[PATCH net 4/4] mptcp: fix disconnect vs accept race
From: Matthieu Baerts
Date: Thu Aug 03 2023 - 12:28:15 EST
From: Paolo Abeni <pabeni@xxxxxxxxxx>
Despite commit 0ad529d9fd2b ("mptcp: fix possible divide by zero in
recvmsg()"), the mptcp protocol is still prone to a race between
disconnect() (or shutdown) and accept.
The root cause is that the mentioned commit checks the msk-level
flag, but mptcp_stream_accept() does acquire the msk-level lock,
as it can rely directly on the first subflow lock.
As reported by Christoph than can lead to a race where an msk
socket is accepted after that mptcp_subflow_queue_clean() releases
the listener socket lock and just before it takes destructive
actions leading to the following splat:
BUG: kernel NULL pointer dereference, address: 0000000000000012
PGD 5a4ca067 P4D 5a4ca067 PUD 37d4c067 PMD 0
Oops: 0000 [#1] PREEMPT SMP
CPU: 2 PID: 10955 Comm: syz-executor.5 Not tainted 6.5.0-rc1-gdc7b257ee5dd #37
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2.el7 04/01/2014
RIP: 0010:mptcp_stream_accept+0x1ee/0x2f0 include/net/inet_sock.h:330
Code: 0a 09 00 48 8b 1b 4c 39 e3 74 07 e8 bc 7c 7f fe eb a1 e8 b5 7c 7f fe 4c 8b 6c 24 08 eb 05 e8 a9 7c 7f fe 49 8b 85 d8 09 00 00 <0f> b6 40 12 88 44 24 07 0f b6 6c 24 07 bf 07 00 00 00 89 ee e8 89
RSP: 0018:ffffc90000d07dc0 EFLAGS: 00010293
RAX: 0000000000000000 RBX: ffff888037e8d020 RCX: ffff88803b093300
RDX: 0000000000000000 RSI: ffffffff833822c5 RDI: ffffffff8333896a
RBP: 0000607f82031520 R08: ffff88803b093300 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000003e83 R12: ffff888037e8d020
R13: ffff888037e8c680 R14: ffff888009af7900 R15: ffff888009af6880
FS: 00007fc26d708640(0000) GS:ffff88807dd00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000000000000012 CR3: 0000000066bc5001 CR4: 0000000000370ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
do_accept+0x1ae/0x260 net/socket.c:1872
__sys_accept4+0x9b/0x110 net/socket.c:1913
__do_sys_accept4 net/socket.c:1954 [inline]
__se_sys_accept4 net/socket.c:1951 [inline]
__x64_sys_accept4+0x20/0x30 net/socket.c:1951
do_syscall_x64 arch/x86/entry/common.c:50 [inline]
do_syscall_64+0x47/0xa0 arch/x86/entry/common.c:80
entry_SYSCALL_64_after_hwframe+0x6e/0xd8
Address the issue by temporary removing the pending request socket
from the accept queue, so that racing accept() can't touch them.
After depleting the msk - the ssk still exists, as plain TCP sockets,
re-insert them into the accept queue, so that later inet_csk_listen_stop()
will complete the tcp socket disposal.
