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Date: Mon, 8 Apr 2024 23:30:29 +0900
From: Jeongjun Park <aha310510@...il.com>
To: edumazet@...gle.com
Cc: daan.j.demeyer@...il.com,
davem@...emloft.net,
dsahern@...nel.org,
eric.dumazet@...il.com,
kuba@...nel.org,
kuniyu@...zon.com,
martin.lau@...nel.org,
netdev@...r.kernel.org,
pabeni@...hat.com,
willemdebruijn.kernel@...il.com
Subject: Re: [PATCH net] net: implement lockless setsockopt(SO_PEEK_OFF)
Eric Dumazet wrote:
> 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.
The method employed in this patch, which avoids locking u->iolock in
SO_PEEK_OFF, appears to have effectively remedied the immediate vulnerability,
and the patch itself seems robust.
However, if a future scenario arises where mutex_lock(&u->iolock) is required
after sk_setsockopt(sk), this patch would become ineffective.
In practical testing within my environment, I observed that reintroducing
mutex_lock(&u->iolock) within sk_setsockopt() triggered the vulnerability once again.
Therefore, I believe it's crucial to address the fundamental cause triggering this vulnerability
alongside the current patch.
[ 30.537400] ======================================================
[ 30.537765] WARNING: possible circular locking dependency detected
[ 30.538237] 6.9.0-rc1-00058-g4076fa161217-dirty #8 Not tainted
[ 30.538541] ------------------------------------------------------
[ 30.538791] poc/209 is trying to acquire lock:
[ 30.539008] ffff888007a8cd58 (sk_lock-AF_UNIX){+.+.}-{0:0}, at: __unix_dgram_recvmsg+0x37e/0x550
[ 30.540060]
[ 30.540060] but task is already holding lock:
[ 30.540482] ffff888007a8d070 (&u->iolock){+.+.}-{3:3}, at: __unix_dgram_recvmsg+0xec/0x550
[ 30.540871]
[ 30.540871] which lock already depends on the new lock.
[ 30.540871]
[ 30.541341]
[ 30.541341] the existing dependency chain (in reverse order) is:
[ 30.541816]
[ 30.541816] -> #1 (&u->iolock){+.+.}-{3:3}:
[ 30.542411] lock_acquire+0xc0/0x2e0
[ 30.542650] __mutex_lock+0x91/0x4b0
[ 30.542830] sk_setsockopt+0xae2/0x1510
[ 30.543009] do_sock_setsockopt+0x14e/0x180
[ 30.543443] __sys_setsockopt+0x73/0xc0
[ 30.543635] __x64_sys_setsockopt+0x1a/0x30
[ 30.543859] do_syscall_64+0xc9/0x1e0
[ 30.544057] entry_SYSCALL_64_after_hwframe+0x6d/0x75
[ 30.544652]
[ 30.544652] -> #0 (sk_lock-AF_UNIX){+.+.}-{0:0}:
[ 30.544987] check_prev_add+0xeb/0xa20
[ 30.545174] __lock_acquire+0x12fb/0x1740
[ 30.545516] lock_acquire+0xc0/0x2e0
[ 30.545692] lock_sock_nested+0x2d/0x80
[ 30.545871] __unix_dgram_recvmsg+0x37e/0x550
[ 30.546066] sock_recvmsg+0xbf/0xd0
[ 30.546419] ____sys_recvmsg+0x85/0x1d0
[ 30.546653] ___sys_recvmsg+0x77/0xc0
[ 30.546971] __sys_recvmsg+0x55/0xa0
[ 30.547149] do_syscall_64+0xc9/0x1e0
[ 30.547428] entry_SYSCALL_64_after_hwframe+0x6d/0x75
[ 30.547740]
[ 30.547740] other info that might help us debug this:
[ 30.547740]
[ 30.548217] Possible unsafe locking scenario:
[ 30.548217]
[ 30.548502] CPU0 CPU1
[ 30.548713] ---- ----
[ 30.548926] lock(&u->iolock);
[ 30.549234] lock(sk_lock-AF_UNIX);
[ 30.549535] lock(&u->iolock);
[ 30.549798] lock(sk_lock-AF_UNIX);
[ 30.549970]
[ 30.549970] *** DEADLOCK ***
[ 30.549970]
[ 30.550504] 1 lock held by poc/209:
[ 30.550681] #0: ffff888007a8d070 (&u->iolock){+.+.}-{3:3}, at: __unix_dgram_recvmsg+0xec/0x550
[ 30.551100]
[ 30.551100] stack backtrace:
[ 30.551532] CPU: 1 PID: 209 Comm: poc Not tainted 6.9.0-rc1-00058-g4076fa161217-dirty #8
[ 30.551910] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014
[ 30.552539] Call Trace:
[ 30.552788] <TASK>
[ 30.552987] dump_stack_lvl+0x68/0xa0
[ 30.553429] check_noncircular+0x135/0x150
[ 30.553626] check_prev_add+0xeb/0xa20
[ 30.553811] __lock_acquire+0x12fb/0x1740
[ 30.553993] lock_acquire+0xc0/0x2e0
[ 30.554234] ? __unix_dgram_recvmsg+0x37e/0x550
[ 30.554543] ? __skb_try_recv_datagram+0xb2/0x190
[ 30.554752] lock_sock_nested+0x2d/0x80
[ 30.554912] ? __unix_dgram_recvmsg+0x37e/0x550
[ 30.555097] __unix_dgram_recvmsg+0x37e/0x550
[ 30.555498] sock_recvmsg+0xbf/0xd0
[ 30.555661] ____sys_recvmsg+0x85/0x1d0
[ 30.555826] ? __import_iovec+0x177/0x1d0
[ 30.555998] ? import_iovec+0x1a/0x20
[ 30.556401] ? copy_msghdr_from_user+0x68/0xa0
[ 30.556676] ___sys_recvmsg+0x77/0xc0
[ 30.556856] ? __fget_files+0xc8/0x1a0
[ 30.557612] ? lock_release+0xbd/0x290
[ 30.557799] ? __fget_files+0xcd/0x1a0
[ 30.557969] __sys_recvmsg+0x55/0xa0
[ 30.558284] do_syscall_64+0xc9/0x1e0
[ 30.558455] entry_SYSCALL_64_after_hwframe+0x6d/0x75
[ 30.558740] RIP: 0033:0x7f3c14632dad
[ 30.559329] Code: 28 89 54 24 1c 48 89 74 24 10 89 7c 24 08 e8 6a ef ff ff 8b 54 24 1c 48 8b 74 24 10 41 89 c0 8b 7c 24 08 b8 2f 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 33 44 89 c7 48 89 44 24 08 e8 9e ef f8
[ 30.560156] RSP: 002b:00007f3c12c43e60 EFLAGS: 00000293 ORIG_RAX: 000000000000002f
[ 30.560582] RAX: ffffffffffffffda RBX: 0000000000000000 RCX: 00007f3c14632dad
[ 30.560933] RDX: 0000000000000000 RSI: 00007f3c12c44eb0 RDI: 0000000000000005
[ 30.562935] RBP: 00007f3c12c44ef0 R08: 0000000000000000 R09: 00007f3c12c45700
[ 30.565833] R10: fffffffffffff648 R11: 0000000000000293 R12: 00007ffe93a2bfde
[ 30.566161] R13: 00007ffe93a2bfdf R14: 00007f3c12c44fc0 R15: 0000000000802000
[ 30.569456] </TASK>
What are your thoughts on this?
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