Re: [Qemu-devel] [PATCH 8/8] qcow2: Read outside array bounds in qcow2_pre_write_overlap_check()

[Liam Merwick]

On 30/08/18 19:43, Eric Blake wrote:
> On 08/30/2018 10:47 AM, Liam Merwick wrote:
> > The commit for 0e4e4318eaa5 increments QCOW2_OL_MAX_BITNR but does not
> > add an array entry for QCOW2_OL_BITMAP_DIRECTORY_BITNR to 
> > metadata_ol_names[].
> > As a result, an array dereference of metadata_ol_names[8] in
> > qcow2_pre_write_overlap_check() could result in a read outside of the 
> > array bounds.
> >
> > Fixes: 0e4e4318eaa5 ('qcow2: add overlap check for bitmap directory')
> >
> > Cc: Vladimir Sementsov-Ogievskiy <address@hidden>
> > Signed-off-by: Liam Merwick <address@hidden>
> > Reviewed-by: Darren Kenny <address@hidden>
> > Reviewed-by: Mark Kanda <address@hidden>
> > ---
> >   block/qcow2-refcount.c | 17 +++++++++--------
> >   1 file changed, 9 insertions(+), 8 deletions(-)
>
> The fix looks correct, but to prevent the problem from happening again, 
> I'd suggest you also add a compile-time BUG_ON that fails if the array 
> size gets out of sync again due to another addition of another overlap 
> detection bit.

Good idea. There is no generic BUG_ON in QEMU (just a few private 
copies) or BUILD_BUG_ON. I can add a commit that introduces a copy of 
include/linux/build_bug.h from the Linux kernel and use BUILD_BUG_ON in 
this commit.  Is there any reason not to do that?

Regards,
Liam