[PATCH] vhost: fix use-after-free race during cleanup

[PATCH] vhost: fix use-after-free race during cleanup

[Shani Peretz]

This commit fixes a use-after-free that causes the application
to crash on shutdown (detected by ASAN).

The vhost library uses a background event dispatch thread that monitors
fds with epoll. It runs in an infinite loop, waiting for I/O events
and calling callbacks when they occur.

During cleanup, a race condition existed:

  Main Thread:                    Event Dispatch Thread:
  1. Remove fds from fdset        while (1) {
  2. Close file descriptors           epoll_wait() [gets interrupted]
  3. Free fdset memory                [continues loop]
  4. Continue...                      Accesses fdset...   CRASH
                                  }

The main thread would free the fdset memory while the background thread
was still running and using it.

The code had a `destroy` flag that the event dispatch thread checked,
but it was never set during cleanup, and the code never waited for
the thread to actually exit before freeing memory.

This commit implements `fdset_destroy()` that will set the destroy
flag, wait for thread termination, and clean up all resources.
The socket.c is updated to call fdset_destroy() when the last vhost-user
socket is unregistered.

Fixes: 0e38b42bf61c ("vhost: manage FD with epoll")
Cc: stable@dpdk.org

Signed-off-by: Shani Peretz <shperetz@nvidia.com>
---
 lib/vhost/fd_man.c | 38 ++++++++++++++++++++++++++++++++++++++
 lib/vhost/fd_man.h |  1 +
 lib/vhost/socket.c |  7 +++++++
 3 files changed, 46 insertions(+)

diff --git a/lib/vhost/fd_man.c b/lib/vhost/fd_man.c
index f9147edee7..ba1b2ead86 100644
--- a/lib/vhost/fd_man.c
+++ b/lib/vhost/fd_man.c
@@ -393,3 +393,41 @@ fdset_event_dispatch(void *arg)
 
 	return 0;
 }
+
+/**
+ * Destroy the fdset and stop its event dispatch thread.
+ */
+void
+fdset_destroy(struct fdset *pfdset)
+{
+	uint32_t val;
+	int i;
+
+	if (pfdset == NULL)
+		return;
+
+	/* Signal the event dispatch thread to stop */
+	pfdset->destroy = true;
+
+	/* Wait for the event dispatch thread to finish */
+	rte_thread_join(pfdset->tid, &val);
+
+	/* Close the epoll file descriptor */
+	close(pfdset->epfd);
+
+	/* Destroy the mutex */
+	pthread_mutex_destroy(&pfdset->fd_mutex);
+
+	/* Remove from global registry */
+	pthread_mutex_lock(&fdsets_mutex);
+	for (i = 0; i < MAX_FDSETS; i++) {
+		if (fdsets[i] == pfdset) {
+			fdsets[i] = NULL;
+			break;
+		}
+	}
+	pthread_mutex_unlock(&fdsets_mutex);
+
+	/* Free the fdset structure */
+	rte_free(pfdset);
+}
diff --git a/lib/vhost/fd_man.h b/lib/vhost/fd_man.h
index eadcc6fb42..ed2109f3c8 100644
--- a/lib/vhost/fd_man.h
+++ b/lib/vhost/fd_man.h
@@ -21,5 +21,6 @@ int fdset_add(struct fdset *pfdset, int fd,
 
 void fdset_del(struct fdset *pfdset, int fd);
 int fdset_try_del(struct fdset *pfdset, int fd);
+void fdset_destroy(struct fdset *pfdset);
 
 #endif
diff --git a/lib/vhost/socket.c b/lib/vhost/socket.c
index 9b4f332f94..0240da8ff0 100644
--- a/lib/vhost/socket.c
+++ b/lib/vhost/socket.c
@@ -1141,6 +1141,13 @@ rte_vhost_driver_unregister(const char *path)
 		count = --vhost_user.vsocket_cnt;
 		vhost_user.vsockets[i] = vhost_user.vsockets[count];
 		vhost_user.vsockets[count] = NULL;
+
+		/* Check if we need to destroy the vhost fdset */
+		if (vhost_user.vsocket_cnt == 0 && vhost_user.fdset != NULL) {
+			fdset_destroy(vhost_user.fdset);
+			vhost_user.fdset = NULL;
+		}
+
 		pthread_mutex_unlock(&vhost_user.mutex);
 		return 0;
 	}
-- 
2.34.1

