Re: [PATCH 06/23] hw/block/dataplane/virtio-blk: Avoid dynamic stack allocation

[Philippe Mathieu-Daudé]

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 <philmd@redhat.com>
>> ---
>>  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.

> 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?

> 
> Stefan
>