[PATCH AUTOSEL 4.4 23/64] skbuff: fix a data race in skb_queue_len()

Sasha Levin
2020-09-18T02:20:53Z

From: Qian Cai <cai@lca.pw>

[ Upstream commit 86b18aaa2b5b5bb48e609cd591b3d2d0fdbe0442 ]

sk_buff.qlen can be accessed concurrently as noticed by KCSAN,

BUG: KCSAN: data-race in __skb_try_recv_from_queue / unix_dgram_sendmsg

read to 0xffff8a1b1d8a81c0 of 4 bytes by task 5371 on cpu 96:
unix_dgram_sendmsg+0x9a9/0xb70 include/linux/skbuff.h:1821
net/unix/af_unix.c:1761
____sys_sendmsg+0x33e/0x370
___sys_sendmsg+0xa6/0xf0
__sys_sendmsg+0x69/0xf0
__x64_sys_sendmsg+0x51/0x70
do_syscall_64+0x91/0xb47
entry_SYSCALL_64_after_hwframe+0x49/0xbe

write to 0xffff8a1b1d8a81c0 of 4 bytes by task 1 on cpu 99:
__skb_try_recv_from_queue+0x327/0x410 include/linux/skbuff.h:2029
__skb_try_recv_datagram+0xbe/0x220
unix_dgram_recvmsg+0xee/0x850
____sys_recvmsg+0x1fb/0x210
___sys_recvmsg+0xa2/0xf0
__sys_recvmsg+0x66/0xf0
__x64_sys_recvmsg+0x51/0x70
do_syscall_64+0x91/0xb47
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Since only the read is operating as lockless, it could introduce a logic
bug in unix_recvq_full() due to the load tearing. Fix it by adding
a lockless variant of skb_queue_len() and unix_recvq_full() where
READ_ONCE() is on the read while WRITE_ONCE() is on the write similar to
the commit d7d16a89350a ("net: add skb_queue_empty_lockless()").

Signed-off-by: Qian Cai <cai@lca.pw>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/linux/skbuff.h | 14 +++++++++++++-
net/unix/af_unix.c | 11 +++++++++--
2 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 735ff1525f485..ffd698f70a84e 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1438,6 +1438,18 @@ static inline __u32 skb_queue_len(const struct sk_buff_head *list_)
return list_->qlen;
}

+/**
+ * skb_queue_len_lockless - get queue length
+ * @list_: list to measure
+ *
+ * Return the length of an &sk_buff queue.
+ * This variant can be used in lockless contexts.
+ */
+static inline __u32 skb_queue_len_lockless(const struct sk_buff_head *list_)
+{
+ return READ_ONCE(list_->qlen);
+}
+
/**
* __skb_queue_head_init - initialize non-spinlock portions of sk_buff_head
* @list: queue to initialize
@@ -1641,7 +1653,7 @@ static inline void __skb_unlink(struct sk_buff *skb, struct sk_buff_head *list)
{
struct sk_buff *next, *prev;

- list->qlen--;
+ WRITE_ONCE(list->qlen, list->qlen - 1);
next = skb->next;
prev = skb->prev;
skb->next = skb->prev = NULL;
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index b5e2ef242efe7..ac78c5ac82846 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -191,11 +191,17 @@ static inline int unix_may_send(struct sock *sk, struct sock *osk)
return unix_peer(osk) == NULL || unix_our_peer(sk, osk);
}

-static inline int unix_recvq_full(struct sock const *sk)
+static inline int unix_recvq_full(const struct sock *sk)
{
return skb_queue_len(&sk->sk_receive_queue) > sk->sk_max_ack_backlog;
}

+static inline int unix_recvq_full_lockless(const struct sock *sk)
+{
+ return skb_queue_len_lockless(&sk->sk_receive_queue) >
+ READ_ONCE(sk->sk_max_ack_backlog);
+}
+
struct sock *unix_peer_get(struct sock *s)
{
struct sock *peer;
@@ -1792,7 +1798,8 @@ restart_locked:
* - unix_peer(sk) == sk by time of get but disconnected before lock
*/
if (other != sk &&
- unlikely(unix_peer(other) != sk && unix_recvq_full(other))) {
+ unlikely(unix_peer(other) != sk &&
+ unix_recvq_full_lockless(other))) {
if (timeo) {
timeo = unix_wait_for_peer(other, timeo);

--
2.25.1