Re: [Qemu-devel] [PATCH] exec: Fix non-power-of-2 sized accesses

[Laszlo Ersek <lersek@redhat.com>]

On 08/16/13 06:55, Alex Williamson wrote:
> Since commit 23326164 we align access sizes to match the alignment of
> the address, but we don't align the access size itself.  This means we
> let illegal access sizes (ex. 3) slip through if the address is
> sufficiently aligned (ex. 4).  This results in an abort which would be
> easy for a guest to trigger.  Account for aligning the access size.
> 
> Signed-off-by: Alex Williamson <alex.williamson@redhat.com>
> Cc: qemu-stable@nongnu.org
> ---
> 
> In the example I saw the guest was doing a 4-byte read at I/O port
> 0xcd7.  We satisfy the first byte with a 1-byte read leaving 3 bytes
> remaining at an 8-byte aligned address... boom.  ffs() caused weird
> stack smashing errors here, so I just did a loop since it can only
> run for a few iterations max.
> 
>  exec.c |    7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/exec.c b/exec.c
> index 3ca9381..652fc3a 100644
> --- a/exec.c
> +++ b/exec.c
> @@ -1924,6 +1924,13 @@ static int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr)
>          }
>      }
>  
> +    /* Size must be a power of 2 */
> +    if (l & (l - 1)) {
> +        while (l & (access_size_max - 1) && access_size_max > 1) {
> +            access_size_max >>= 1;
> +        }
> +    }
> +
>      /* Don't attempt accesses larger than the maximum.  */
>      if (l > access_size_max) {
>          l = access_size_max;
> 
> 

Assuming that "access_size_max" is positive when reaching the code
you're adding (and it does seem positive at that point), you don't need
"&& access_size_max > 1". That expression won't be evaluated when it
would matter (ie. when access_size_max==1).

Anyway that's not a bug.

Reviewed-by: Laszlo Ersek <lersek@redhat.com>