[PATCH] vhost-vdpa: drop the parent's vq callback before the call fd is released

[PATCH] vhost-vdpa: drop the parent's vq callback before the call fd is released

[Yu Zhang]

VHOST_SET_VRING_CALL releases the previous call eventfd inside
vhost_vring_ioctl() -- it swaps the new context into vq->call_ctx.ctx
and then eventfd_ctx_put()s the old one, which is a synchronous
kfree(). The parent vdpa device is only told about the change
afterwards, when vhost_vdpa_vring_ioctl() reaches ops->set_vq_cb().

Parent drivers cache the pointer handed to them in
vdpa_callback::trigger and do not take a reference on it, so
throughout that window the parent holds a dangling eventfd_ctx and may
signal it. The documentation added with the field describes what
signalling it means but says nothing about how long it stays valid.

This is the same hazard that "vhost_vdpa: assign irq bypass producer
token correctly" addressed for the irq bypass producer token, by
moving vhost_vdpa_unsetup_vq_irq() ahead of the vhost_vring_ioctl()
call. The producer token was only one of the two consumers of that
pointer; the one the parent keeps via ->set_vq_cb() was left behind
the free.

With VDUSE the window is directly reachable from userspace, because
the device emulation daemon can inject an interrupt at any time from a
different fd, and neither side shares a lock with the other: VDUSE
takes vq->irq_lock, vhost takes vhost_dev.mutex + vq->mutex.

  BUG: KASAN: slab-use-after-free in _raw_spin_lock_irqsave+0x76/0xe0
  Write of size 4 at addr ffff8881084e8788 by task vduse_uaf/2987

  _raw_spin_lock_irqsave+0x76/0xe0
  eventfd_signal_mask+0x69/0x120
  vduse_dev_ioctl+0x337/0x1a60     <- vduse_vq_signal_irqfd(), inlined
  __x64_sys_ioctl+0x120/0x170      <- VDUSE_VQ_INJECT_IRQ

  Allocated by task 2986:
   do_eventfd+0x50/0x200
   __x64_sys_eventfd2+0x2e/0x40

  kmalloc-64, freed 64-byte region [ffff8881084e8780, ffff8881084e87c0)

One thread loops VHOST_SET_VRING_CALL on /dev/vhost-vdpa-N with a
fresh eventfd and then unbinds it, while another loops
VDUSE_VQ_INJECT_IRQ on /dev/vduse/<name>. This reproduces in 5 out of
5 ten-second runs on v7.1.6 and 3 out of 3 on v7.2-rc6. With the patch
there are no reports in 3 out of 3 runs on either, while the same
workload still gets ~30000 interrupts per run delivered into live
eventfds, so the path is still being exercised.

Tell the parent to drop the callback before vhost_vring_ioctl() can
free the eventfd, mirroring what is already done for the bypass
producer, and restore it if the ioctl fails -- on failure the swap
never happened, the old context is still installed, and leaving the
parent without a callback would silently drop that vq's interrupts.

Fixes: 5e68470f4e80 ("vdpa: Add eventfd for the vdpa callback")
Signed-off-by: Yu Zhang <yuz08559@gmail.com>
---
 drivers/vhost/vdpa.c | 23 ++++++++++++++++++++++-
 1 file changed, 22 insertions(+), 1 deletion(-)

diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
index ac55275..b7833bd 100644
--- a/drivers/vhost/vdpa.c
+++ b/drivers/vhost/vdpa.c
@@ -714,13 +714,34 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
 			if (ops->get_status(vdpa) &
 			    VIRTIO_CONFIG_S_DRIVER_OK)
 				vhost_vdpa_unsetup_vq_irq(v, idx);
+			/*
+			 * The parent caches call_ctx.ctx in cb.trigger without
+			 * holding a reference, so it has to stop using it
+			 * before vhost_vring_ioctl() drops the last one.
+			 */
+			cb.callback = NULL;
+			cb.private = NULL;
+			cb.trigger = NULL;
+			ops->set_vq_cb(vdpa, idx, &cb);
 		}
 		break;
 	}
 
 	r = vhost_vring_ioctl(&v->vdev, cmd, argp);
-	if (r)
+	if (r) {
+		/*
+		 * A failure here means the swap never happened and the old
+		 * context is still installed, so give the parent back the
+		 * callback that was torn down above.
+		 */
+		if (cmd == VHOST_SET_VRING_CALL && vq->call_ctx.ctx) {
+			cb.callback = vhost_vdpa_virtqueue_cb;
+			cb.private = vq;
+			cb.trigger = vq->call_ctx.ctx;
+			ops->set_vq_cb(vdpa, idx, &cb);
+		}
 		return r;
+	}
 
 	switch (cmd) {
 	case VHOST_SET_VRING_ADDR:
-- 
2.43.0

Re: [PATCH] vhost-vdpa: drop the parent's vq callback before the call fd is released

[sashiko-bot]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [Critical] Torn read causing NULL pointer dereference and incomplete UAF fix for hardware VDPA drivers.
--

--- Patch [1]: [PATCH] vhost-vdpa: drop the parent's vq callback before the call fd is released ---
commit 59fc02f21fdc99bd2ce2389a383eab4badff2c47
Author: Yu Zhang <yuz08559@gmail.com>

vhost-vdpa: drop the parent's vq callback before the call fd is released

This commit clears the vdpa callback pointers before vhost_vring_ioctl()
releases the eventfd to prevent a use-after-free regression with VDUSE. It
aims to ensure the parent driver drops references to the callback context
before it is destroyed.

> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index c3d913bd7cac7..63ffa94bb4793 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -721,13 +721,34 @@ static long vhost_vdpa_vring_ioctl(struct vhost_vdpa *v, unsigned int cmd,
>  			if (ops->get_status(vdpa) &
>  			    VIRTIO_CONFIG_S_DRIVER_OK)
>  				vhost_vdpa_unsetup_vq_irq(v, idx);
> +			/*
> +			 * The parent caches call_ctx.ctx in cb.trigger without
> +			 * holding a reference, so it has to stop using it
> +			 * before vhost_vring_ioctl() drops the last one.
> +			 */
> +			cb.callback = NULL;
> +			cb.private = NULL;
> +			cb.trigger = NULL;
> +			ops->set_vq_cb(vdpa, idx, &cb);

[Severity: Critical]
Can this non-atomic update cause a NULL pointer dereference for hardware
drivers?

For hardware drivers like vp_vdpa, the interrupt handler reads the callback
struct without locks. If a concurrent interrupt occurs during ops->set_vq_cb(),
could it read a torn state where it sees the old valid cb.callback but the
new NULL cb.private?

drivers/vdpa/virtio_pci/vp_vdpa.c:vp_vdpa_vq_handler() {
    ...
	if (vring->cb.callback)
		return vring->cb.callback(vring->cb.private);
    ...
}

If this happens, it would call vhost_vdpa_virtqueue_cb() with a NULL pointer,
causing a panic when attempting to dereference it.

Additionally, since there is no synchronize_irq() here, could an
already-executing hardware interrupt handler still race with the subsequent
vhost_vring_ioctl() freeing the eventfd, leaving the original use-after-free
unfixed for hardware drivers?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260806150323.2154-1-yuz08559@gmail.com?part=1