KASAN: stack-out-of-bounds Write 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=11c4bd39e00000
kernel config:  https://syzkaller.appspot.com/x/.config?x=2b05d5a033f5be50
dashboard link: https://syzkaller.appspot.com/bug?extid=b055b1a6b2b958707a21
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)
syz repro:      https://syzkaller.appspot.com/x/repro.syz?x=13252bf9e00000
C reproducer:   https://syzkaller.appspot.com/x/repro.c?x=1632c813e00000

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+b055b1a6b2b958707a21@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: stack-out-of-bounds in set_bit include/asm-generic/bitops/instrumented-atomic.h:28 [inline]
BUG: KASAN: stack-out-of-bounds in __node_set include/linux/nodemask.h:130 [inline]
BUG: KASAN: stack-out-of-bounds in mpol_to_str+0x377/0x3be mm/mempolicy.c:2962
Write of size 8 at addr ffffc90000c7fb60 by task systemd/1

CPU: 0 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+0x188/0x20d lib/dump_stack.c:118
 print_address_description.constprop.0.cold+0x5/0x315 mm/kasan/report.c:374
 __kasan_report.cold+0x1a/0x32 mm/kasan/report.c:506
 kasan_report+0xe/0x20 mm/kasan/common.c:641
 check_memory_region_inline mm/kasan/generic.c:185 [inline]
 check_memory_region+0x128/0x190 mm/kasan/generic.c:192
 set_bit include/asm-generic/bitops/instrumented-atomic.h:28 [inline]
 __node_set include/linux/nodemask.h:130 [inline]
 mpol_to_str+0x377/0x3be mm/mempolicy.c:2962
 shmem_show_mpol mm/shmem.c:1406 [inline]
 shmem_show_options+0x418/0x630 mm/shmem.c:3609
 show_mountinfo+0x616/0x900 fs/proc_namespace.c:187
 seq_read+0xad0/0x1160 fs/seq_file.c:268
 __vfs_read+0x76/0x100 fs/read_write.c:425
 vfs_read+0x1ea/0x430 fs/read_write.c:461
 ksys_read+0x127/0x250 fs/read_write.c:587
 do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x7f17892db92d
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:00007ffc058b86b0 EFLAGS: 00000293 ORIG_RAX: 0000000000000000
RAX: ffffffffffffffda RBX: 0000556657fec630 RCX: 00007f17892db92d
RDX: 0000000000000400 RSI: 0000556657fec860 RDI: 000000000000002c
RBP: 0000000000000d68 R08: 00007f178ad11500 R09: 00000000000000e0
R10: 0000556657fecc47 R11: 0000000000000293 R12: 00007f1789596440
R13: 00007f1789595900 R14: 0000000000000019 R15: 0000000000000000


addr ffffc90000c7fb60 is located in stack of task systemd/1 at offset 40 in frame:
 mpol_to_str+0x0/0x3be mm/mempolicy.c:2924

this frame has 1 object:
 [32, 40) 'nodes'

Memory state around the buggy address:
 ffffc90000c7fa00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
 ffffc90000c7fa80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffffc90000c7fb00: 00 00 00 00 00 00 00 f1 f1 f1 f1 00 f3 f3 f3 00
                                                       ^
 ffffc90000c7fb80: 00 00 00 00 00 00 00 00 00 00 f1 f1 f1 f1 00 00
 ffffc90000c7fc00: 00 00 00 00 00 00 f3 f3 f3 f3 00 00 00 00 00 00
==================================================================


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