net: implement lockless setsockopt(SO_PEEK_OFF)

In the Linux kernel, the following vulnerability has been resolved:

net: implement lockless setsockopt(SO_PEEK_OFF)

syzbot reported a lockdep violation [1] involving af_unix

support of SO_PEEK_OFF.

Since SO_PEEK_OFF is inherently not thread safe (it uses a per-socket

sk_peek_off field), there is really no point to enforce a pointless

thread safety in the kernel.

After this patch :

- setsockopt(SO_PEEK_OFF) no longer acquires the socket lock.

- skb_consume_udp() no longer has to acquire the socket lock.

- af_unix no longer needs a special version of sk_set_peek_off(),

because it does not lock u->iolock anymore.

As a followup, we could replace prot->set_peek_off to be a boolean

and avoid an indirect call, since we always use sk_set_peek_off().

[1]

WARNING: possible circular locking dependency detected

6.8.0-rc4-syzkaller-00267-g0f1dd5e91e2b #0 Not tainted

syz-executor.2/30025 is trying to acquire lock:

ffff8880765e7d80 (&u->iolock){+.+.}-{3:3}, at: unix_set_peek_off+0x26/0xa0 net/unix/af_unix.c:789

but task is already holding lock:

ffff8880765e7930 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1691 [inline]

ffff8880765e7930 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: sockopt_lock_sock net/core/sock.c:1060 [inline]

ffff8880765e7930 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: sk_setsockopt+0xe52/0x3360 net/core/sock.c:1193

which lock already depends on the new lock.

the existing dependency chain (in reverse order) is:

-> #1 (sk_lock-AF_UNIX){+.+.}-{0:0}:

lock_acquire+0x1e3/0x530 kernel/locking/lockdep.c:5754

lock_sock_nested+0x48/0x100 net/core/sock.c:3524

lock_sock include/net/sock.h:1691 [inline]

__unix_dgram_recvmsg+0x1275/0x12c0 net/unix/af_unix.c:2415

sock_recvmsg_nosec+0x18e/0x1d0 net/socket.c:1046

____sys_recvmsg+0x3c0/0x470 net/socket.c:2801

___sys_recvmsg net/socket.c:2845 [inline]

do_recvmmsg+0x474/0xae0 net/socket.c:2939

__sys_recvmmsg net/socket.c:3018 [inline]

__do_sys_recvmmsg net/socket.c:3041 [inline]

__se_sys_recvmmsg net/socket.c:3034 [inline]

__x64_sys_recvmmsg+0x199/0x250 net/socket.c:3034

do_syscall_64+0xf9/0x240

entry_SYSCALL_64_after_hwframe+0x6f/0x77

-> #0 (&u->iolock){+.+.}-{3:3}:

check_prev_add kernel/locking/lockdep.c:3134 [inline]

check_prevs_add kernel/locking/lockdep.c:3253 [inline]

validate_chain+0x18ca/0x58e0 kernel/locking/lockdep.c:3869

__lock_acquire+0x1345/0x1fd0 kernel/locking/lockdep.c:5137

lock_acquire+0x1e3/0x530 kernel/locking/lockdep.c:5754

__mutex_lock_common kernel/locking/mutex.c:608 [inline]

__mutex_lock+0x136/0xd70 kernel/locking/mutex.c:752

unix_set_peek_off+0x26/0xa0 net/unix/af_unix.c:789

sk_setsockopt+0x207e/0x3360

do_sock_setsockopt+0x2fb/0x720 net/socket.c:2307

__sys_setsockopt+0x1ad/0x250 net/socket.c:2334

__do_sys_setsockopt net/socket.c:2343 [inline]

__se_sys_setsockopt net/socket.c:2340 [inline]

__x64_sys_setsockopt+0xb5/0xd0 net/socket.c:2340

do_syscall_64+0xf9/0x240

entry_SYSCALL_64_after_hwframe+0x6f/0x77

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0 CPU1

---- ----

lock(sk_lock-AF_UNIX);

lock(&u->iolock);

lock(sk_lock-AF_UNIX);

lock(&u->iolock);

*** DEADLOCK ***

1 lock held by syz-executor.2/30025:

#0: ffff8880765e7930 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: lock_sock include/net/sock.h:1691 [inline]

#0: ffff8880765e7930 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: sockopt_lock_sock net/core/sock.c:1060 [inline]

#0: ffff8880765e7930 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: sk_setsockopt+0xe52/0x3360 net/core/sock.c:1193

stack backtrace:

CPU: 0 PID: 30025 Comm: syz-executor.2 Not tainted 6.8.0-rc4-syzkaller-00267-g0f1dd5e91e2b #0

Hardware name: Google Google C

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