Subject: WARNING: refcount bug in tcindex_data_put

From: syzbot <[email-masked]>
Date: Thu, 02 Apr 2020 11:23:11 -0700

Hello,

syzbot found the following crash on:

HEAD commit:    304e0242 net_sched: add a temporary refcnt for struct tcin..
git tree:       net
console output: https://syzkaller.appspot.com/x/log.txt?x=13471edbe00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=8c1e98458335a7d1
dashboard link: https://syzkaller.appspot.com/bug?extid=8325e509a1bf83ec741d
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=11c6940be00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=13c74ba3e00000

The bug was bisected to:

commit 304e024216a802a7dc8ba75d36de82fa136bbf3e
Author: Cong Wang <[email-masked]>
Date:   Sat Mar 28 19:12:59 2020 +0000

    net_sched: add a temporary refcnt for struct tcindex_data

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=141434b7e00000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=161434b7e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=121434b7e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: [email-masked]
Fixes: 304e024216a8 ("net_sched: add a temporary refcnt for struct tcindex_data")

------------[ cut here ]------------
refcount_t: underflow; use-after-free.
WARNING: CPU: 0 PID: 7 at lib/refcount.c:28 refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Kernel panic - not syncing: panic_on_warn set ...
CPU: 0 PID: 7 Comm: kworker/u4:0 Not tainted 5.6.0-syzkaller #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011
Workqueue: tc_filter_workqueue tcindex_destroy_work
Call Trace:
 __dump_stack lib/dump_stack.c:77 [inline]
 dump_stack+0x188/0x20d lib/dump_stack.c:118
 panic+0x2e3/0x75c kernel/panic.c:221
 __warn.cold+0x2f/0x35 kernel/panic.c:582
 report_bug+0x27b/0x2f0 lib/bug.c:195
 fixup_bug arch/x86/kernel/traps.c:175 [inline]
 fixup_bug arch/x86/kernel/traps.c:170 [inline]
 do_error_trap+0x12b/0x220 arch/x86/kernel/traps.c:267
 do_invalid_op+0x32/0x40 arch/x86/kernel/traps.c:286
 invalid_op+0x23/0x30 arch/x86/entry/entry_64.S:1027
RIP: 0010:refcount_warn_saturate+0x1d1/0x1e0 lib/refcount.c:28
Code: e9 db fe ff ff 48 89 df e8 2c 95 1e fe e9 8a fe ff ff e8 c2 81 e1 fd 48 c7 c7 40 c6 71 88 c6 05 42 be f1 06 01 e8 17 f6 b2 fd <0f> 0b e9 af fe ff ff 0f 1f 84 00 00 00 00 00 41 56 41 55 41 54 55
RSP: 0018:ffffc90000cdfcf0 EFLAGS: 00010282
RAX: 0000000000000000 RBX: 0000000000000000 RCX: 0000000000000000
RDX: 0000000000000000 RSI: ffffffff815ca861 RDI: fffff5200019bf90
RBP: 0000000000000003 R08: ffff8880a95a41c0 R09: ffffed1015cc66a1
R10: ffffed1015cc66a0 R11: ffff8880ae633507 R12: ffff8880a81c8c2c
R13: ffff8880a81c8c40 R14: ffff8880a9580e00 R15: ffff8880aa034800
 refcount_sub_and_test include/linux/refcount.h:261 [inline]
 refcount_dec_and_test include/linux/refcount.h:281 [inline]
 tcindex_data_put+0xd1/0xf0 net/sched/cls_tcindex.c:72
 process_one_work+0x965/0x16a0 kernel/workqueue.c:2266
 worker_thread+0x96/0xe20 kernel/workqueue.c:2412
 kthread+0x388/0x470 kernel/kthread.c:268
 ret_from_fork+0x24/0x30 arch/x86/entry/entry_64.S:352
Kernel Offset: disabled
Rebooting in 86400 seconds..

---
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