CVE-2024-26732: 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 ---truncated---