[syzbot] [can?] WARNING: refcount bug in sk_skb_reason_drop

[syzbot]

Hello,

syzbot found the following issue on:

HEAD commit: a430d95c5efa Merge tag 'lsm-pr-20240911' of git://git.kern..
git tree: upstream
console+strace: https://syzkaller.appspot.com/x/log.txt?x=11260ca9980000
kernel config: https://syzkaller.appspot.com/x/.config?x=d85e1e571a820894
dashboard link: https://syzkaller.appspot.com/bug?extid=d4e8dc385d9258220c31
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40
syz repro: https://syzkaller.appspot.com/x/repro.syz?x=11903177980000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15b9a607980000

Downloadable assets:
disk image: https://storage.googleapis.com/syzbot-assets/4faaa939b3a4/disk-a430d95c.raw.xz
vmlinux: https://storage.googleapis.com/syzbot-assets/83c722076440/vmlinux-a430d95c.xz
kernel image: https://storage.googleapis.com/syzbot-assets/24c938d49c40/bzImage-a430d95c.xz

The issue was bisected to:

commit c9c0ee5f20c593faf289fa8850c3ed84031dd12a
Author: Breno Leitao <lei...@debian.org>
Date: Mon Jul 29 10:47:40 2024 +0000

net: skbuff: Skip early return in skb_unref when debugging

bisection log: https://syzkaller.appspot.com/x/bisect.txt?x=15e6f69f980000
final oops: https://syzkaller.appspot.com/x/report.txt?x=17e6f69f980000
console output: https://syzkaller.appspot.com/x/log.txt?x=13e6f69f980000

IMPORTANT: if you fix the issue, please add the following tag to the commit:
Reported-by: syzbot+d4e8dc...@syzkaller.appspotmail.com
Fixes: c9c0ee5f20c5 ("net: skbuff: Skip early return in skb_unref when debugging")

vxcan1: j1939_tp_rxtimer: 0xffff8880326cb400: rx timeout, send abort
vxcan1: j1939_xtp_rx_abort_one: 0xffff8880326cb400: 0x00000: (3) A timeout occurred and this is the connection abort to close the session.
------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 16 at lib/refcount.c:28 refcount_warn_saturate+0x14a/0x210 lib/refcount.c:28
Modules linked in:
CPU: 0 UID: 0 PID: 16 Comm: ksoftirqd/0 Not tainted 6.11.0-syzkaller-02574-ga430d95c5efa #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 08/06/2024
RIP: 0010:refcount_warn_saturate+0x14a/0x210 lib/refcount.c:28
Code: ff 89 de e8 58 80 04 fd 84 db 0f 85 66 ff ff ff e8 6b 7e 04 fd c6 05 59 3d 7f 0b 01 90 48 c7 c7 00 ba b0 8b e8 67 d1 c6 fc 90 <0f> 0b 90 90 e9 43 ff ff ff e8 48 7e 04 fd 0f b6 1d 34 3d 7f 0b 31
RSP: 0018:ffffc900001577c8 EFLAGS: 00010286
RAX: 0000000000000000 RBX: 0000000000000000 RCX: ffffffff814e2c79
RDX: ffff88801cadda00 RSI: ffffffff814e2c86 RDI: 0000000000000001
RBP: ffff88802e405224 R08: 0000000000000001 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000002
R13: 0000000000000000 R14: ffff88802e405224 R15: 0000000000000001
FS: 0000000000000000(0000) GS:ffff8880b8800000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 000055772d12ee40 CR3: 000000002853a000 CR4: 0000000000350ef0
Call Trace:
<TASK>
__refcount_sub_and_test include/linux/refcount.h:275 [inline]
__refcount_dec_and_test include/linux/refcount.h:307 [inline]
refcount_dec_and_test include/linux/refcount.h:325 [inline]
skb_unref include/linux/skbuff.h:1232 [inline]
__sk_skb_reason_drop net/core/skbuff.c:1213 [inline]
sk_skb_reason_drop+0x183/0x1a0 net/core/skbuff.c:1241
kfree_skb_reason include/linux/skbuff.h:1262 [inline]
kfree_skb include/linux/skbuff.h:1271 [inline]
j1939_session_destroy+0x163/0x460 net/can/j1939/transport.c:282
__j1939_session_release net/can/j1939/transport.c:294 [inline]
kref_put include/linux/kref.h:65 [inline]
j1939_session_put net/can/j1939/transport.c:299 [inline]
j1939_xtp_rx_abort_one+0x3f9/0x560 net/can/j1939/transport.c:1354
j1939_xtp_rx_abort net/can/j1939/transport.c:1362 [inline]
j1939_tp_cmd_recv net/can/j1939/transport.c:2128 [inline]
j1939_tp_recv+0xcf8/0xf50 net/can/j1939/transport.c:2161
j1939_can_recv+0x78f/0xa50 net/can/j1939/main.c:108
deliver net/can/af_can.c:572 [inline]
can_rcv_filter+0x2ab/0x900 net/can/af_can.c:606
can_receive+0x320/0x5c0 net/can/af_can.c:663
can_rcv+0x1e2/0x280 net/can/af_can.c:687
__netif_receive_skb_one_core+0x1b4/0x1e0 net/core/dev.c:5662
__netif_receive_skb+0x1d/0x160 net/core/dev.c:5775
process_backlog+0x443/0x15f0 net/core/dev.c:6107
__napi_poll.constprop.0+0xba/0x550 net/core/dev.c:6771
napi_poll net/core/dev.c:6840 [inline]
net_rx_action+0xa92/0x1010 net/core/dev.c:6962
handle_softirqs+0x219/0x8f0 kernel/softirq.c:554
run_ksoftirqd kernel/softirq.c:928 [inline]
run_ksoftirqd+0x3a/0x60 kernel/softirq.c:920
smpboot_thread_fn+0x664/0xa10 kernel/smpboot.c:164
kthread+0x2c4/0x3a0 kernel/kthread.c:389
ret_from_fork+0x48/0x80 arch/x86/kernel/process.c:147
ret_from_fork_asm+0x1a/0x30 arch/x86/entry/entry_64.S:244
</TASK>


