6.8-stable review patch. If anyone has any objections, please let me know.
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From: Gavin Shan gshan@redhat.com
commit 22e1992cf7b034db5325660e98c41ca5afa5f519 upstream.
A smp_rmb() has been missed in vhost_vq_avail_empty(), spotted by Will. Otherwise, it's not ensured the available ring entries pushed by guest can be observed by vhost in time, leading to stale available ring entries fetched by vhost in vhost_get_vq_desc(), as reported by Yihuang Yu on NVidia's grace-hopper (ARM64) platform.
/home/gavin/sandbox/qemu.main/build/qemu-system-aarch64 \ -accel kvm -machine virt,gic-version=host -cpu host \ -smp maxcpus=1,cpus=1,sockets=1,clusters=1,cores=1,threads=1 \ -m 4096M,slots=16,maxmem=64G \ -object memory-backend-ram,id=mem0,size=4096M \ : \ -netdev tap,id=vnet0,vhost=true \ -device virtio-net-pci,bus=pcie.8,netdev=vnet0,mac=52:54:00:f1:26:b0 : guest# netperf -H 10.26.1.81 -l 60 -C -c -t UDP_STREAM virtio_net virtio0: output.0:id 100 is not a head!
Add the missed smp_rmb() in vhost_vq_avail_empty(). When tx_can_batch() returns true, it means there's still pending tx buffers. Since it might read indices, so it still can bypass the smp_rmb() in vhost_get_vq_desc(). Note that it should be safe until vq->avail_idx is changed by commit 275bf960ac697 ("vhost: better detection of available buffers").
Fixes: 275bf960ac69 ("vhost: better detection of available buffers") Cc: stable@kernel.org # v4.11+ Reported-by: Yihuang Yu yihyu@redhat.com Suggested-by: Will Deacon will@kernel.org Signed-off-by: Gavin Shan gshan@redhat.com Acked-by: Jason Wang jasowang@redhat.com Message-Id: 20240328002149.1141302-2-gshan@redhat.com Signed-off-by: Michael S. Tsirkin mst@redhat.com Reviewed-by: Stefano Garzarella sgarzare@redhat.com Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- drivers/vhost/vhost.c | 12 +++++++++++- 1 file changed, 11 insertions(+), 1 deletion(-)
--- a/drivers/vhost/vhost.c +++ b/drivers/vhost/vhost.c @@ -2799,9 +2799,19 @@ bool vhost_vq_avail_empty(struct vhost_d r = vhost_get_avail_idx(vq, &avail_idx); if (unlikely(r)) return false; + vq->avail_idx = vhost16_to_cpu(vq, avail_idx); + if (vq->avail_idx != vq->last_avail_idx) { + /* Since we have updated avail_idx, the following + * call to vhost_get_vq_desc() will read available + * ring entries. Make sure that read happens after + * the avail_idx read. + */ + smp_rmb(); + return false; + }
- return vq->avail_idx == vq->last_avail_idx; + return true; } EXPORT_SYMBOL_GPL(vhost_vq_avail_empty);