[syzbot] WARNING: refcount bug in l2cap_chan_hold

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: 9be9ed2612b5 Merge tag 'platform-drivers-x86-v5.18-4' of g..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=164544f1f00000
kernel config: https://syzkaller.appspot.com/x/.config?x=3afe599903e2c6b6
dashboard link: https://syzkaller.appspot.com/bug?extid=fd86a41b17a416a37a75
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=17285deaf00000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15bd5d3ef00000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+fd86a4...@syzkaller.appspotmail.com

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.
WARNING: CPU: 2 PID: 3674 at lib/refcount.c:25 refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
Modules linked in:
CPU: 2 PID: 3674 Comm: kworker/u19:1 Not tainted 5.18.0-rc6-syzkaller-00007-g9be9ed2612b5 #0
Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
Workqueue: hci0 hci_rx_work
RIP: 0010:refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
Code: 09 31 ff 89 de e8 97 44 81 fd 84 db 0f 85 36 ff ff ff e8 aa 40 81 fd 48 c7 c7 80 61 26 8a c6 05 0f 00 ad 09 01 e8 e7 f5 30 05 <0f> 0b e9 17 ff ff ff e8 8b 40 81 fd 0f b6 1d f4 ff ac 09 31 ff 89
RSP: 0018:ffffc90002e17960 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: ffff8880187b2180 RSI: ffffffff815f4a18 RDI: fffff520005c2f1e
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000001
R10: ffffffff815ef3ee R11: 0000000000000000 R12: ffff88801c4a1000
R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000003
FS: 0000000000000000(0000) GS:ffff88802cc00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 0000555556dd1608 CR3: 00000000234d6000 CR4: 0000000000150ee0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__refcount_add include/linux/refcount.h:199 [inline]
__refcount_inc include/linux/refcount.h:250 [inline]
refcount_inc include/linux/refcount.h:267 [inline]
kref_get include/linux/kref.h:45 [inline]
l2cap_chan_hold+0xff/0x120 net/bluetooth/l2cap_core.c:497
l2cap_global_chan_by_psm+0x3b1/0x4a0 net/bluetooth/l2cap_core.c:1986
l2cap_conless_channel net/bluetooth/l2cap_core.c:7611 [inline]
l2cap_recv_frame+0x68d/0x8e60 net/bluetooth/l2cap_core.c:7681
l2cap_recv_acldata+0xa96/0xc10 net/bluetooth/l2cap_core.c:8394
hci_acldata_packet net/bluetooth/hci_core.c:3633 [inline]
hci_rx_work+0x4d5/0xd60 net/bluetooth/hci_core.c:3824
process_one_work+0x996/0x1610 kernel/workqueue.c:2289
worker_thread+0x665/0x1080 kernel/workqueue.c:2436
kthread+0x2e9/0x3a0 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298
</TASK>


---
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syzbot can test patches for this issue, for details see:
https://goo.gl/tpsmEJ#testing-patches

[Hillf Danton]

On Sat, 14 May 2022 20:20:20 -0700

Fix uaf by acquiring chan_list_lock after channel is fully initialized.

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ 9be9ed2612b5

--- y/net/bluetooth/l2cap_core.c
+++ x/net/bluetooth/l2cap_core.c
@@ -458,10 +458,6 @@ struct l2cap_chan *l2cap_chan_create(voi
/* Set default lock nesting level */
atomic_set(&chan->nesting, L2CAP_NESTING_NORMAL);

- write_lock(&chan_list_lock);
- list_add(&chan->global_l, &chan_list);
- write_unlock(&chan_list_lock);
-
INIT_DELAYED_WORK(&chan->chan_timer, l2cap_chan_timeout);

chan->state = BT_OPEN;
@@ -473,6 +469,10 @@ struct l2cap_chan *l2cap_chan_create(voi

BT_DBG("chan %p", chan);

+ write_lock(&chan_list_lock);
+ list_add(&chan->global_l, &chan_list);
+ write_unlock(&chan_list_lock);
+
return chan;
}
EXPORT_SYMBOL_GPL(l2cap_chan_create);
--

[syzbot]

Hello,

syzbot has tested the proposed patch but the reproducer is still triggering an issue:
WARNING: refcount bug in l2cap_chan_hold

------------[ cut here ]------------
refcount_t: addition on 0; use-after-free.

WARNING: CPU: 3 PID: 3688 at lib/refcount.c:25 refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
Modules linked in:
CPU: 3 PID: 3688 Comm: kworker/u19:2 Not tainted 5.18.0-rc6-syzkaller-00007-g9be9ed2612b5-dirty #0

Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 1.14.0-2 04/01/2014
Workqueue: hci0 hci_rx_work
RIP: 0010:refcount_warn_saturate+0x169/0x1e0 lib/refcount.c:25
Code: 09 31 ff 89 de e8 97 44 81 fd 84 db 0f 85 36 ff ff ff e8 aa 40 81 fd 48 c7 c7 80 61 26 8a c6 05 0f 00 ad 09 01 e8 e7 f5 30 05 <0f> 0b e9 17 ff ff ff e8 8b 40 81 fd 0f b6 1d f4 ff ac 09 31 ff 89

