netdev - [PATCH net] af_packet: fix data-race in packet_setsockopt / packet

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Message-Id: <20220201022358.330621-1-eric.dumazet@gmail.com>
Date:   Mon, 31 Jan 2022 18:23:58 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     "David S . Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>
Cc:     netdev <netdev@...r.kernel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Willem de Bruijn <willemb@...gle.com>,
        syzbot <syzkaller@...glegroups.com>
Subject: [PATCH net] af_packet: fix data-race in packet_setsockopt / packet_setsockopt

From: Eric Dumazet <edumazet@...gle.com>

When packet_setsockopt( PACKET_FANOUT_DATA ) reads po->fanout,
no lock is held, meaning that another thread can change po->fanout.

Given that po->fanout can only be set once during the socket lifetime
(it is only cleared from fanout_release()), we can use
READ_ONCE()/WRITE_ONCE() to document the race.

BUG: KCSAN: data-race in packet_setsockopt / packet_setsockopt

write to 0xffff88813ae8e300 of 8 bytes by task 14653 on cpu 0:
 fanout_add net/packet/af_packet.c:1791 [inline]
 packet_setsockopt+0x22fe/0x24a0 net/packet/af_packet.c:3931
 __sys_setsockopt+0x209/0x2a0 net/socket.c:2180
 __do_sys_setsockopt net/socket.c:2191 [inline]
 __se_sys_setsockopt net/socket.c:2188 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2188
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

read to 0xffff88813ae8e300 of 8 bytes by task 14654 on cpu 1:
 packet_setsockopt+0x691/0x24a0 net/packet/af_packet.c:3935
 __sys_setsockopt+0x209/0x2a0 net/socket.c:2180
 __do_sys_setsockopt net/socket.c:2191 [inline]
 __se_sys_setsockopt net/socket.c:2188 [inline]
 __x64_sys_setsockopt+0x62/0x70 net/socket.c:2188
 do_syscall_x64 arch/x86/entry/common.c:50 [inline]
 do_syscall_64+0x44/0xd0 arch/x86/entry/common.c:80
 entry_SYSCALL_64_after_hwframe+0x44/0xae

value changed: 0x0000000000000000 -> 0xffff888106f8c000

Reported by Kernel Concurrency Sanitizer on:
CPU: 1 PID: 14654 Comm: syz-executor.3 Not tainted 5.16.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011

Fixes: 47dceb8ecdc1 ("packet: add classic BPF fanout mode")
Signed-off-by: Eric Dumazet <edumazet@...gle.com>
Cc: Willem de Bruijn <willemb@...gle.com>
Reported-by: syzbot <syzkaller@...glegroups.com>
---
 net/packet/af_packet.c | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/net/packet/af_packet.c b/net/packet/af_packet.c
index 85ea7ddb48db6a50228ae2b9a255bd161d5b12ed..ab87f22cc7ecde517ba4cd0b3804a28c3cccfc85 100644
--- a/net/packet/af_packet.c
+++ b/net/packet/af_packet.c
@@ -1789,7 +1789,10 @@ static int fanout_add(struct sock *sk, struct fanout_args *args)
 		err = -ENOSPC;
 		if (refcount_read(&match->sk_ref) < match->max_num_members) {
 			__dev_remove_pack(&po->prot_hook);
-			po->fanout = match;
+
+			/* Paired with packet_setsockopt(PACKET_FANOUT_DATA) */
+			WRITE_ONCE(po->fanout, match);
+
 			po->rollover = rollover;
 			rollover = NULL;
 			refcount_set(&match->sk_ref, refcount_read(&match->sk_ref) + 1);
@@ -3934,7 +3937,8 @@ packet_setsockopt(struct socket *sock, int level, int optname, sockptr_t optval,
 	}
 	case PACKET_FANOUT_DATA:
 	{
-		if (!po->fanout)
+		/* Paired with the WRITE_ONCE() in fanout_add() */
+		if (!READ_ONCE(po->fanout))
 			return -EINVAL;
 
 		return fanout_set_data(po, optval, optlen);
-- 
2.35.0.rc2.247.g8bbb082509-goog