INFO: task hung in blk_freeze_queue

[syzbot]

Hello,

syzbot hit the following crash on upstream commit
e237f98a9c134c3d600353f21e07db915516875b (Mon Feb 5 21:35:56 2018 +0000)
Merge tag 'xfs-4.16-merge-5' of
git://git.kernel.org/pub/scm/fs/xfs/xfs-linux

So far this crash happened 2 times on upstream.
Unfortunately, I don't have any reproducer for this crash yet.
Raw console output is attached.
compiler: gcc (GCC) 7.1.1 20170620
.config is attached.
user-space arch: i386

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+2ab52b...@syzkaller.appspotmail.com
It will help syzbot understand when the bug is fixed. See footer for
details.
If you forward the report, please keep this part and the footer.

Buffer I/O error on dev loop0, logical block 0, lost async page write
INFO: task syz-executor1:14117 blocked for more than 120 seconds.
Not tainted 4.15.0+ #210
"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
syz-executor1 D26432 14117 4227 0x20020004
Call Trace:
context_switch kernel/sched/core.c:2845 [inline]
__schedule+0x8eb/0x2060 kernel/sched/core.c:3421
schedule+0xf5/0x430 kernel/sched/core.c:3480
blk_mq_freeze_queue_wait+0x1bb/0x400 block/blk-mq.c:136
blk_freeze_queue+0x4a/0x80 block/blk-mq.c:165
blk_mq_freeze_queue+0x15/0x20 block/blk-mq.c:174
loop_change_fd drivers/block/loop.c:667 [inline]
lo_ioctl+0x10ce/0x1b70 drivers/block/loop.c:1361
lo_compat_ioctl+0xab/0x140 drivers/block/loop.c:1556
compat_blkdev_ioctl+0x3ae/0x1840 block/compat_ioctl.c:406
C_SYSC_ioctl fs/compat_ioctl.c:1473 [inline]
compat_SyS_ioctl+0x151/0x2a30 fs/compat_ioctl.c:1419
do_syscall_32_irqs_on arch/x86/entry/common.c:330 [inline]
do_fast_syscall_32+0x3ee/0xfa1 arch/x86/entry/common.c:392
entry_SYSENTER_compat+0x54/0x63 arch/x86/entry/entry_64_compat.S:129
RIP: 0023:0xf7f44c79
RSP: 002b:00000000f774008c EFLAGS: 00000296 ORIG_RAX: 0000000000000036
RAX: ffffffffffffffda RBX: 0000000000000013 RCX: 0000000000004c06
RDX: 0000000000000013 RSI: 0000000000000000 RDI: 0000000000000000
RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000000 R12: 0000000000000000
R13: 0000000000000000 R14: 0000000000000000 R15: 0000000000000000

Showing all locks held in the system:
2 locks held by khungtaskd/759:
#0: (rcu_read_lock){....}, at: [<00000000b3b533f6>]
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: (rcu_read_lock){....}, at: [<00000000b3b533f6>] watchdog+0x1c5/0xd60
kernel/hung_task.c:249
#1: (tasklist_lock){.+.+}, at: [<00000000440cd9e2>]
debug_show_all_locks+0xd3/0x3d0 kernel/locking/lockdep.c:4470
1 lock held by rsyslogd/4029:
#0: (&f->f_pos_lock){+.+.}, at: [<000000007729bd98>]
__fdget_pos+0x12b/0x190 fs/file.c:765
2 locks held by getty/4151:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4152:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4153:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4154:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4155:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4156:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
2 locks held by getty/4157:
#0: (&tty->ldisc_sem){++++}, at: [<00000000903280c4>]
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: (&ldata->atomic_read_lock){+.+.}, at: [<00000000fb09d1cd>]
n_tty_read+0x2ef/0x1a00 drivers/tty/n_tty.c:2131
1 lock held by syz-executor1/14117:
#0: (&lo->lo_ctl_mutex/1){+.+.}, at: [<00000000b6589539>]
lo_ioctl+0x8b/0x1b70 drivers/block/loop.c:1355

