Re: WARNING: bad unlock balance in __get_user_pages_remote

[syzbot]

syzbot has found a reproducer for the following crash on:

HEAD commit:    763dede1 Merge tag 'for-linus-5.7-rc1' of git://git.kernel..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=17731b0be00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=12205d036cec317f
dashboard link: https://syzkaller.appspot.com/bug?extid=a8c70b7f3579fc0587dc
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
userspace arch: i386
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=135855cde00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=149ea07de00000

The bug was bisected to:

commit 71335f37c5e8ec9225285206f7f875057b9737ad
Author: Peter Xu <peterx@xxxxxxxxxx>
Date:   Thu Apr 2 04:08:53 2020 +0000

    mm/gup: allow to react to fatal signals

bisection log:  https://syzkaller.appspot.com/x/bisect.txt?x=17dba9b3e00000
final crash:    https://syzkaller.appspot.com/x/report.txt?x=143ba9b3e00000
console output: https://syzkaller.appspot.com/x/log.txt?x=103ba9b3e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+a8c70b7f3579fc0587dc@xxxxxxxxxxxxxxxxxxxxxxxxx
Fixes: 71335f37c5e8 ("mm/gup: allow to react to fatal signals")

=====================================
WARNING: bad unlock balance detected!
5.6.0-syzkaller #0 Not tainted
-------------------------------------
syz-executor151/7052 is trying to release lock (&mm->mmap_sem) at:
[<ffffffff819fbf60>] __get_user_pages_locked mm/gup.c:1366 [inline]
[<ffffffff819fbf60>] __get_user_pages_remote mm/gup.c:1831 [inline]
[<ffffffff819fbf60>] __get_user_pages_remote+0x540/0x740 mm/gup.c:1806
but there are no more locks to release!

other info that might help us debug this:
no locks held by syz-executor151/7052.

stack backtrace:
CPU: 0 PID: 7052 Comm: syz-executor151 Not tainted 5.6.0-syzkaller #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+0x188/0x20d lib/dump_stack.c:118
 __lock_release kernel/locking/lockdep.c:4633 [inline]
 lock_release+0x586/0x800 kernel/locking/lockdep.c:4941
 up_read+0x79/0x750 kernel/locking/rwsem.c:1573
 __get_user_pages_locked mm/gup.c:1366 [inline]
 __get_user_pages_remote mm/gup.c:1831 [inline]
 __get_user_pages_remote+0x540/0x740 mm/gup.c:1806
 pin_user_pages_remote+0x67/0xa0 mm/gup.c:2897
 process_vm_rw_single_vec mm/process_vm_access.c:108 [inline]
 process_vm_rw_core.isra.0+0x423/0x940 mm/process_vm_access.c:218
 compat_process_vm_rw+0x21f/0x240 mm/process_vm_access.c:343
 __do_compat_sys_process_vm_writev mm/process_vm_access.c:370 [inline]
 __se_compat_sys_process_vm_writev mm/process_vm_access.c:363 [inline]
 __ia32_compat_sys_process_vm_writev+0xdc/0x1b0 mm/process_vm_access.c:363
 do_syscall_32_irqs_on arch/x86/entry/common.c:337 [inline]
 do_fast_syscall_32+0x270/0xe90 arch/x86/entry/common.c:396
 entry_SYSENTER_compat+0x70/0x7f arch/x86/entry/entry_64_compat.S:139
------------[ cut here ]------------
DEBUG_RWSEMS_WARN_ON(tmp < 0): count = 0xffffffffffffff00, magic = 0xffff88809e2392f8, owner = 0x3, curr 0xffff88809a6c6240, list empty
WARNING: CPU: 1 PID: 7052 at kernel/locking/rwsem.c:1435 __up_read kernel/locking/rwsem.c:1435 [inline]
WARNING: CPU: 1 PID: 7052 at kernel/locking/rwsem.c:1435 up_read+0x5f9/0x750 kernel/locking/rwsem.c:1574