[PATCH v3] exec: set map length to zero when returning NULL

[PATCH v3] exec: set map length to zero when returning NULL

[P J P]

From: Prasad J Pandit <pjp@fedoraproject.org>

When mapping physical memory into host's virtual address space,
'address_space_map' may return NULL if BounceBuffer is in_use.
Set and return '*plen = 0' to avoid later NULL pointer dereference.

Reported-by: Alexander Bulekov <alxndr@bu.edu>
Fixes: https://bugs.launchpad.net/qemu/+bug/1878259
Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
---
 exec.c | 1 +
 1 file changed, 1 insertion(+)

Update v3: set *plen = 0;
  -> https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg06249.html

diff --git a/exec.c b/exec.c
index 5162f0d12f..4eea84bf66 100644
--- a/exec.c
+++ b/exec.c
@@ -3538,6 +3538,7 @@ void *address_space_map(AddressSpace *as,
 
     if (!memory_access_is_direct(mr, is_write)) {
         if (atomic_xchg(&bounce.in_use, true)) {
+            *plen = 0;
             return NULL;
         }
         /* Avoid unbounded allocations */
-- 
2.26.2

Re: [PATCH v3] exec: set map length to zero when returning NULL

[Philippe Mathieu-Daudé]

On 5/26/20 9:50 AM, P J P wrote:
> From: Prasad J Pandit <pjp@fedoraproject.org>
> 
> When mapping physical memory into host's virtual address space,
> 'address_space_map' may return NULL if BounceBuffer is in_use.
> Set and return '*plen = 0' to avoid later NULL pointer dereference.
> 
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1878259
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  exec.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> Update v3: set *plen = 0;
>   -> https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg06249.html
> 
> diff --git a/exec.c b/exec.c
> index 5162f0d12f..4eea84bf66 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -3538,6 +3538,7 @@ void *address_space_map(AddressSpace *as,
>  
>      if (!memory_access_is_direct(mr, is_write)) {
>          if (atomic_xchg(&bounce.in_use, true)) {
> +            *plen = 0;
>              return NULL;
>          }
>          /* Avoid unbounded allocations */
> 

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>

Re: [PATCH v3] exec: set map length to zero when returning NULL

[Philippe Mathieu-Daudé]

On Tue, May 26, 2020 at 9:53 AM P J P <ppandit@redhat.com> wrote:
>
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> When mapping physical memory into host's virtual address space,
> 'address_space_map' may return NULL if BounceBuffer is in_use.
> Set and return '*plen = 0' to avoid later NULL pointer dereference.
>
> Reported-by: Alexander Bulekov <alxndr@bu.edu>

Suggested-by: Paolo Bonzini <pbonzini@redhat.com>

> Fixes: https://bugs.launchpad.net/qemu/+bug/1878259
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>
> ---
>  exec.c | 1 +
>  1 file changed, 1 insertion(+)
>
> Update v3: set *plen = 0;
>   -> https://lists.gnu.org/archive/html/qemu-devel/2020-05/msg06249.html
>
> diff --git a/exec.c b/exec.c
> index 5162f0d12f..4eea84bf66 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -3538,6 +3538,7 @@ void *address_space_map(AddressSpace *as,
>
>      if (!memory_access_is_direct(mr, is_write)) {
>          if (atomic_xchg(&bounce.in_use, true)) {
> +            *plen = 0;
>              return NULL;
>          }
>          /* Avoid unbounded allocations */
> --
> 2.26.2
>

Re: [PATCH v3] exec: set map length to zero when returning NULL

[Peter Maydell]

On Tue, 26 May 2020 at 08:53, P J P <ppandit@redhat.com> wrote:
>
> From: Prasad J Pandit <pjp@fedoraproject.org>
>
> When mapping physical memory into host's virtual address space,
> 'address_space_map' may return NULL if BounceBuffer is in_use.
> Set and return '*plen = 0' to avoid later NULL pointer dereference.
>
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> Fixes: https://bugs.launchpad.net/qemu/+bug/1878259
> Signed-off-by: Prasad J Pandit <pjp@fedoraproject.org>

The doc comment for this function in include/exec/memory.h
currently says
"May return %NULL if resources needed to perform the mapping
 are exhausted."

If we're tightening the API to guarantee to also set *plen to 0
in this failure case it would be good to capture that in the
documentation, eg:

"May return %NULL (and set *@plen to 0) if resources needed
 to perform the mapping are exhausted.)"

thanks
-- PMM

Re: [PATCH v3] exec: set map length to zero when returning NULL

[P J P]

+-- On Tue, 26 May 2020, Peter Maydell wrote --+
| The doc comment for this function in include/exec/memory.h currently says
| 
| "May return %NULL if resources needed to perform the mapping
|  are exhausted."
| 
| If we're tightening the API to guarantee to also set *plen to 0 in this 
| failure case it would be good to capture that in the documentation, eg:
| 
| "May return %NULL (and set *@plen to 0) if resources needed
|  to perform the mapping are exhausted.)"

Sending patch v4. Thank you.
--
Prasad J Pandit / Red Hat Product Security Team
8685 545E B54C 486B C6EB 271E E285 8B5A F050 DE8D