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

[Alex Williamson]

On Fri, 2013-08-16 at 09:10 +0200, Laszlo Ersek wrote:
> 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 <address@hidden>
> > Cc: address@hidden
> > ---
> > 
> > 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 <address@hidden>

I realized this after I went to bed too.  I'll send a v2 w/o the second
condition.  Thanks,

Alex