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Tsirkin" , Jason Wang , =?UTF-8?q?Eugenio=20P=C3=A9rez?= Cc: virtualization@lists.linux.dev, kvm@vger.kernel.org, netdev@vger.kernel.org, linux-kernel@vger.kernel.org, Yu Zhang Subject: [PATCH] vhost-vdpa: drop the parent's vq callback before the call fd is released Date: Fri, 7 Aug 2026 01:03:23 +1000 Message-ID: <20260806150323.2154-1-yuz08559@gmail.com> X-Mailer: git-send-email 2.55.0 Precedence: bulk X-Mailing-List: virtualization@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 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/. 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 --- 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