KASAN: out-of-bounds Read in unwind_next_frame

[syzbot]

Hello,

syzbot found the following crash on:

HEAD commit:    b5aef86e089a Merge tag 'docs-5.0-fixes' of git://git.lwn.n..
git tree:       upstream
console output: https://syzkaller.appspot.com/x/log.txt?x=1034c54b400000
kernel config:  https://syzkaller.appspot.com/x/.config?x=d55557b516c45456
dashboard link: https://syzkaller.appspot.com/bug?extid=2fdf0ed2b3bc8fc51fc6
compiler:       gcc (GCC) 9.0.0 20181231 (experimental)

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+2fdf0ed2b3bc8fc51fc6@xxxxxxxxxxxxxxxxxxxxxxxxx

==================================================================
BUG: KASAN: out-of-bounds in unwind_next_frame.part.0+0x756/0xa90  
arch/x86/kernel/unwind_frame.c:324
Read of size 8 at addr ffff88805082fb68 by task syz-executor2/21500

CPU: 0 PID: 21500 Comm: syz-executor2 Not tainted 4.20.0+ #12
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+0x1db/0x2d0 lib/dump_stack.c:113
 print_address_description.cold+0x7c/0x20d mm/kasan/report.c:187
 kasan_report.cold+0x1b/0x40 mm/kasan/report.c:317
 __asan_report_load8_noabort+0x14/0x20 mm/kasan/generic_report.c:135
 unwind_next_frame.part.0+0x756/0xa90 arch/x86/kernel/unwind_frame.c:324
 unwind_next_frame+0x3b/0x50 arch/x86/kernel/unwind_frame.c:287
 __save_stack_trace+0x7a/0xf0 arch/x86/kernel/stacktrace.c:44
 save_stack_trace_tsk arch/x86/kernel/stacktrace.c:76 [inline]
 save_stack_trace_tsk+0x9e/0xd0 arch/x86/kernel/stacktrace.c:69
 proc_pid_stack+0x272/0x430 fs/proc/base.c:438
 proc_single_show+0xf6/0x180 fs/proc/base.c:739
 seq_read+0x4db/0x1130 fs/seq_file.c:229
 do_loop_readv_writev fs/read_write.c:700 [inline]
 do_loop_readv_writev fs/read_write.c:687 [inline]
 do_iter_read+0x4a9/0x660 fs/read_write.c:921
 vfs_readv+0x175/0x1c0 fs/read_write.c:983
 do_preadv+0x1c4/0x280 fs/read_write.c:1067
 __do_sys_preadv fs/read_write.c:1117 [inline]
 __se_sys_preadv fs/read_write.c:1112 [inline]
 __x64_sys_preadv+0x9a/0xf0 fs/read_write.c:1112
 do_syscall_64+0x1a3/0x800 arch/x86/entry/common.c:290
 entry_SYSCALL_64_after_hwframe+0x49/0xbe
RIP: 0033:0x457ec9
Code: 6d b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00 00 66 90 48 89 f8 48 89 f7  
48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff  
ff 0f 83 3b b7 fb ff c3 66 2e 0f 1f 84 00 00 00 00
RSP: 002b:00007f8d6c5e5c78 EFLAGS: 00000246 ORIG_RAX: 0000000000000127
RAX: ffffffffffffffda RBX: 0000000000000004 RCX: 0000000000457ec9
RDX: 00000000000001a1 RSI: 00000000200017c0 RDI: 0000000000000005
RBP: 000000000073bf00 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000246 R12: 00007f8d6c5e66d4
R13: 00000000004c4861 R14: 00000000004d7d20 R15: 00000000ffffffff

The buggy address belongs to the page:
page:ffffea0001420bc0 count:0 mapcount:0 mapping:0000000000000000 index:0x0
flags: 0x1fffc0000000000()
raw: 01fffc0000000000 0000000000000000 ffffffff01420101 0000000000000000
raw: 0000000000000000 0000000000000000 00000000ffffffff 0000000000000000
page dumped because: kasan: bad access detected

Memory state around the buggy address:
 ffff88805082fa00: 00 f2 f2 00 f2 f2 f2 00 f2 f2 f2 00 f2 f2 f2 f8
 ffff88805082fa80: f3 f3 f3 00 00 00 00 00 00 00 00 00 00 00 00 00
> ffff88805082fb00: f1 f1 f1 f1 00 f2 f2 f2 00 f3 f3 f3 00 00 00 00
                                                          ^
 ffff88805082fb80: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 f1 f1
 ffff88805082fc00: f1 f1 00 f2 f2 f2 00 00 f2 f2 04 f3 f3 f3 00 00
==================================================================


---
This bug is generated by a bot. It may contain errors.
See https://goo.gl/tpsmEJ for more information about syzbot.
syzbot engineers can be reached at syzkaller@xxxxxxxxxxxxxxxxx

syzbot will keep track of this bug report. See:
https://goo.gl/tpsmEJ#bug-status-tracking for how to communicate with  
syzbot.