[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 at dpdk.org

Signed-off-by: Shani Peretz <shperetz at 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