=============================================

NMI backtrace for cpu 1
CPU: 1 PID: 759 Comm: khungtaskd Not tainted 4.15.0+ #210
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011
Call Trace:
__dump_stack lib/dump_stack.c:17 [inline]
dump_stack+0x194/0x257 lib/dump_stack.c:53
nmi_cpu_backtrace+0x1d2/0x210 lib/nmi_backtrace.c:103
nmi_trigger_cpumask_backtrace+0x122/0x180 lib/nmi_backtrace.c:62
arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
check_hung_task kernel/hung_task.c:132 [inline]
check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]
watchdog+0x90c/0xd60 kernel/hung_task.c:249
kthread+0x33c/0x400 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:429
Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0 skipped: idling at native_safe_halt+0x6/0x10
arch/x86/include/asm/irqflags.h:54


---
This bug is generated by a dumb bot. It may contain errors.
See https://goo.gl/tpsmEJ for details.
Direct all questions to syzk...@googlegroups.com.

syzbot will keep track of this bug report.
If you forgot to add the Reported-by tag, once the fix for this bug is
merged
into any tree, please reply to this email with:
#syz fix: exact-commit-title
To mark this as a duplicate of another syzbot report, please reply with:
#syz dup: exact-subject-of-another-report
If it's a one-off invalid bug report, please reply with:
#syz invalid
Note: if the crash happens again, it will cause creation of a new bug
report.
Note: all commands must start from beginning of the line in the email body.

[Dmitry Vyukov]

On Wed, Feb 7, 2018 at 8:46 AM, syzbot
<syzbot+2ab52b...@syzkaller.appspotmail.com> wrote:
> Hello,
>
> syzbot hit the following crash on upstream commit
> e237f98a9c134c3d600353f21e07db915516875b (Mon Feb 5 21:35:56 2018 +0000)
> Merge tag 'xfs-4.16-merge-5' of
> git://git.kernel.org/pub/scm/fs/xfs/xfs-linux
>
> So far this crash happened 2 times on upstream.
> Unfortunately, I don't have any reproducer for this crash yet.
> Raw console output is attached.
> compiler: gcc (GCC) 7.1.1 20170620
> .config is attached.
> user-space arch: i386

Looks similar to this one:

#syz dup: INFO: task hung in lo_ioctl

> --
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> "syzkaller-bugs" group.
> To unsubscribe from this group and stop receiving emails from it, send an
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> https://groups.google.com/d/msgid/syzkaller-bugs/001a113f64ee97b27105649a7cb2%40google.com.
> For more options, visit https://groups.google.com/d/optout.

[Tetsuo Handa]

A patch for this specific report is ready. I don't know whether other
"dup" reports will be solved by this patch. Thus, I "undup" this report.

#syz undup

From eed54c6ae475860a9c63b37b58f34735e792eef7 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
Date: Sat, 5 May 2018 12:59:12 +0900
Subject: [PATCH] block/loop: Add recursion check for LOOP_CHANGE_FD request.

syzbot is reporting hung tasks at blk_freeze_queue() [1]. This is
due to ioctl(loop_fd, LOOP_CHANGE_FD, loop_fd) request which should be
rejected. Fix this by adding same recursion check which is used by
LOOP_SET_FD request.

Signed-off-by: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+2ab52b...@syzkaller.appspotmail.com>
Cc: Jens Axboe <ax...@kernel.dk>
---
drivers/block/loop.c | 59 ++++++++++++++++++++++++++++++++--------------------
1 file changed, 37 insertions(+), 22 deletions(-)

diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 5d4e316..cee3c01 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -644,6 +644,34 @@ static void loop_reread_partitions(struct loop_device *lo,
__func__, lo->lo_number, lo->lo_file_name, rc);
}

