In virtqueue_{split,packed}_get_avail_bytes() descriptors are read
in a loop via MemoryRegionCache regions and calls to
vring_{split,packed}_desc_read() - these take a region cache and the
index of the descriptor to be read.
For direct descriptors we use a cache provided by the caller, whose
size matches that of the virtqueue vring. We limit the number of
descriptors we can read by the size of that vring:
max = vq->vring.num;
...
MemoryRegionCache *desc_cache = &caches->desc;
For indirect descriptors, we initialize a new cache and limit the
number of descriptors by the size of the intermediate descriptor:
len = address_space_cache_init(&indirect_desc_cache,
vdev->dma_as,
desc.addr, desc.len, false);
desc_cache = &indirect_desc_cache;
...
max = desc.len / sizeof(VRingDesc);
However, the first initialization of `max` is done outside the loop
where we process guest descriptors, while the second one is done
inside. This means that a sequence of an indirect descriptor followed
by a direct one will leave a stale value in `max`. If the second
descriptor's `next` field is smaller than the stale value, but
greater than the size of the virtqueue ring (and thus the cached
region), a failed assertion will be triggered in
address_space_read_cached() down the call chain.
Fix this by initializing `max` inside the loop in both functions.
Fixes: 9796d0ac8fb0 ("virtio: use address_space_map/unmap to access descriptors")
Signed-off-by: Carlos López <clopez@suse.de>
---
v2: simplify patch by removing variable cleanups
hw/virtio/virtio.c | 11 +++++------
1 file changed, 5 insertions(+), 6 deletions(-)
diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
index f35178f5fc..98c4819fcc 100644
--- a/hw/virtio/virtio.c
+++ b/hw/virtio/virtio.c
@@ -1069,7 +1069,7 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq,
VRingMemoryRegionCaches *caches)
{
VirtIODevice *vdev = vq->vdev;
- unsigned int max, idx;
+ unsigned int idx;
unsigned int total_bufs, in_total, out_total;
MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
int64_t len = 0;
@@ -1078,13 +1078,12 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq,
idx = vq->last_avail_idx;
total_bufs = in_total = out_total = 0;
- max = vq->vring.num;
-
while ((rc = virtqueue_num_heads(vq, idx)) > 0) {
MemoryRegionCache *desc_cache = &caches->desc;
unsigned int num_bufs;
VRingDesc desc;
unsigned int i;
+ unsigned int max = vq->vring.num;
num_bufs = total_bufs;
@@ -1206,7 +1205,7 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue *vq,
VRingMemoryRegionCaches *caches)
{
VirtIODevice *vdev = vq->vdev;
- unsigned int max, idx;
+ unsigned int idx;
unsigned int total_bufs, in_total, out_total;
MemoryRegionCache *desc_cache;
MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID;
@@ -1218,14 +1217,14 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue *vq,
wrap_counter = vq->last_avail_wrap_counter;
total_bufs = in_total = out_total = 0;
- max = vq->vring.num;
-
for (;;) {
unsigned int num_bufs = total_bufs;
unsigned int i = idx;
int rc;
+ unsigned int max = vq->vring.num;
desc_cache = &caches->desc;
+
vring_packed_desc_read(vdev, &desc, desc_cache, idx, true);
if (!is_desc_avail(desc.flags, wrap_counter)) {
break;
--
2.35.3
On 230302 1103, Carlos López wrote: > In virtqueue_{split,packed}_get_avail_bytes() descriptors are read > in a loop via MemoryRegionCache regions and calls to > vring_{split,packed}_desc_read() - these take a region cache and the > index of the descriptor to be read. > > For direct descriptors we use a cache provided by the caller, whose > size matches that of the virtqueue vring. We limit the number of > descriptors we can read by the size of that vring: > > max = vq->vring.num; > ... > MemoryRegionCache *desc_cache = &caches->desc; > > For indirect descriptors, we initialize a new cache and limit the > number of descriptors by the size of the intermediate descriptor: > > len = address_space_cache_init(&indirect_desc_cache, > vdev->dma_as, > desc.addr, desc.len, false); > desc_cache = &indirect_desc_cache; > ... > max = desc.len / sizeof(VRingDesc); > > However, the first initialization of `max` is done outside the loop > where we process guest descriptors, while the second one is done > inside. This means that a sequence of an indirect descriptor followed > by a direct one will leave a stale value in `max`. If the second > descriptor's `next` field is smaller