[Qemu-devel] [PATCH 2/4] virtiofsd: prevent FUSE_INIT/FUSE_DESTROY races

[Stefan Hajnoczi <stefanha@redhat.com>]

When running with multiple threads it can be tricky to handle
FUSE_INIT/FUSE_DESTROY in parallel with other request types or in
parallel with themselves.  Serialize FUSE_INIT and FUSE_DESTROY so that
malicious clients cannot trigger race conditions.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 contrib/virtiofsd/fuse_i.h        |  1 +
 contrib/virtiofsd/fuse_lowlevel.c | 17 +++++++++++++++++
 2 files changed, 18 insertions(+)

diff --git a/contrib/virtiofsd/fuse_i.h b/contrib/virtiofsd/fuse_i.h
index ff6e1b75ba..dcde9feb97 100644
--- a/contrib/virtiofsd/fuse_i.h
+++ b/contrib/virtiofsd/fuse_i.h
@@ -63,6 +63,7 @@ struct fuse_session {
 	struct fuse_req list;
 	struct fuse_req interrupts;
 	pthread_mutex_t lock;
+	pthread_rwlock_t init_rwlock;
 	int got_destroy;
 	int broken_splice_nonblock;
 	uint64_t notify_ctr;
diff --git a/contrib/virtiofsd/fuse_lowlevel.c b/contrib/virtiofsd/fuse_lowlevel.c
index 6ebf237baa..483a1bc9be 100644
--- a/contrib/virtiofsd/fuse_lowlevel.c
+++ b/contrib/virtiofsd/fuse_lowlevel.c
@@ -2473,6 +2473,18 @@ void fuse_session_process_buf_int(struct fuse_session *se,
 	req->ctx.pid = in->pid;
 	req->ch = ch ? fuse_chan_get(ch) : NULL;
 
+	/* INIT and DESTROY requests are serialized, all other request types
+	 * run in parallel.  This prevents races between FUSE_INIT and ordinary
+	 * requests, FUSE_INIT and FUSE_INIT, FUSE_INIT and FUSE_DESTROY, and
+	 * FUSE_DESTROY and FUSE_DESTROY.
+	 */
+	if (in->opcode == FUSE_INIT || in->opcode == CUSE_INIT ||
+	    in->opcode == FUSE_DESTROY) {
+		pthread_rwlock_wrlock(&se->init_rwlock);
+	} else {
+		pthread_rwlock_rdlock(&se->init_rwlock);
+	}
+
 	err = EIO;
 	if (!se->got_init) {
 		enum fuse_opcode expected;
@@ -2524,10 +2536,13 @@ void fuse_session_process_buf_int(struct fuse_session *se,
 		do_write_buf(req, in->nodeid, &iter, bufv);
 	else
 		fuse_ll_ops[in->opcode].func(req, in->nodeid, &iter);
+
+	pthread_rwlock_unlock(&se->init_rwlock);
 	return;
 
 reply_err:
 	fuse_reply_err(req, err);
+	pthread_rwlock_unlock(&se->init_rwlock);
 }
 
 #define LL_OPTION(n,o,v) \
@@ -2569,6 +2584,7 @@ void fuse_session_destroy(struct fuse_session *se)
 		if (se->op.destroy)
 			se->op.destroy(se->userdata, se);
 	}
+	pthread_rwlock_destroy(&se->init_rwlock);
 	pthread_mutex_destroy(&se->lock);
 	free(se->cuse_data);
 	if (se->fd != -1)
@@ -2656,6 +2672,7 @@ struct fuse_session *fuse_session_new(struct fuse_args *args,
 	list_init_req(&se->list);
 	list_init_req(&se->interrupts);
 	fuse_mutex_init(&se->lock);
+	pthread_rwlock_init(&se->init_rwlock, NULL);
 
 	memcpy(&se->op, op, op_size);
 	se->owner = getuid();
-- 
2.21.0