From: Mingyu Wang <25181214217@stu.xidian.edu.cn> In qrtr_port_remove(), the socket reference count is decremented via __sock_put() before the port is removed from the qrtr_ports XArray and before the RCU grace period elapses. This breaks the fundamental RCU update paradigm. It exposes a race window where a concurrent RCU reader (such as qrtr_reset_ports() or qrtr_port_lookup()) can obtain a pointer to the socket from the XArray, and attempt to call sock_hold() on a socket whose reference count has already dropped to zero. This exact race condition was hit during syzkaller fuzzing, leading to the following refcount saturation warning and a potential Use-After-Free: refcount_t: saturated; leaking memory. WARNING: CPU: 3 PID: 1273 at lib/refcount.c:22 refcount_warn_saturate+0xae/0x1d0 Modules linked in: qrtr(+) bochs drm_shmem_helper ... Call Trace: qrtr_reset_ports net/qrtr/af_qrtr.c:768 [inline] [qrtr] __qrtr_bind.isra.0+0x48b/0x570 net/qrtr/af_qrtr.c:805 [qrtr] qrtr_bind+0x17d/0x210 net/qrtr/af_qrtr.c:901 [qrtr] kernel_bind+0xe4/0x120 net/socket.c:3592 qrtr_ns_init+0x1a6/0x380 net/qrtr/ns.c:715 [qrtr] qrtr_proto_init+0x3b/0xff0 net/qrtr/af_qrtr.c:169 [qrtr] do_one_initcall+0xf5/0x5e0 init/main.c:1283 ... Fix this by deferring the reference count decrement until after the xa_erase() and the synchronize_rcu() complete. (Note: The v1 of this patch incorrectly replaced __sock_put() with sock_put(). As Simon Horman pointed out, the callers of qrtr_port_remove() still hold a reference to the socket, so freeing the socket memory here would lead to a subsequent UAF in the caller. Thus, the __sock_put() is kept, but only repositioned to close the RCU race.) Fixes: bdabad3e363d ("net: Add Qualcomm IPC router") Signed-off-by: Mingyu Wang <25181214217@stu.xidian.edu.cn> --- v1: https://lore.kernel.org/netdev/20260530082243.1123402-1-w15303746062@163.com/ Changes in v2: - Reverted the change from __sock_put() to sock_put() to avoid UAF in callers like qrtr_release(), as suggested by Simon Horman. - Removed misleading "memory leak" wording from commit message. - Corrected the Fixes tag to point to the initial commit introducing the race (bdabad3e363d). net/qrtr/af_qrtr.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/net/qrtr/af_qrtr.c b/net/qrtr/af_qrtr.c index 7cec6a7859b0..db823177e636 100644 --- a/net/qrtr/af_qrtr.c +++ b/net/qrtr/af_qrtr.c @@ -707,13 +707,13 @@ static void qrtr_port_remove(struct qrtr_sock *ipc) if (port == QRTR_PORT_CTRL) port = 0; - __sock_put(&ipc->sk); - xa_erase(&qrtr_ports, port); /* Ensure that if qrtr_port_lookup() did enter the RCU read section we * wait for it to up increment the refcount */ synchronize_rcu(); + + __sock_put(&ipc->sk); } /* Assign port number to socket. -- 2.34.1