Re: [PATCH v2] virtio: gracefully handle invalid region caches

["Michael S. Tsirkin" <mst@redhat.com>]

On Fri, Feb 07, 2020 at 10:46:19AM +0000, Stefan Hajnoczi wrote:
> The virtqueue code sets up MemoryRegionCaches to access the virtqueue
> guest RAM data structures.  The code currently assumes that
> VRingMemoryRegionCaches is initialized before device emulation code
> accesses the virtqueue.  An assertion will fail in
> vring_get_region_caches() when this is not true.  Device fuzzing found a
> case where this assumption is false (see below).
> 
> Virtqueue guest RAM addresses can also be changed from a vCPU thread
> while an IOThread is accessing the virtqueue.  This breaks the same
> assumption but this time the caches could become invalid partway through
> the virtqueue code.  The code fetches the caches RCU pointer multiple
> times so we will need to validate the pointer every time it is fetched.
> 
> Add checks each time we call vring_get_region_caches() and treat invalid
> caches as a nop: memory stores are ignored and memory reads return 0.
> 
> The fuzz test failure is as follows:
> 
>   $ qemu -M pc -device virtio-blk-pci,id=drv0,drive=drive0,addr=4.0 \
>          -drive if=none,id=drive0,file=null-co://,format=raw,auto-read-only=off \
>          -drive if=none,id=drive1,file=null-co://,file.read-zeroes=on,format=raw \
>          -display none \
>          -qtest stdio
>   endianness
>   outl 0xcf8 0x80002020
>   outl 0xcfc 0xe0000000
>   outl 0xcf8 0x80002004
>   outw 0xcfc 0x7
>   write 0xe0000000 0x24 0x00ffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffab5cffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffabffffffab0000000001
>   inb 0x4
>   writew 0xe000001c 0x1
>   write 0xe0000014 0x1 0x0d
> 
> The following error message is produced:
> 
>   qemu-system-x86_64: /home/stefanha/qemu/hw/virtio/virtio.c:286: vring_get_region_caches: Assertion `caches != NULL' failed.
> 
> The backtrace looks like this:
> 
>   #0  0x00007ffff5520625 in raise () at /lib64/libc.so.6
>   #1  0x00007ffff55098d9 in abort () at /lib64/libc.so.6
>   #2  0x00007ffff55097a9 in _nl_load_domain.cold () at /lib64/libc.so.6
>   #3  0x00007ffff5518a66 in annobin_assert.c_end () at /lib64/libc.so.6
>   #4  0x00005555559073da in vring_get_region_caches (vq=<optimized out>) at qemu/hw/virtio/virtio.c:286
>   #5  vring_get_region_caches (vq=<optimized out>) at qemu/hw/virtio/virtio.c:283
>   #6  0x000055555590818d in vring_used_flags_set_bit (mask=1, vq=0x5555575ceea0) at qemu/hw/virtio/virtio.c:398
>   #7  virtio_queue_split_set_notification (enable=0, vq=0x5555575ceea0) at qemu/hw/virtio/virtio.c:398
>   #8  virtio_queue_set_notification (vq=vq@entry=0x5555575ceea0, enable=enable@entry=0) at qemu/hw/virtio/virtio.c:451
>   #9  0x0000555555908512 in virtio_queue_set_notification (vq=vq@entry=0x5555575ceea0, enable=enable@entry=0) at qemu/hw/virtio/virtio.c:444
>   #10 0x00005555558c697a in virtio_blk_handle_vq (s=0x5555575c57e0, vq=0x5555575ceea0) at qemu/hw/block/virtio-blk.c:775
>   #11 0x0000555555907836 in virtio_queue_notify_aio_vq (vq=0x5555575ceea0) at qemu/hw/virtio/virtio.c:2244
>   #12 0x0000555555cb5dd7 in aio_dispatch_handlers (ctx=ctx@entry=0x55555671a420) at util/aio-posix.c:429
>   #13 0x0000555555cb67a8 in aio_dispatch (ctx=0x55555671a420) at util/aio-posix.c:460
>   #14 0x0000555555cb307e in aio_ctx_dispatch (source=<optimized out>, callback=<optimized out>, user_data=<optimized out>) at util/async.c:260
>   #15 0x00007ffff7bbc510 in g_main_context_dispatch () at /lib64/libglib-2.0.so.0
>   #16 0x0000555555cb5848 in glib_pollfds_poll () at util/main-loop.c:219
>   #17 os_host_main_loop_wait (timeout=<optimized out>) at util/main-loop.c:242
>   #18 main_loop_wait (nonblocking=<optimized out>) at util/main-loop.c:518
>   #19 0x00005555559b20c9 in main_loop () at vl.c:1683
>   #20 0x0000555555838115 in main (argc=<optimized out>, argv=<optimized out>, envp=<optimized out>) at vl.c:4441
> 
> Reported-by: Alexander Bulekov <alxndr@bu.edu>
> Cc: Michael Tsirkin <mst@redhat.com>
> Cc: Cornelia Huck <cohuck@redhat.com>
> Cc: Paolo Bonzini <pbonzini@redhat.com>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

I sent a pull already, but I just realized it's needed for stable.