Re: [PATCH] vhost: fix use-after-free race during cleanup

[fengchengwen]

On 11/4/2025 4:09 PM, Shani Peretz wrote:
> This commit fixes a use-after-free that causes the application
> to crash on shutdown (detected by ASAN).
> 
> The vhost library uses a background event dispatch thread that monitors
> fds with epoll. It runs in an infinite loop, waiting for I/O events
> and calling callbacks when they occur.
> 
> During cleanup, a race condition existed:
> 
>   Main Thread:                    Event Dispatch Thread:
>   1. Remove fds from fdset        while (1) {
>   2. Close file descriptors           epoll_wait() [gets interrupted]
>   3. Free fdset memory                [continues loop]
>   4. Continue...                      Accesses fdset...   CRASH
>                                   }
> 
> The main thread would free the fdset memory while the background thread
> was still running and using it.

Who will free fdset memory ? I check the lib/vhost/socket.c and found there are no explicit free.

I think it maybe the hugepage free because the fdset use rte_zmalloc(). If it's, please explicit
add it into the commit log.

> 
> The code had a `destroy` flag that the event dispatch thread checked,
> but it was never set during cleanup, and the code never waited for
> the thread to actually exit before freeing memory.
> 
> This commit implements `fdset_destroy()` that will set the destroy
> flag, wait for thread termination, and clean up all resources.
> The socket.c is updated to call fdset_destroy() when the last vhost-user
> socket is unregistered.
> 
> Fixes: 0e38b42bf61c ("vhost: manage FD with epoll")
> Cc: stable@dpdk.org
> 
> Signed-off-by: Shani Peretz <shperetz@nvidia.com>

Re: [PATCH] vhost: fix use-after-free race during cleanup

[Maxime Coquelin]

Hi Shani,

Thanks for the fix, more comments below:

On Tue, Nov 4, 2025 at 10:50 AM fengchengwen <fengchengwen@huawei.com> wrote:
>
> On 11/4/2025 4:09 PM, Shani Peretz wrote:
> > This commit fixes a use-after-free that causes the application
> > to crash on shutdown (detected by ASAN).
> >
> > The vhost library uses a background event dispatch thread that monitors
> > fds with epoll. It runs in an infinite loop, waiting for I/O events
> > and calling callbacks when they occur.
> >
> > During cleanup, a race condition existed:
> >
> >   Main Thread:                    Event Dispatch Thread:
> >   1. Remove fds from fdset        while (1) {
> >   2. Close file descriptors           epoll_wait() [gets interrupted]
> >   3. Free fdset memory                [continues loop]
> >   4. Continue...                      Accesses fdset...   CRASH
> >                                   }
> >
> > The main thread would free the fdset memory while the background thread
> > was still running and using it.
>
> Who will free fdset memory ? I check the lib/vhost/socket.c and found there are no explicit free.
>
> I think it maybe the hugepage free because the fdset use rte_zmalloc(). If it's, please explicit
> add it into the commit log.

I agree with Feng, it would be good to provide more information on who
is freeing the memory.

> >
> > The code had a `destroy` flag that the event dispatch thread checked,
> > but it was never set during cleanup, and the code never waited for
> > the thread to actually exit before freeing memory.
> >
> > This commit implements `fdset_destroy()` that will set the destroy
> > flag, wait for thread termination, and clean up all resources.
> > The socket.c is updated to call fdset_destroy() when the last vhost-user
> > socket is unregistered.
> >
> > Fixes: 0e38b42bf61c ("vhost: manage FD with epoll")
> > Cc: stable@dpdk.org
> >
> > Signed-off-by: Shani Peretz <shperetz@nvidia.com>
>
>

We also need to call fdset_destroy in vduse_device_destroy() if it is
destorying the last VDUSE device.
We might need to add a counter to struct vduse to know whether this is
the last device.

Other than that, the patch looks good to me.

Thanks,
Maxime