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Re: [PATCH v2] virtio: gracefully handle invalid region caches
From: |
Stefan Hajnoczi |
Subject: |
Re: [PATCH v2] virtio: gracefully handle invalid region caches |
Date: |
Mon, 24 Feb 2020 13:35:54 +0000 |
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 <address@hidden>
> Cc: Michael Tsirkin <address@hidden>
> Cc: Cornelia Huck <address@hidden>
> Cc: Paolo Bonzini <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
> 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(-)
Ping?
> 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
>
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