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.william...@redhat.com>
> Cc: qemu-sta...@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 <ler...@redhat.com>
