[PATCH AUTOSEL 5.12 19/39] inet: annotate date races around sk->sk_txhash

Sasha Levin
2021-06-21T17:53:18Z

From: Eric Dumazet <edumazet@google.com>

[ Upstream commit b71eaed8c04f72a919a9c44e83e4ee254e69e7f3 ]

UDP sendmsg() path can be lockless, it is possible for another
thread to re-connect an change sk->sk_txhash under us.

There is no serious impact, but we can use READ_ONCE()/WRITE_ONCE()
pair to document the race.

BUG: KCSAN: data-race in __ip4_datagram_connect / skb_set_owner_w

write to 0xffff88813397920c of 4 bytes by task 30997 on cpu 1:
sk_set_txhash include/net/sock.h:1937 [inline]
__ip4_datagram_connect+0x69e/0x710 net/ipv4/datagram.c:75
__ip6_datagram_connect+0x551/0x840 net/ipv6/datagram.c:189
ip6_datagram_connect+0x2a/0x40 net/ipv6/datagram.c:272
inet_dgram_connect+0xfd/0x180 net/ipv4/af_inet.c:580
__sys_connect_file net/socket.c:1837 [inline]
__sys_connect+0x245/0x280 net/socket.c:1854
__do_sys_connect net/socket.c:1864 [inline]
__se_sys_connect net/socket.c:1861 [inline]
__x64_sys_connect+0x3d/0x50 net/socket.c:1861
do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88813397920c of 4 bytes by task 31039 on cpu 0:
skb_set_hash_from_sk include/net/sock.h:2211 [inline]
skb_set_owner_w+0x118/0x220 net/core/sock.c:2101
sock_alloc_send_pskb+0x452/0x4e0 net/core/sock.c:2359
sock_alloc_send_skb+0x2d/0x40 net/core/sock.c:2373
__ip6_append_data+0x1743/0x21a0 net/ipv6/ip6_output.c:1621
ip6_make_skb+0x258/0x420 net/ipv6/ip6_output.c:1983
udpv6_sendmsg+0x160a/0x16b0 net/ipv6/udp.c:1527
inet6_sendmsg+0x5f/0x80 net/ipv6/af_inet6.c:642
sock_sendmsg_nosec net/socket.c:654 [inline]
sock_sendmsg net/socket.c:674 [inline]
____sys_sendmsg+0x360/0x4d0 net/socket.c:2350
___sys_sendmsg net/socket.c:2404 [inline]
__sys_sendmmsg+0x315/0x4b0 net/socket.c:2490
__do_sys_sendmmsg net/socket.c:2519 [inline]
__se_sys_sendmmsg net/socket.c:2516 [inline]
__x64_sys_sendmmsg+0x53/0x60 net/socket.c:2516
do_syscall_64+0x4a/0x90 arch/x86/entry/common.c:47
entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0xbca3c43d -> 0xfdb309e0

Reported by Kernel Concurrency Sanitizer on:
CPU: 0 PID: 31039 Comm: syz-executor.2 Not tainted 5.13.0-rc3-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
include/net/sock.h | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index b98c80a7c7ae..b9bdeca1d784 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1928,7 +1928,8 @@ static inline u32 net_tx_rndhash(void)

static inline void sk_set_txhash(struct sock *sk)
{
- sk->sk_txhash = net_tx_rndhash();
+ /* This pairs with READ_ONCE() in skb_set_hash_from_sk() */
+ WRITE_ONCE(sk->sk_txhash, net_tx_rndhash());
}

static inline bool sk_rethink_txhash(struct sock *sk)
@@ -2200,9 +2201,12 @@ static inline void sock_poll_wait(struct file *filp, struct socket *sock,

static inline void skb_set_hash_from_sk(struct sk_buff *skb, struct sock *sk)
{
- if (sk->sk_txhash) {
+ /* This pairs with WRITE_ONCE() in sk_set_txhash() */
+ u32 txhash = READ_ONCE(sk->sk_txhash);
+
+ if (txhash) {
skb->l4_hash = 1;
- skb->hash = sk->sk_txhash;
+ skb->hash = txhash;
}
}

--
2.30.2