/xenbus_probe.c
+++ b/drivers/xen/xenbus/xenbus_probe.c
@@ -951,6 +951,20 @@ static int __init xenbus_init(void)
err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
if (err)
goto out_error;
+ /* Uninitialized. */
+ if (v == 0 || v == ULLONG_MAX) {
+ err = -ENOENT;
+ goto out_error;
+ }
+ /* Avoid truncation on 32-bit. */
+#if BITS_PER_LONG == 32
+ if (v > ULONG_MAX) {
+ pr_err("%s: cannot handle HVM_PARAM_STORE_PFN=%llx > ULONG_MAX\n",
+ __func__, v);
+ err = -EINVAL;
+ goto out_error;
+ }
+#endif
xen_store_gfn = (unsigned long)v;
xen_store_interface =
xen_remap(xen_store_gfn << XEN_PAGE_SHIFT,
--
2.25.1
On 11/19/21 3:29 PM, Stefano Stabellini wrote:
> From: Stefano Stabellini <[email protected]>
>
> If the xenstore page hasn't been allocated properly, reading the value
> of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually return
> error. Instead, it will succeed and return zero. Instead of attempting
> to xen_remap a bad guest physical address, detect this condition and
> return early.
>
> Note that although a guest physical address of zero for
> HVM_PARAM_STORE_PFN is theoretically possible, it is not a good choice
> and zero has never been validly used in that capacity.
>
> Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
>
> For 32-bit Linux, any pfn above ULONG_MAX would get truncated. Pfns
> above ULONG_MAX should never be passed by the Xen tools to HVM guests
> anyway, so check for this condition and return early.
>
> Cc: [email protected]
> Signed-off-by: Stefano Stabellini <[email protected]>
Reviewed-by: Boris Ostrovsky <[email protected]>
On 19.11.2021 21:29, Stefano Stabellini wrote:
> --- a/drivers/xen/xenbus/xenbus_probe.c
> +++ b/drivers/xen/xenbus/xenbus_probe.c
> @@ -951,6 +951,20 @@ static int __init xenbus_init(void)
> err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
> if (err)
> goto out_error;
> + /* Uninitialized. */
> + if (v == 0 || v == ULLONG_MAX) {
Didn't you have a comment in v1 here regarding the check against 0? Or was that
just like now only in the description? IOW I think there ought to be a code
comment justifying the theoretically wrong check ...
Also, while I realize there are various other similar assumptions elsewhere, I
would generally recommend to avoid such: There's no guarantee that now and
forever unsigned long long and uint64_t are the same thing. And it's easy in
cases like this one:
if (!v || !(v + 1)) {
Jan
On Mon, 22 Nov 2021, Jan Beulich wrote:
> On 19.11.2021 21:29, Stefano Stabellini wrote:
> > --- a/drivers/xen/xenbus/xenbus_probe.c
> > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > @@ -951,6 +951,20 @@ static int __init xenbus_init(void)
> > err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
> > if (err)
> > goto out_error;
> > + /* Uninitialized. */
> > + if (v == 0 || v == ULLONG_MAX) {
>
> Didn't you have a comment in v1 here regarding the check against 0? Or was that
> just like now only in the description? IOW I think there ought to be a code
> comment justifying the theoretically wrong check ...
Yeah, I added all the info in the commit message and shortened the
in-code comment this time. I am also happy to keep the details in the
in-code comment, e.g.:
/*
* If the xenstore page hasn't been allocated properly, reading the
* value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually
* return error. Instead, it will succeed and return zero. Instead of
* attempting to xen_remap a bad guest physical address, detect this
* condition and return early.
*
* Note that although a guest physical address of zero for
* HVM_PARAM_STORE_PFN is theoretically possible, it is not a good
* choice and zero has never been validly used in that capacity.
*
* Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
*/
> Also, while I realize there are various other similar assumptions elsewhere, I
> would generally recommend to avoid such: There's no guarantee that now and
> forever unsigned long long and uint64_t are the same thing. And it's easy in
> cases like this one:
>
> if (!v || !(v + 1)) {
I am happy to use this.
On Mon, 22 Nov 2021, Stefano Stabellini wrote:
> On Mon, 22 Nov 2021, Jan Beulich wrote:
> > On 19.11.2021 21:29, Stefano Stabellini wrote:
> > > --- a/drivers/xen/xenbus/xenbus_probe.c
> > > +++ b/drivers/xen/xenbus/xenbus_probe.c
> > > @@ -951,6 +951,20 @@ static int __init xenbus_init(void)
> > > err = hvm_get_parameter(HVM_PARAM_STORE_PFN, &v);
> > > if (err)
> > > goto out_error;
> > > + /* Uninitialized. */
> > > + if (v == 0 || v == ULLONG_MAX) {
> >
> > Didn't you have a comment in v1 here regarding the check against 0? Or was that
> > just like now only in the description? IOW I think there ought to be a code
> > comment justifying the theoretically wrong check ...
>
> Yeah, I added all the info in the commit message and shortened the
> in-code comment this time. I am also happy to keep the details in the
> in-code comment, e.g.:
>
> /*
> * If the xenstore page hasn't been allocated properly, reading the
> * value of the related hvm_param (HVM_PARAM_STORE_PFN) won't actually
> * return error. Instead, it will succeed and return zero. Instead of
> * attempting to xen_remap a bad guest physical address, detect this
> * condition and return early.
> *
> * Note that although a guest physical address of zero for
> * HVM_PARAM_STORE_PFN is theoretically possible, it is not a good
> * choice and zero has never been validly used in that capacity.
> *
> * Also recognize the invalid value of INVALID_PFN which is ULLONG_MAX.
> */
I sent a new version of the patch with the check below and slightly more
concise version of this comment.
> > Also, while I realize there are various other similar assumptions elsewhere, I
> > would generally recommend to avoid such: There's no guarantee that now and
> > forever unsigned long long and uint64_t are the same thing. And it's easy in
> > cases like this one:
> >
> > if (!v || !(v + 1)) {
>
> I am happy to use this.