kernel panic: stack is corrupted in mpol_to_str

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    e2cf67f6 Merge tag 'zonefs-5.6-rc7' of git://git.kernel.or..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1217e987e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=90791b5aec4dc636
dashboard link: https://syzkaller.appspot.com/bug?extid=b636fe4123f39f69c35b
compiler:       clang version 10.0.0 (https://github.com/llvm/llvm-project/ c2443155a0fb245c8f17f2c1c72b6ea391e86e81)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=16773d8de00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1309b523e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+b636fe4123f39f69c35b@xxxxxxxxxxxxxxxxxxxxxxxxx

Kernel panic - not syncing: stack-protector: Kernel stack is corrupted in: mpol_to_str+0x3ec/0x3f0 mm/mempolicy.c:2962
CPU: 1 PID: 1 Comm: systemd Not tainted 5.6.0-rc7-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+0x1e9/0x30e lib/dump_stack.c:118
 panic+0x264/0x7a0 kernel/panic.c:221
 __stack_chk_fail+0x1c/0x20 kernel/panic.c:667
 mpol_to_str+0x3ec/0x3f0 mm/mempolicy.c:2962
 shmem_show_mpol mm/shmem.c:1406 [inline]
 shmem_show_options+0x3d9/0x4c0 mm/shmem.c:3609
 show_mountinfo+0x68d/0x790 fs/proc_namespace.c:187
 seq_read+0x9df/0xd20 fs/seq_file.c:268
 __vfs_read+0xa7/0x6f0 fs/read_write.c:425
 vfs_read+0x1c0/0x3f0 fs/read_write.c:461
 ksys_read+0x115/0x220 fs/read_write.c:587
 do_syscall_64+0xf3/0x1b0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7f83e3f3f92d
Code: 2d 2c 00 00 75 10 b8 00 00 00 00 0f 05 48 3d 01 f0 ff ff 73 31 c3 48 83 ec 08 e8 de 9b 01 00 48 89 04 24 b8 00 00 00 00 0f 05 <48> 8b 3c 24 48 89 c2 e8 27 9c 01 00 48 89 d0 48 83 c4 08 48 3d 01
RSP: 002b:00007ffe6d4b18c0 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 000055840db5f6d0 RCX: 00007f83e3f3f92d
RDX: 0000000000000400 RSI: 000055840db5f900 RDI: 000000000000002b
RBP: 0000000000000d68 R08: 00007f83e5975500 R09: 00000000000000e0
R10: 000055840db5fce2 R11: 0000000000000293 R12: 00007f83e41fa440
R13: 00007f83e41f9900 R14: 000000000000001e R15: 0000000000000000
Kernel Offset: disabled
Rebooting in 86400 seconds..


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