On Thu, May 06, 2021 at 11:01:47AM +0200, Philippe Mathieu-Daudé wrote: > On 5/6/21 10:53 AM, Stefan Hajnoczi wrote: > > On Wed, May 05, 2021 at 11:10:30PM +0200, Philippe Mathieu-Daudé wrote: > >> Use autofree heap allocation instead of variable-length > >> array on the stack. > >> > >> Signed-off-by: Philippe Mathieu-Daudé <[email protected]> > >> --- > >> hw/block/dataplane/virtio-blk.c | 7 ++++--- > >> 1 file changed, 4 insertions(+), 3 deletions(-) > > > > Why? > > The motivation behind removing all variable-length allocations > (and adding CPPFLAG+=-Wvla at the end) is to avoid security > vulnerabilities such CVE-2021-3527.
I see. Please mention it in the commit description. There could be other
reasons for this change, like minimizing stack usage, so I wasn't sure
why.
> > This is a performance-critical code path and BITS_TO_LONGS(nvqs) is
> > small.
>
> OK, having looked better at nvqs, I suppose this is preferred:
>
> -- >8 --
> @@ -60,7 +60,7 @@ static void notify_guest_bh(void *opaque)
> {
> VirtIOBlockDataPlane *s = opaque;
> unsigned nvqs = s->conf->num_queues;
> - unsigned long bitmap[BITS_TO_LONGS(nvqs)];
> + unsigned long bitmap[BITS_TO_LONGS(VIRTIO_QUEUE_MAX)];
> unsigned j;
>
> memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
> ---
>
> Would that work for you?
It's a little risky since s->batch_notify_vqs does not have
sizeof(bitmap). That makes uninitialized data and buffer overflows more
likely. Your example has the bug:
memcpy(bitmap, s->batch_notify_vqs, sizeof(bitmap));
^^^^^^^^^^^^^^
Accesses beyond the end of s->batch_notify_vqs[].
Can we eliminate bitmap[] entirely by using bitops.h APIs on
s->batch_notify_vqs instead?
Stefan
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