KASAN: out-of-bounds Read in prepare_exit_to_usermode

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit: c2003765 Merge tag 'io_uring-5.6-2020-03-07' of git://git...
git tree: upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=12d05f29e00000
kernel config: https://syzkaller.appspot.com/x/.config?x=4527d1e2fb19fd5c
dashboard link: https://syzkaller.appspot.com/bug?extid=825d6b18fa37b8742731
compiler: gcc (GCC) 9.0.0 20181231 (experimental)
CC: [b...@alien8.de h...@zytor.com linux-...@vger.kernel.org lu...@kernel.org mi...@redhat.com tg...@linutronix.de x...@kernel.org]

Unfortunately, I don't have any reproducer for this crash yet.

IMPORTANT: if you fix the bug, please add the following tag to the commit:
Reported-by: syzbot+825d6b...@syzkaller.appspotmail.com

==================================================================
BUG: KASAN: out-of-bounds in prepare_exit_to_usermode+0x369/0x3d0 arch/x86/entry/common.c:189
Read of size 4 at addr ffff88804f288c94 by task udevd/20194

CPU: 1 PID: 20194 Comm: udevd Not tainted 5.6.0-rc4-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+0xd3/0x315 mm/kasan/report.c:374
__kasan_report.cold+0x1a/0x32 mm/kasan/report.c:506
kasan_report+0xe/0x20 mm/kasan/common.c:641
prepare_exit_to_usermode+0x369/0x3d0 arch/x86/entry/common.c:189
ret_from_intr+0x26/0x36
RIP: 0033:0x7fd02d1f90a5
Code: 41 5c 41 5d 41 5e c9 c3 48 8b 15 8e 8d 2d 00 f7 d8 41 83 cc ff 64 89 02 0f 1f 44 00 00 64 8b 04 25 d0 02 00 00 41 39 c1 75 7a <64> 44 89 04 25 d4 02 00 00 e8 0d 60 fc ff 48 85 db 74 bb 41 bd ca
RSP: 002b:00007ffec6ec6090 EFLAGS: 00010246
RAX: 0000000000004ee2 RBX: 00007ffec6ec6090 RCX: 00007fd02d1f8f46
RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000001200011
RBP: 00007ffec6ec60f0 R08: 0000000000004ee2 R09: 0000000000004ee2
R10: 00007fd02db15a70 R11: 0000000000000246 R12: 000000000000556c
R13: 00007ffec6ec60b0 R14: 0000000000000005 R15: 0000000000000005

Allocated by task 4208:
save_stack+0x1b/0x80 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
__kasan_kmalloc mm/kasan/common.c:515 [inline]
__kasan_kmalloc.constprop.0+0xbf/0xd0 mm/kasan/common.c:488
slab_post_alloc_hook mm/slab.h:584 [inline]
slab_alloc_node mm/slab.c:3263 [inline]
kmem_cache_alloc_node+0x13c/0x760 mm/slab.c:3575
alloc_task_struct_node kernel/fork.c:169 [inline]
dup_task_struct kernel/fork.c:868 [inline]
copy_process+0x5e7/0x7290 kernel/fork.c:1920
_do_fork+0x12d/0x1010 kernel/fork.c:2430
__do_sys_clone kernel/fork.c:2585 [inline]
__se_sys_clone kernel/fork.c:2566 [inline]
__x64_sys_clone+0x182/0x210 kernel/fork.c:2566
do_syscall_64+0xf6/0x7d0 arch/x86/entry/common.c:294
entry_SYSCALL_64_after_hwframe+0x49/0xbe

Freed by task 16:
save_stack+0x1b/0x80 mm/kasan/common.c:72
set_track mm/kasan/common.c:80 [inline]
kasan_set_free_info mm/kasan/common.c:337 [inline]
__kasan_slab_free+0xf7/0x140 mm/kasan/common.c:476
__cache_free mm/slab.c:3426 [inline]
kmem_cache_free+0x7f/0x320 mm/slab.c:3694
__put_task_struct+0x22d/0x520 kernel/fork.c:751
put_task_struct include/linux/sched/task.h:122 [inline]
delayed_put_task_struct+0x248/0x3b0 kernel/exit.c:182
rcu_do_batch kernel/rcu/tree.c:2186 [inline]
rcu_core+0x5a4/0x12d0 kernel/rcu/tree.c:2410
__do_softirq+0x26c/0x99d kernel/softirq.c:292

The buggy address belongs to the object at ffff88804f288400
which belongs to the cache task_struct of size 6272
The buggy address is located 2196 bytes inside of
6272-byte region [ffff88804f288400, ffff88804f289c80)
The buggy address belongs to the page:
page:ffffea00013ca200 refcount:1 mapcount:0 mapping:ffff88821bc46700 index:0x0 compound_mapcount: 0
flags: 0xfffe0000010200(slab|head)
raw: 00fffe0000010200 ffffea0000ed1508 ffffea0000ea4508 ffff88821bc46700
raw: 0000000000000000 ffff88804f288400 0000000100000001 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
ffff88804f288b80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff88804f288c00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff88804f288c80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
^
ffff88804f288d00: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
ffff88804f288d80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
==================================================================


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[syzbot]

Auto-closing this bug as obsolete.
Crashes did not happen for a while, no reproducer and no activity.