Fixes: 2a6a870e44dd ("mptcp: stops worker on unaccepted sockets at listener close")
Cc: stable@xxxxxxxxxxxxxxx
Reported-by: Christoph Paasch <cpaasch@xxxxxxxxx>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/423
Signed-off-by: Paolo Abeni <pabeni@xxxxxxxxxx>
Reviewed-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
Signed-off-by: Matthieu Baerts <matthieu.baerts@xxxxxxxxxxxx>
---
net/mptcp/protocol.h | 1 -
net/mptcp/subflow.c | 60 ++++++++++++++++++++++++++--------------------------
2 files changed, 30 insertions(+), 31 deletions(-)
diff --git a/net/mptcp/protocol.h b/net/mptcp/protocol.h
index 37fbe22e2433..ba2a873a4d2e 100644
--- a/net/mptcp/protocol.h
+++ b/net/mptcp/protocol.h
@@ -325,7 +325,6 @@ struct mptcp_sock {
u32 subflow_id;
u32 setsockopt_seq;
char ca_name[TCP_CA_NAME_MAX];
- struct mptcp_sock *dl_next;
};
#define mptcp_data_lock(sk) spin_lock_bh(&(sk)->sk_lock.slock)
diff --git a/net/mptcp/subflow.c b/net/mptcp/subflow.c
index 9ee3b7abbaf6..94ae7dd01c65 100644
--- a/net/mptcp/subflow.c
+++ b/net/mptcp/subflow.c
@@ -1793,34 +1793,21 @@ static void subflow_state_change(struct sock *sk)
void mptcp_subflow_queue_clean(struct sock *listener_sk, struct sock *listener_ssk)
{
struct request_sock_queue *queue = &inet_csk(listener_ssk)->icsk_accept_queue;
- struct mptcp_sock *msk, *next, *head = NULL;
- struct request_sock *req;
- struct sock *sk;
+ struct request_sock *req, *head, *tail;
+ struct mptcp_subflow_context *subflow;
+ struct sock *sk, *ssk;
- /* build a list of all unaccepted mptcp sockets */
+ /* Due to lock dependencies no relevant lock can be acquired under rskq_lock.
+ * Splice the req list, so that accept() can not reach the pending ssk after
+ * the listener socket is released below.
+ */
spin_lock_bh(&queue->rskq_lock);
- for (req = queue->rskq_accept_head; req; req = req->dl_next) {
- struct mptcp_subflow_context *subflow;
- struct sock *ssk = req->sk;
-
- if (!sk_is_mptcp(ssk))
- continue;
-
- subflow = mptcp_subflow_ctx(ssk);
- if (!subflow || !subflow->conn)
- continue;
-
- /* skip if already in list */
- sk = subflow->conn;
- msk = mptcp_sk(sk);
- if (msk->dl_next || msk == head)
- continue;
-
- sock_hold(sk);
- msk->dl_next = head;
- head = msk;
- }
+ head = queue->rskq_accept_head;
+ tail = queue->rskq_accept_tail;
+ queue->rskq_accept_head = NULL;
+ queue->rskq_accept_tail = NULL;
spin_unlock_bh(&queue->rskq_lock);
+
if (!head)
return;
@@ -1829,13 +1816,19 @@ void mptcp_subflow_queue_clean(struct sock *listener_sk, struct sock *listener_s
*/
release_sock(listener_ssk);
- for (msk = head; msk; msk = next) {
- sk = (struct sock *)msk;
+ for (req = head; req; req = req->dl_next) {
+ ssk = req->sk;
+ if (!sk_is_mptcp(ssk))
+ continue;
+
+ subflow = mptcp_subflow_ctx(ssk);
+ if (!subflow || !subflow->conn)
+ continue;
+
+ sk = subflow->conn;
+ sock_hold(sk);
lock_sock_nested(sk, SINGLE_DEPTH_NESTING);
- next = msk->dl_next;
- msk->dl_next = NULL;
-
__mptcp_unaccepted_force_close(sk);
release_sock(sk);
@@ -1859,6 +1852,13 @@ void mptcp_subflow_queue_clean(struct sock *listener_sk, struct sock *listener_s
/* we are still under the listener msk socket lock */
lock_sock_nested(listener_ssk, SINGLE_DEPTH_NESTING);
+
+ /* restore the listener queue, to let the TCP code clean it up */
+ spin_lock_bh(&queue->rskq_lock);
+ WARN_ON_ONCE(queue->rskq_accept_head);
+ queue->rskq_accept_head = head;
+ queue->rskq_accept_tail = tail;
+ spin_unlock_bh(&queue->rskq_lock);
}
static int subflow_ulp_init(struct sock *sk)
--
2.40.1