In case it gets merged earlier, CC stable now.

> ---
> An alternative solution is to keep the vring.caches pointer non-NULL all
> the time so no checks are necessary.  We would need to initialize it to
> a VRingMemoryRegionCaches object that points to unassigned_mem.  This
> way virtio.c never hits NULL pointers and all memory loads/stores become
> nop when caches are invalid.
> 
> I think this solution is cleaner but couldn't see a reasonable way of
> initializing MemoryRegionCache objects so that they point to a 64-bit
> unassigned_mem MemoryRegion.  Maybe someone who knows the memory API
> better knows whether this is doable?
> 
> Michael: We discussed changing vring.desc checks, but I think that's no
> longer necessary with this patch.  If a guest gets past a vring.desc
> check then it can no longer trigger the assertion failure.
> ---
>  hw/virtio/virtio.c | 99 ++++++++++++++++++++++++++++++++++++++++++----
>  1 file changed, 91 insertions(+), 8 deletions(-)
> 
> diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> index 2c5410e981..00d444699d 100644
> --- a/hw/virtio/virtio.c
> +++ b/hw/virtio/virtio.c
> @@ -282,15 +282,19 @@ static void vring_packed_flags_write(VirtIODevice *vdev,
>  /* Called within rcu_read_lock().  */
>  static VRingMemoryRegionCaches *vring_get_region_caches(struct VirtQueue *vq)
>  {
> -    VRingMemoryRegionCaches *caches = atomic_rcu_read(&vq->vring.caches);
> -    assert(caches != NULL);
> -    return caches;
> +    return atomic_rcu_read(&vq->vring.caches);
>  }
> +
>  /* Called within rcu_read_lock().  */
>  static inline uint16_t vring_avail_flags(VirtQueue *vq)
>  {
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      hwaddr pa = offsetof(VRingAvail, flags);
> +
> +    if (!caches) {
> +        return 0;
> +    }
> +
>      return virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
>  }
>  
> @@ -299,6 +303,11 @@ static inline uint16_t vring_avail_idx(VirtQueue *vq)
>  {
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      hwaddr pa = offsetof(VRingAvail, idx);
> +
> +    if (!caches) {
> +        return 0;
> +    }
> +
>      vq->shadow_avail_idx = virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
>      return vq->shadow_avail_idx;
>  }
> @@ -308,6 +317,11 @@ static inline uint16_t vring_avail_ring(VirtQueue *vq, int i)
>  {
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      hwaddr pa = offsetof(VRingAvail, ring[i]);
> +
> +    if (!caches) {
> +        return 0;
> +    }
> +
>      return virtio_lduw_phys_cached(vq->vdev, &caches->avail, pa);
>  }
>  
> @@ -323,6 +337,11 @@ static inline void vring_used_write(VirtQueue *vq, VRingUsedElem *uelem,
>  {
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      hwaddr pa = offsetof(VRingUsed, ring[i]);
> +
> +    if (!caches) {
> +        return;
> +    }
> +
>      virtio_tswap32s(vq->vdev, &uelem->id);
>      virtio_tswap32s(vq->vdev, &uelem->len);
>      address_space_write_cached(&caches->used, pa, uelem, sizeof(VRingUsedElem));
> @@ -334,6 +353,11 @@ static uint16_t vring_used_idx(VirtQueue *vq)
>  {
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      hwaddr pa = offsetof(VRingUsed, idx);
> +
> +    if (!caches) {
> +        return 0;
> +    }
> +
>      return virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
>  }
>  
> @@ -342,8 +366,12 @@ static inline void vring_used_idx_set(VirtQueue *vq, uint16_t val)
>  {
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      hwaddr pa = offsetof(VRingUsed, idx);
> -    virtio_stw_phys_cached(vq->vdev, &caches->used, pa, val);
> -    address_space_cache_invalidate(&caches->used, pa, sizeof(val));
> +
> +    if (caches) {
> +        virtio_stw_phys_cached(vq->vdev, &caches->used, pa, val);
> +        address_space_cache_invalidate(&caches->used, pa, sizeof(val));
> +    }
> +
>      vq->used_idx = val;
>  }
>  
> @@ -353,8 +381,13 @@ static inline void vring_used_flags_set_bit(VirtQueue *vq, int mask)
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      VirtIODevice *vdev = vq->vdev;
>      hwaddr pa = offsetof(VRingUsed, flags);
> -    uint16_t flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
> +    uint16_t flags;
>  
> +    if (!caches) {
> +        return;
> +    }
> +
> +    flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
>      virtio_stw_phys_cached(vdev, &caches->used, pa, flags | mask);
>      address_space_cache_invalidate(&caches->used, pa, sizeof(flags));