than the stale value, but > greater than the size of the virtqueue ring (and thus the cached > region), a failed assertion will be triggered in > address_space_read_cached() down the call chain. > > Fix this by initializing `max` inside the loop in both functions. > > Fixes: 9796d0ac8fb0 ("virtio: use address_space_map/unmap to access descriptors") > Signed-off-by: Carlos López <clopez@suse.de> OSS-Fuzz confirmed this fixed an issue: https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=52152 Never made a gitlab report for it because, it seemed very similar to some other issues https://gitlab.com/qemu-project/qemu/-/issues/301 https://gitlab.com/qemu-project/qemu/-/issues/781 https://gitlab.com/qemu-project/qemu/-/issues/300 But apparently it was a different one. Thank you -Alex
On Thu, Mar 02, 2023 at 11:03:59AM +0100, Carlos López wrote: > In virtqueue_{split,packed}_get_avail_bytes() descriptors are read > in a loop via MemoryRegionCache regions and calls to > vring_{split,packed}_desc_read() - these take a region cache and the > index of the descriptor to be read. > > For direct descriptors we use a cache provided by the caller, whose > size matches that of the virtqueue vring. We limit the number of > descriptors we can read by the size of that vring: > > max = vq->vring.num; > ... > MemoryRegionCache *desc_cache = &caches->desc; > > For indirect descriptors, we initialize a new cache and limit the > number of descriptors by the size of the intermediate descriptor: > > len = address_space_cache_init(&indirect_desc_cache, > vdev->dma_as, > desc.addr, desc.len, false); > desc_cache = &indirect_desc_cache; > ... > max = desc.len / sizeof(VRingDesc); > > However, the first initialization of `max` is done outside the loop > where we process guest descriptors, while the second one is done > inside. This means that a sequence of an indirect descriptor followed > by a direct one will leave a stale value in `max`. If the second > descriptor's `next` field is smaller than the stale value, but > greater than the size of the virtqueue ring (and thus the cached > region), a failed assertion will be triggered in > address_space_read_cached() down the call chain. > > Fix this by initializing `max` inside the loop in both functions. > > Fixes: 9796d0ac8fb0 ("virtio: use address_space_map/unmap to access descriptors") > Signed-off-by: Carlos López <clopez@suse.de> > --- > v2: simplify patch by removing variable cleanups > > hw/virtio/virtio.c | 11 +++++------ > 1 file changed, 5 insertions(+), 6 deletions(-) > > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c > index f35178f5fc..98c4819fcc 100644 > --- a/hw/virtio/virtio.c > +++ b/hw/virtio/virtio.c > @@ -1069,7 +1069,7 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq, > VRingMemoryRegionCaches *caches) > { > VirtIODevice *vdev = vq->vdev; > - unsigned int max, idx; > + unsigned int idx; > unsigned int total_bufs, in_total, out_total; > MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID; > int64_t len = 0; > @@ -1078,13 +1078,12 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq, > idx = vq->last_avail_idx; > total_bufs = in_total = out_total = 0; > > - max = vq->vring.num; > - > while ((rc = virtqueue_num_heads(vq, idx)) > 0) { > MemoryRegionCache *desc_cache = &caches->desc; > unsigned int num_bufs; > VRingDesc desc; > unsigned int i; > + unsigned int max = vq->vring.num; > > num_bufs = total_bufs; > > @@ -1206,7 +1205,7 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue *vq, > VRingMemoryRegionCaches *caches) > { > VirtIODevice *vdev = vq->vdev; > - unsigned int max, idx; > + unsigned int idx; > unsigned int total_bufs, in_total, out_total; > MemoryRegionCache *desc_cache; > MemoryRegionCache indirect_desc_cache = MEMORY_REGION_CACHE_INVALID; > @@ -1218,14 +1217,14 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue *vq, > wrap_counter = vq->last_avail_wrap_counter; > total_bufs = in_total = out_total = 0; > > - max = vq->vring.num; > - > for (;;) { > unsigned int num_bufs = total_bufs; > unsigned int i = idx; > int rc; > + unsigned int max = vq->vring.num; > > desc_cache = &caches->desc; > + OK and let's not add cosmetics with a bugfix unnecessarily? > vring_packed_desc_read(vdev, &desc, desc_cache, idx, true); > if (!is_desc_avail(desc.flags, wrap_counter)) { > break; > -- > 2.35.3
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