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Subject[PATCH 4.19 087/114] net: add a READ_ONCE() in skb_peek_tail()
From: Eric Dumazet <>

commit f8cc62ca3e660ae3fdaee533b1d554297cd2ae82 upstream.

skb_peek_tail() can be used without protection of a lock,
as spotted by KCSAN [1]

In order to avoid load-stearing, add a READ_ONCE()

Note that the corresponding WRITE_ONCE() are already there.

BUG: KCSAN: data-race in sk_wait_data / skb_queue_tail

read to 0xffff8880b36a4118 of 8 bytes by task 20426 on cpu 1:
skb_peek_tail include/linux/skbuff.h:1784 [inline]
sk_wait_data+0x15b/0x250 net/core/sock.c:2477
kcm_wait_data+0x112/0x1f0 net/kcm/kcmsock.c:1103
kcm_recvmsg+0xac/0x320 net/kcm/kcmsock.c:1130
sock_recvmsg_nosec net/socket.c:871 [inline]
sock_recvmsg net/socket.c:889 [inline]
sock_recvmsg+0x92/0xb0 net/socket.c:885
___sys_recvmsg+0x1a0/0x3e0 net/socket.c:2480
do_recvmmsg+0x19a/0x5c0 net/socket.c:2601
__sys_recvmmsg+0x1ef/0x200 net/socket.c:2680
__do_sys_recvmmsg net/socket.c:2703 [inline]
__se_sys_recvmmsg net/socket.c:2696 [inline]
__x64_sys_recvmmsg+0x89/0xb0 net/socket.c:2696
do_syscall_64+0xcc/0x370 arch/x86/entry/common.c:290

write to 0xffff8880b36a4118 of 8 bytes by task 451 on cpu 0:
__skb_insert include/linux/skbuff.h:1852 [inline]
__skb_queue_before include/linux/skbuff.h:1958 [inline]
__skb_queue_tail include/linux/skbuff.h:1991 [inline]
skb_queue_tail+0x7e/0xc0 net/core/skbuff.c:3145
kcm_queue_rcv_skb+0x202/0x310 net/kcm/kcmsock.c:206
kcm_rcv_strparser+0x74/0x4b0 net/kcm/kcmsock.c:370
__strp_recv+0x348/0xf50 net/strparser/strparser.c:309
strp_recv+0x84/0xa0 net/strparser/strparser.c:343
tcp_read_sock+0x174/0x5c0 net/ipv4/tcp.c:1639
strp_read_sock+0xd4/0x140 net/strparser/strparser.c:366
do_strp_work net/strparser/strparser.c:414 [inline]
strp_work+0x9a/0xe0 net/strparser/strparser.c:423
process_one_work+0x3d4/0x890 kernel/workqueue.c:2269
worker_thread+0xa0/0x800 kernel/workqueue.c:2415
kthread+0x1d4/0x200 drivers/block/aoe/aoecmd.c:1253
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:352

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 451 Comm: kworker/u4:3 Not tainted 5.4.0-rc3+ #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: kstrp strp_work

Signed-off-by: Eric Dumazet <>
Reported-by: syzbot <>
Signed-off-by: David S. Miller <>
Signed-off-by: Greg Kroah-Hartman <>

include/linux/skbuff.h | 6 ++++--
1 file changed, 4 insertions(+), 2 deletions(-)

--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -1669,7 +1669,7 @@ static inline struct sk_buff *skb_peek_n
static inline struct sk_buff *skb_peek_tail(const struct sk_buff_head *list_)
- struct sk_buff *skb = list_->prev;
+ struct sk_buff *skb = READ_ONCE(list_->prev);

if (skb == (struct sk_buff *)list_)
skb = NULL;
@@ -1737,7 +1737,9 @@ static inline void __skb_insert(struct s
struct sk_buff *prev, struct sk_buff *next,
struct sk_buff_head *list)
- /* see skb_queue_empty_lockless() for the opposite READ_ONCE() */
+ /* See skb_queue_empty_lockless() and skb_peek_tail()
+ * for the opposite READ_ONCE()
+ */
WRITE_ONCE(newsk->next, next);
WRITE_ONCE(newsk->prev, prev);
WRITE_ONCE(next->prev, newsk);

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