+static inline int is_loop_device(struct file *file)
+{
+ struct inode *i = file->f_mapping->host;
+
+ return i && S_ISBLK(i->i_mode) && MAJOR(i->i_rdev) == LOOP_MAJOR;
+}
+
+static int check_loop_recursion(struct file *f, struct block_device *bdev)
+{
+ /*
+ * FIXME: Traversing on other loop devices without corresponding
+ * lo_ctl_mutex is not safe. l->lo_state can become Lo_rundown and
+ * l->lo_backing_file can become NULL when raced with LOOP_CLR_FD.
+ */
+ while (is_loop_device(f)) {
+ struct loop_device *l;
+
+ if (f->f_mapping->host->i_bdev == bdev)
+ return -EBUSY;
+
+ l = f->f_mapping->host->i_bdev->bd_disk->private_data;
+ if (l->lo_state == Lo_unbound)
+ return -EINVAL;
+ f = l->lo_backing_file;
+ }
+ return 0;
+}
+
/*
* loop_change_fd switched the backing store of a loopback device to
* a new file. This is useful for operating system installers to free up
@@ -673,6 +701,11 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
if (!file)
goto out;

+ /* Avoid recursion */
+ error = check_loop_recursion(file, bdev);
+ if (error)
+ goto out_putf;
+
inode = file->f_mapping->host;
old_file = lo->lo_backing_file;

@@ -706,13 +739,6 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
return error;
}

-static inline int is_loop_device(struct file *file)
-{
- struct inode *i = file->f_mapping->host;
-
- return i && S_ISBLK(i->i_mode) && MAJOR(i->i_rdev) == LOOP_MAJOR;
-}
-
/* loop sysfs attributes */

static ssize_t loop_attr_show(struct device *dev, char *page,
@@ -877,7 +903,7 @@ static int loop_prepare_queue(struct loop_device *lo)
static int loop_set_fd(struct loop_device *lo, fmode_t mode,
struct block_device *bdev, unsigned int arg)
{
- struct file *file, *f;
+ struct file *file;
struct inode *inode;
struct address_space *mapping;
int lo_flags = 0;
@@ -897,20 +923,9 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
goto out_putf;

/* Avoid recursion */
- f = file;
- while (is_loop_device(f)) {
- struct loop_device *l;
-
- if (f->f_mapping->host->i_bdev == bdev)
- goto out_putf;
-
- l = f->f_mapping->host->i_bdev->bd_disk->private_data;
- if (l->lo_state == Lo_unbound) {
- error = -EINVAL;
- goto out_putf;
- }
- f = l->lo_backing_file;
- }
+ error = check_loop_recursion(file, bdev);
+ if (error)
+ goto out_putf;

mapping = file->f_mapping;
inode = mapping->host;
--
1.8.3.1

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit: cdface520934 Merge tag 'for_linus_stable' of git://git.ker..
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=136c8ee7800000
kernel config: https://syzkaller.appspot.com/x/.config?x=61c12b53c2a25ec4
dashboard link: https://syzkaller.appspot.com/bug?extid=2ab52b8d94df5e2eaa01
compiler: gcc (GCC) 8.0.1 20180413 (experimental)
syzkaller repro:https://syzkaller.appspot.com/x/repro.syz?x=15afa247800000
C reproducer: https://syzkaller.appspot.com/x/repro.c?x=16f07717800000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+2ab52b...@syzkaller.appspotmail.com

INFO: task syz-executor148:4500 blocked for more than 120 seconds.
Not tainted 4.17.0-rc2+ #23

"echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.

syz-executor148 D16648 4500 4481 0x00000000
Call Trace:
context_switch kernel/sched/core.c:2848 [inline]
__schedule+0x801/0x1e30 kernel/sched/core.c:3490
schedule+0xef/0x430 kernel/sched/core.c:3549
blk_mq_freeze_queue_wait+0x1ce/0x460 block/blk-mq.c:136

blk_freeze_queue+0x4a/0x80 block/blk-mq.c:165
blk_mq_freeze_queue+0x15/0x20 block/blk-mq.c:174

