WARNING: refcount bug in sock_wfree (2)

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: 442630f6 Add linux-next specific files for 20191008
git tree: linux-next
console output: https://syzkaller.appspot.com/x/log.txt?x=158fa6bf600000
kernel config: https://syzkaller.appspot.com/x/.config?x=af1bfeef713eefdd
dashboard link: https://syzkaller.appspot.com/bug?extid=c0ba5b9e742f049a2edf
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=15a861b3600000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=1769f48b600000

The bug was bisected to:

commit 6c080ff07363389cb4092193eb333639f0392b8c
Author: Jens Axboe <ax...@kernel.dk>
Date: Thu Oct 3 14:11:03 2019 +0000

io_uring: allow sparse fixed file sets

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=1344b993600000
final crash: https://syzkaller.appspot.com/x/report.txt?x=10c4b993600000
console output: https://syzkaller.appspot.com/x/log.txt?x=1744b993600000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+c0ba5b...@syzkaller.appspotmail.com
Fixes: 6c080ff07363 ("io_uring: allow sparse fixed file sets")

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 8691 at lib/refcount.c:190
refcount_sub_and_test_checked lib/refcount.c:190 [inline]
WARNING: CPU: 0 PID: 8691 at lib/refcount.c:190
refcount_sub_and_test_checked+0x1d0/0x200 lib/refcount.c:180
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 8691 Comm: syz-executor322 Not tainted 5.4.0-rc2-next-20191008
#0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:77 [inline]
dump_stack+0x172/0x1f0 lib/dump_stack.c:113
panic+0x2e3/0x75c kernel/panic.c:221
__warn.cold+0x2f/0x35 kernel/panic.c:582
report_bug+0x289/0x300 lib/bug.c:195
fixup_bug arch/x86/kernel/traps.c:174 [inline]
fixup_bug arch/x86/kernel/traps.c:169 [inline]
do_error_trap+0x11b/0x200 arch/x86/kernel/traps.c:267
do_invalid_op+0x37/0x50 arch/x86/kernel/traps.c:286
invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1028
RIP: 0010:refcount_sub_and_test_checked lib/refcount.c:190 [inline]
RIP: 0010:refcount_sub_and_test_checked+0x1d0/0x200 lib/refcount.c:180
Code: 1d b0 f0 7f 06 31 ff 89 de e8 6c d7 30 fe 84 db 75 94 e8 23 d6 30 fe
48 c7 c7 20 81 e6 87 c6 05 90 f0 7f 06 01 e8 e8 15 02 fe <0f> 0b e9 75 ff
ff ff e8 04 d6 30 fe e9 6e ff ff ff 48 89 df e8 e7
RSP: 0018:ffff8880a8267a28 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815cb676 RDI: ffffed101504cf37
RBP: ffff8880a8267ac0 R08: ffff888090e363c0 R09: fffffbfff14eeb42
R10: fffffbfff14eeb41 R11: ffffffff8a775a0f R12: 00000000fffffd02
R13: 0000000000000001 R14: ffff8880a8267a98 R15: 0000000000000001
sock_wfree+0x10c/0x190 net/core/sock.c:1958
unix_destruct_scm+0x115/0x170 net/unix/scm.c:149
io_destruct_skb+0x62/0x80 fs/io_uring.c:2995
skb_release_head_state+0xeb/0x260 net/core/skbuff.c:652
skb_release_all+0x16/0x60 net/core/skbuff.c:663
__kfree_skb net/core/skbuff.c:679 [inline]
kfree_skb net/core/skbuff.c:697 [inline]
kfree_skb+0x101/0x3c0 net/core/skbuff.c:691
__io_sqe_files_scm+0x429/0x640 fs/io_uring.c:3049
io_sqe_files_scm fs/io_uring.c:3071 [inline]
io_sqe_files_register fs/io_uring.c:3154 [inline]
__io_uring_register+0x1f69/0x2d70 fs/io_uring.c:4152
__do_sys_io_uring_register fs/io_uring.c:4204 [inline]
__se_sys_io_uring_register fs/io_uring.c:4186 [inline]
__x64_sys_io_uring_register+0x193/0x1f0 fs/io_uring.c:4186
do_syscall_64+0xfa/0x760 arch/x86/entry/common.c:290
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x440279
Code: 18 89 d0 c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 48 89 f8 48 89 f7
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff
ff 0f 83 fb 13 fc ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007ffeeedc31e8 EFLAGS: 00000246 ORIG_RAX: 00000000000001ab
RAX: ffffffffffffffda RBX: 00000000004002c8 RCX: 0000000000440279
RDX: 0000000020000280 RSI: 0000000000000002 RDI: 0000000000000003
RBP: 00000000006ca018 R08: 0000000000000000 R09: 00000000004002c8
R10: 0000000000000001 R11: 0000000000000246 R12: 0000000000401b00
R13: 0000000000401b90 R14: 0000000000000000 R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


---
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For information about bisection process see: https://goo.gl/tpsmEJ#bisection
syzbot can test patches for this bug, for details see:
https://goo.gl/tpsmEJ#testing-patches

[Jens Axboe]

I think this will do it, but that's just from looking at the code,
haven't tried to reproduce anything yet.

I'll take a closer look tomorrow and verify.


diff --git a/fs/io_uring.c b/fs/io_uring.c
index 0b20b3599b01..52daeb71ee27 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -3016,7 +3016,6 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
}

skb->sk = sk;
- skb->destructor = io_destruct_skb;

nr_files = 0;
fpl->user = get_uid(ctx->user);
@@ -3031,6 +3030,7 @@ static int __io_sqe_files_scm(struct io_ring_ctx *ctx, int nr, int offset)
if (nr_files) {
fpl->max = fpl->count = nr_files;
UNIXCB(skb).fp = fpl;
+ skb->destructor = io_destruct_skb;
refcount_add(skb->truesize, &sk->sk_wmem_alloc);
skb_queue_head(&sk->sk_receive_queue, skb);


--
Jens Axboe

[Jens Axboe]

Took a closer look, and verified that this is indeed the issue, and that
the patch does fix it. I'm going to apply this to my for-5.5/io_uring
branch, which is where the offending commit it queued up.

--
Jens Axboe

[Eric Biggers]

The fix apparently got folded into the original patch
"io_uring: allow sparse fixed file sets", so telling syzbot:

#syz invalid