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***

Subject: Re: [syzbot] [can?] WARNING: refcount bug in sk_skb_reason_drop
Author: dman...@yandex.ru

#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git 7ec462100ef9142344ddbf86f2c3008b97acddbe

From eb58509f460225df7cf181a07772d577a270091b Mon Sep 17 00:00:00 2001
From: Dmitry Antipov <dman...@yandex.ru>
Date: Thu, 3 Oct 2024 20:24:12 +0300
Subject: [PATCH] can: fix skb reference counting in j1939_session_new()

Since 'j1939_session_skb_queue()' do an extra 'skb_get()' for each
new skb, I assume that the same should be done for an initial one
in 'j1939_session_new()' just to avoid refcount underflow.

Reported-by: syzbot+d4e8dc...@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d4e8dc385d9258220c31
Fixes: 9d71dd0c7009 ("can: add support of SAE J1939 protocol")
Signed-off-by: Dmitry Antipov <dman...@yandex.ru>
---
net/can/j1939/transport.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/net/can/j1939/transport.c b/net/can/j1939/transport.c
index 319f47df3330..fecee36dbb70 100644
--- a/net/can/j1939/transport.c
+++ b/net/can/j1939/transport.c
@@ -1505,7 +1505,8 @@ static struct j1939_session *j1939_session_new(struct j1939_priv *priv,
session->state = J1939_SESSION_NEW;

skb_queue_head_init(&session->skb_queue);
- skb_queue_tail(&session->skb_queue, skb);
+ /* Take an extra ref just like in j1939_session_skb_queue(). */
+ skb_queue_tail(&session->skb_queue, skb_get(skb));

skcb = j1939_skb_to_cb(skb);
memcpy(&session->skcb, skcb, sizeof(session->skcb));
--
2.46.2

[syzbot]

Hello,

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

Reported-by: syzbot+d4e8dc...@syzkaller.appspotmail.com
Tested-by: syzbot+d4e8dc...@syzkaller.appspotmail.com

Tested on:

commit: 7ec46210 Merge tag 'pull-work.unaligned' of git://git...
git tree: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
console output: https://syzkaller.appspot.com/x/log.txt?x=12ff5527980000
kernel config: https://syzkaller.appspot.com/x/.config?x=2ee84829c35501a2

dashboard link: https://syzkaller.appspot.com/bug?extid=d4e8dc385d9258220c31
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

patch: https://syzkaller.appspot.com/x/patch.diff?x=14e66307980000

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

[Hillf Danton]

On Thu, 03 Oct 2024 09:14:26 -0700

> syzbot found the following issue on:
>
> HEAD commit: a430d95c5efa Merge tag 'lsm-pr-20240911' of git://git.kern..
> git tree: upstream

> C reproducer: https://syzkaller.appspot.com/x/repro.c?x=15b9a607980000

#syz test

--- x/net/can/j1939/transport.c
+++ y/net/can/j1939/transport.c
@@ -1506,6 +1506,7 @@ static struct j1939_session *j1939_sessi

skb_queue_head_init(&session->skb_queue);
skb_queue_tail(&session->skb_queue, skb);
+ skb_get(skb);

[syzbot]

Hello,

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

Reported-by: syzbot+d4e8dc...@syzkaller.appspotmail.com
Tested-by: syzbot+d4e8dc...@syzkaller.appspotmail.com

Tested on:

commit: 3840cbe2 sched: psi: fix bogus pressure spikes from ag..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=128dd3d0580000

kernel config: https://syzkaller.appspot.com/x/.config?x=2ee84829c35501a2
dashboard link: https://syzkaller.appspot.com/bug?extid=d4e8dc385d9258220c31
compiler: gcc (Debian 12.2.0-14) 12.2.0, GNU ld (GNU Binutils for Debian) 2.40

patch: https://syzkaller.appspot.com/x/patch.diff?x=17122b9f980000