loop_clr_fd+0x226/0xb80 drivers/block/loop.c:1047
lo_ioctl+0x642/0x2130 drivers/block/loop.c:1404
__blkdev_driver_ioctl block/ioctl.c:303 [inline]
blkdev_ioctl+0x9b6/0x2020 block/ioctl.c:601
block_ioctl+0xee/0x130 fs/block_dev.c:1877
vfs_ioctl fs/ioctl.c:46 [inline]
file_ioctl fs/ioctl.c:500 [inline]
do_vfs_ioctl+0x1cf/0x16a0 fs/ioctl.c:684
ksys_ioctl+0xa9/0xd0 fs/ioctl.c:701
__do_sys_ioctl fs/ioctl.c:708 [inline]
__se_sys_ioctl fs/ioctl.c:706 [inline]
__x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:706
do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x449789
RSP: 002b:00007f210fae5da8 EFLAGS: 00000297 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 00000000006dac3c RCX: 0000000000449789
RDX: 0000000000449789 RSI: 0000000000004c01 RDI: 0000000000000003

RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000297 R12: 00000000006dac38
R13: 0030656c69662f2e R14: 6f6f6c2f7665642f R15: 0000000000000007

Showing all locks held in the system:

2 locks held by khungtaskd/893:
#0: 0000000045f40930 (rcu_read_lock){....}, at:
check_hung_uninterruptible_tasks kernel/hung_task.c:175 [inline]
#0: 0000000045f40930 (rcu_read_lock){....}, at: watchdog+0x1ff/0xf60
kernel/hung_task.c:249
#1: 0000000081898718 (tasklist_lock){.+.+}, at:
debug_show_all_locks+0xde/0x34a kernel/locking/lockdep.c:4470
1 lock held by rsyslogd/4362:
#0: 000000002e322c73 (&f->f_pos_lock){+.+.}, at: __run_timers+0x16e/0xc50
kernel/time/timer.c:1658
2 locks held by getty/4452:
#0: 000000003abe4bd2 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 0000000035e35fb8 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4453:
#0: 000000004e78faf9 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 0000000044d079f2 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4454:
#0: 0000000037bf7fca (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000fc65c2e0 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4455:
#0: 00000000650b41ff (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 00000000f8a69a89 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4456:
#0: 0000000033547e18 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000000c85318d (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4457:
#0: 00000000e5cb3908 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000009fc1aed4 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
2 locks held by getty/4458:
#0: 0000000055360c24 (&tty->ldisc_sem){++++}, at:
ldsem_down_read+0x37/0x40 drivers/tty/tty_ldsem.c:365
#1: 000000002bcd4fa8 (&ldata->atomic_read_lock){+.+.}, at:
n_tty_read+0x321/0x1cc0 drivers/tty/n_tty.c:2131
1 lock held by syz-executor148/4486:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/4500:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/4514:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/4515:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/4517:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/5006:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/5008:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/5009:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/5011:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/5012:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/5014:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391
1 lock held by syz-executor148/4483:
#0: 0000000004c28cff (&rq->lock){-.-.}, at: rq_lock
kernel/sched/sched.h:1799 [inline]
#0: 0000000004c28cff (&rq->lock){-.-.}, at: __schedule+0x246/0x1e30
kernel/sched/core.c:3428
1 lock held by blkid/5016:
#0: 00000000bf14345a (&lo->lo_ctl_mutex/1){+.+.}, at: lo_ioctl+0x8d/0x2130
drivers/block/loop.c:1391

=============================================

NMI backtrace for cpu 1

CPU: 1 PID: 893 Comm: khungtaskd Not tainted 4.17.0-rc2+ #23

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+0x1b9/0x294 lib/dump_stack.c:113
nmi_cpu_backtrace.cold.4+0x19/0xce lib/nmi_backtrace.c:103
nmi_trigger_cpumask_backtrace+0x151/0x192 lib/nmi_backtrace.c:62

arch_trigger_cpumask_backtrace+0x14/0x20 arch/x86/kernel/apic/hw_nmi.c:38
trigger_all_cpu_backtrace include/linux/nmi.h:138 [inline]
check_hung_task kernel/hung_task.c:132 [inline]
check_hung_uninterruptible_tasks kernel/hung_task.c:190 [inline]

watchdog+0xc10/0xf60 kernel/hung_task.c:249
kthread+0x345/0x410 kernel/kthread.c:238
ret_from_fork+0x3a/0x50 arch/x86/entry/entry_64.S:412