>  }
> @@ -365,8 +398,13 @@ static inline void vring_used_flags_unset_bit(VirtQueue *vq, int mask)
>      VRingMemoryRegionCaches *caches = vring_get_region_caches(vq);
>      VirtIODevice *vdev = vq->vdev;
>      hwaddr pa = offsetof(VRingUsed, flags);
> -    uint16_t flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
> +    uint16_t flags;
>  
> +    if (!caches) {
> +        return;
> +    }
> +
> +    flags = virtio_lduw_phys_cached(vq->vdev, &caches->used, pa);
>      virtio_stw_phys_cached(vdev, &caches->used, pa, flags & ~mask);
>      address_space_cache_invalidate(&caches->used, pa, sizeof(flags));
>  }
> @@ -381,6 +419,10 @@ static inline void vring_set_avail_event(VirtQueue *vq, uint16_t val)
>      }
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        return;
> +    }
> +
>      pa = offsetof(VRingUsed, ring[vq->vring.num]);
>      virtio_stw_phys_cached(vq->vdev, &caches->used, pa, val);
>      address_space_cache_invalidate(&caches->used, pa, sizeof(val));
> @@ -410,7 +452,11 @@ static void virtio_queue_packed_set_notification(VirtQueue *vq, int enable)
>      VRingMemoryRegionCaches *caches;
>  
>      RCU_READ_LOCK_GUARD();
> -    caches  = vring_get_region_caches(vq);
> +    caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        return;
> +    }
> +
>      vring_packed_event_read(vq->vdev, &caches->used, &e);
>  
>      if (!enable) {
> @@ -597,6 +643,10 @@ static int virtio_queue_packed_empty_rcu(VirtQueue *vq)
>      }
>  
>      cache = vring_get_region_caches(vq);
> +    if (!cache) {
> +        return 1;
> +    }
> +
>      vring_packed_desc_read_flags(vq->vdev, &desc.flags, &cache->desc,
>                                   vq->last_avail_idx);
>  
> @@ -777,6 +827,10 @@ static void virtqueue_packed_fill_desc(VirtQueue *vq,
>      }
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        return;
> +    }
> +
>      vring_packed_desc_write(vq->vdev, &desc, &caches->desc, head, strict_order);
>  }
>  
> @@ -949,6 +1003,10 @@ static void virtqueue_split_get_avail_bytes(VirtQueue *vq,
>  
>      max = vq->vring.num;
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        goto err;
> +    }
> +
>      while ((rc = virtqueue_num_heads(vq, idx)) > 0) {
>          MemoryRegionCache *desc_cache = &caches->desc;
>          unsigned int num_bufs;
> @@ -1089,6 +1147,9 @@ static void virtqueue_packed_get_avail_bytes(VirtQueue *vq,
>  
>      max = vq->vring.num;
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        goto err;
> +    }
>  
>      for (;;) {
>          unsigned int num_bufs = total_bufs;
> @@ -1194,6 +1255,10 @@ void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
>      }
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        goto err;
> +    }
> +
>      desc_size = virtio_vdev_has_feature(vq->vdev, VIRTIO_F_RING_PACKED) ?
>                                  sizeof(VRingPackedDesc) : sizeof(VRingDesc);
>      if (caches->desc.len < vq->vring.num * desc_size) {
> @@ -1387,6 +1452,11 @@ static void *virtqueue_split_pop(VirtQueue *vq, size_t sz)
>      i = head;
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        virtio_error(vdev, "Region caches not initialized");
> +        goto done;
> +    }
> +
>      if (caches->desc.len < max * sizeof(VRingDesc)) {
>          virtio_error(vdev, "Cannot map descriptor ring");
>          goto done;
> @@ -1509,6 +1579,11 @@ static void *virtqueue_packed_pop(VirtQueue *vq, size_t sz)
>      i = vq->last_avail_idx;
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        virtio_error(vdev, "Region caches not initialized");
> +        goto done;
> +    }
> +
>      if (caches->desc.len < max * sizeof(VRingDesc)) {
>          virtio_error(vdev, "Cannot map descriptor ring");
>          goto done;
> @@ -1628,6 +1703,10 @@ static unsigned int virtqueue_packed_drop_all(VirtQueue *vq)
>      VRingPackedDesc desc;
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        return 0;
> +    }
> +
>      desc_cache = &caches->desc;
>  
>      virtio_queue_set_notification(vq, 0);
> @@ -2412,6 +2491,10 @@ static bool virtio_packed_should_notify(VirtIODevice *vdev, VirtQueue *vq)
>      VRingMemoryRegionCaches *caches;
>  
>      caches = vring_get_region_caches(vq);
> +    if (!caches) {
> +        return false;
> +    }
> +
>      vring_packed_event_read(vdev, &caches->avail, &e);
>  
>      old = vq->signalled_used;
> -- 
> 2.24.1