bpf, sockmap: Fix af_unix iter deadlock

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

bpf, sockmap: Fix af_unix iter deadlock

bpf_iter_unix_seq_show() may deadlock when lock_sock_fast() takes the fast

path and the iter prog attempts to update a sockmap. Which ends up spinning

at sock_map_update_elem()'s bh_lock_sock():

WARNING: possible recursive locking detected

test_progs/1393 is trying to acquire lock:

ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: sock_map_update_elem+0xdb/0x1f0

but task is already holding lock:

ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: __lock_sock_fast+0x37/0xe0

other info that might help us debug this:

Possible unsafe locking scenario:

CPU0

----

lock(slock-AF_UNIX);

lock(slock-AF_UNIX);

*** DEADLOCK ***

May be due to missing lock nesting notation

4 locks held by test_progs/1393:

#0: ffff88814b59c790 (&p->lock){+.+.}-{4:4}, at: bpf_seq_read+0x59/0x10d0

#1: ffff88811ec25fd8 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: bpf_seq_read+0x42c/0x10d0

#2: ffff88811ec25f58 (slock-AF_UNIX){+...}-{3:3}, at: __lock_sock_fast+0x37/0xe0

#3: ffffffff85a6a7c0 (rcu_read_lock){....}-{1:3}, at: bpf_iter_run_prog+0x51d/0xb00

Call Trace:

dump_stack_lvl+0x5d/0x80

print_deadlock_bug.cold+0xc0/0xce

__lock_acquire+0x130f/0x2590

lock_acquire+0x14e/0x2b0

_raw_spin_lock+0x30/0x40

sock_map_update_elem+0xdb/0x1f0

bpf_prog_2d0075e5d9b721cd_dump_unix+0x55/0x4f4

bpf_iter_run_prog+0x5b9/0xb00

bpf_iter_unix_seq_show+0x1f7/0x2e0

bpf_seq_read+0x42c/0x10d0

vfs_read+0x171/0xb20

ksys_read+0xff/0x200

do_syscall_64+0x6b/0x3a0

entry_SYSCALL_64_after_hwframe+0x76/0x7e