RSP: 0018:ffffc90002ef7960 EFLAGS: 00010282

RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000

RDX: ffff888021eb8080 RSI: ffffffff815f4a18 RDI: fffff520005def1e
RBP: 0000000000000002 R08: 0000000000000000 R09: 0000000000000000
R10: ffffffff815ef3ee R11: 0000000000000000 R12: ffff888022671000

R13: 0000000000000001 R14: 0000000000000000 R15: 0000000000000003

FS: 0000000000000000(0000) GS:ffff88802cd00000(0000) knlGS:0000000000000000

CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033

CR2: 0000555555842708 CR3: 0000000019825000 CR4: 0000000000150ee0

DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
<TASK>
__refcount_add include/linux/refcount.h:199 [inline]
__refcount_inc include/linux/refcount.h:250 [inline]
refcount_inc include/linux/refcount.h:267 [inline]
kref_get include/linux/kref.h:45 [inline]
l2cap_chan_hold+0xff/0x120 net/bluetooth/l2cap_core.c:497
l2cap_global_chan_by_psm+0x3b1/0x4a0 net/bluetooth/l2cap_core.c:1986
l2cap_conless_channel net/bluetooth/l2cap_core.c:7611 [inline]
l2cap_recv_frame+0x68d/0x8e60 net/bluetooth/l2cap_core.c:7681
l2cap_recv_acldata+0xa96/0xc10 net/bluetooth/l2cap_core.c:8394
hci_acldata_packet net/bluetooth/hci_core.c:3633 [inline]
hci_rx_work+0x4d5/0xd60 net/bluetooth/hci_core.c:3824
process_one_work+0x996/0x1610 kernel/workqueue.c:2289
worker_thread+0x665/0x1080 kernel/workqueue.c:2436
kthread+0x2e9/0x3a0 kernel/kthread.c:376
ret_from_fork+0x1f/0x30 arch/x86/entry/entry_64.S:298
</TASK>

Tested on:

commit: 9be9ed26 Merge tag 'platform-drivers-x86-v5.18-4' of g..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/
console output: https://syzkaller.appspot.com/x/log.txt?x=1449db8ef00000

kernel config: https://syzkaller.appspot.com/x/.config?x=3afe599903e2c6b6
dashboard link: https://syzkaller.appspot.com/bug?extid=fd86a41b17a416a37a75
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

patch: https://syzkaller.appspot.com/x/patch.diff?x=159a21f1f00000

[Hillf Danton]

On Sat, 14 May 2022 22:24:10 -0700

Fix uaf by trying to bump up refcount not zero.

#syz test https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/ 9be9ed2612b5

--- y/net/bluetooth/l2cap_core.c
+++ x/net/bluetooth/l2cap_core.c
@@ -458,10 +458,6 @@ struct l2cap_chan *l2cap_chan_create(voi
/* Set default lock nesting level */
atomic_set(&chan->nesting, L2CAP_NESTING_NORMAL);

- write_lock(&chan_list_lock);
- list_add(&chan->global_l, &chan_list);
- write_unlock(&chan_list_lock);
-
INIT_DELAYED_WORK(&chan->chan_timer, l2cap_chan_timeout);

chan->state = BT_OPEN;
@@ -473,6 +469,10 @@ struct l2cap_chan *l2cap_chan_create(voi

BT_DBG("chan %p", chan);

+ write_lock(&chan_list_lock);
+ list_add(&chan->global_l, &chan_list);
+ write_unlock(&chan_list_lock);
+
return chan;
}
EXPORT_SYMBOL_GPL(l2cap_chan_create);

@@ -497,6 +497,14 @@ void l2cap_chan_hold(struct l2cap_chan *
kref_get(&c->kref);
}

+static struct l2cap_chan *l2cap_chan_hold_not_zero(struct l2cap_chan *c)
+{
+ if (kref_get_unless_zero(&c->kref))
+ return c;
+ else
+ return NULL;
+}
+
void l2cap_chan_put(struct l2cap_chan *c)
{
BT_DBG("chan %p orig refcnt %u", c, kref_read(&c->kref));
@@ -1983,7 +1991,7 @@ static struct l2cap_chan *l2cap_global_c
}

if (c1)
- l2cap_chan_hold(c1);
+ c1 = l2cap_chan_hold_not_zero(c1);

read_unlock(&chan_list_lock);

--

[syzbot]

Hello,

syzbot has tested the proposed patch and the reproducer did not trigger any issue:

Reported-and-tested-by: syzbot+fd86a4...@syzkaller.appspotmail.com

Tested on:

commit: 9be9ed26 Merge tag 'platform-drivers-x86-v5.18-4' of g..
git tree: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/

kernel config: https://syzkaller.appspot.com/x/.config?x=3afe599903e2c6b6
dashboard link: https://syzkaller.appspot.com/bug?extid=fd86a41b17a416a37a75
compiler: gcc (Debian 10.2.1-6) 10.2.1 20210110, GNU ld (GNU Binutils for Debian) 2.35.2

patch: https://syzkaller.appspot.com/x/patch.diff?x=12d8a5bef00000

Note: testing is done by a robot and is best-effort only.

[syzbot]

Auto-closing this bug as obsolete.
No recent activity, existing reproducers are no longer triggering the issue.