Sending NMI from CPU 1 to CPUs 0:
NMI backtrace for cpu 0

CPU: 0 PID: 4495 Comm: syz-executor148 Not tainted 4.17.0-rc2+ #23

Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
Google 01/01/2011

RIP: 0010:debug_lockdep_rcu_enabled.part.1+0x3b/0x60 kernel/rcu/update.c:301
RSP: 0018:ffff8801d3ccefe0 EFLAGS: 00000046
RAX: 0000000000000007 RBX: ffff8801b142c200 RCX: 0000000000000000
RDX: 0000000000000000 RSI: 00000000ffffffff RDI: ffff8801b142ca34
RBP: ffff8801d3ccefe8 R08: ffffed003b5c46c3 R09: ffffed003b5c46c2
R10: ffffed003b5c46c2 R11: ffff8801dae23613 R12: 1ffff1003a799e26
R13: ffff8801d3ccf170 R14: 0000000000000000 R15: ffff8801b142c200
FS: 00007f210fb07700(0000) GS:ffff8801dae00000(0000) knlGS:0000000000000000
CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
CR2: 00007f16b38df9ce CR3: 00000001b19db000 CR4: 00000000001406f0
DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
Call Trace:
debug_lockdep_rcu_enabled+0x77/0x90 kernel/rcu/update.c:300
trace_rcu_utilization include/trace/events/rcu.h:21 [inline]
rcu_note_context_switch+0x554/0x710 kernel/rcu/tree.c:456
__schedule+0x23e/0x1e30 kernel/sched/core.c:3418
schedule+0xef/0x430 kernel/sched/core.c:3549
freezable_schedule include/linux/freezer.h:172 [inline]
futex_wait_queue_me+0x3e8/0x820 kernel/futex.c:2530
futex_wait+0x450/0x9f0 kernel/futex.c:2645
do_futex+0x336/0x27d0 kernel/futex.c:3527
__do_sys_futex kernel/futex.c:3587 [inline]
__se_sys_futex kernel/futex.c:3555 [inline]
__x64_sys_futex+0x46a/0x680 kernel/futex.c:3555
do_syscall_64+0x1b1/0x800 arch/x86/entry/common.c:287
entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x449789
RSP: 002b:00007f210fb06da8 EFLAGS: 00000246 ORIG_RAX: 00000000000000ca
RAX: ffffffffffffffda RBX: 00000000006dac24 RCX: 0000000000449789
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 00000000006dac24

RBP: 0000000000000000 R08: 0000000000000000 R09: 0000000000000000

R10: 0000000000000000 R11: 0000000000000246 R12: 00000000006dac20
R13: 0030656c69662f2e R14: 6f6f6c2f7665642f R15: 0000000000000007
Code: 48 8b 1c 25 c0 ed 01 00 48 8d bb 34 08 00 00 48 89 fa 48 c1 ea 03 0f
b6 14 02 48 89 f8 83 e0 07 83 c0 03 38 d0 7c 04 84 d2 75 10 <8b> 93 34 08
00 00 31 c0 5b 5d 85 d2 0f 94 c0 c3 e8 70 ed 51 00

[Tetsuo Handa]

Oops, I forgot to add the link. Reposting.

From eed54c6ae475860a9c63b37b58f34735e792eef7 Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin...@I-love.SAKURA.ne.jp>
Date: Sat, 5 May 2018 12:59:12 +0900
Subject: [PATCH] block/loop: Add recursion check for LOOP_CHANGE_FD request.

syzbot is reporting hung tasks at blk_freeze_queue() [1]. This is
due to ioctl(loop_fd, LOOP_CHANGE_FD, loop_fd) request which should be
rejected. Fix this by adding same recursion check which is used by
LOOP_SET_FD request.

[1] https://syzkaller.appspot.com/bug?id=078805a692853552e08154b1bcd2bc2c729eda88

[Tetsuo Handa]

OK. Patch is in ext4.git#loop-fix as 170785a9cc72e8e1.

#syz fix: loop: add recursion validation to LOOP_CHANGE_FD