Re: BUG: unable to handle kernel paging request from pty_write [was: Linux 4.4.2]

[Peter Hurley]

On 02/26/2016 12:56 AM, Jiri Slaby wrote:
> On 02/26/2016, 01:38 AM, Linus Torvalds wrote:
>> On Thu, Feb 25, 2016 at 1:32 PM, Jiri Slaby <jslaby@xxxxxxx> wrote:
>>>
>>> Interestingly, RBP contains address inside try_to_wake_up --
>>> ffffffff810a535a (dunno why) which is:
>>> ffffffff810a5355:       e8 66 a0 ff ff          callq  ffffffff8109f3c0
>>> <ttwu_stat>
>>> ffffffff810a535a:       e9 9d fe ff ff          jmpq   ffffffff810a51fc
>>> <try_to_wake_up+0x3c>
>>>
>>> ttwu_stat does in the begginning:
>>> mov    $0x16e80,%r14
>>>
>>> which is what we actually still have in r14 when it crashes. The first
>>> ttwu_stat's "if" has to go through the true branch (otherwise r14 would
>>> be overwritten).
>>
>> Hmm. That does sound very much like it might be ttwu_stat() that has
>> gotten the stack frame wrong, and when finishes exits, it does
>>
>>         popq    %rbp
>>         ret
>>
>> but in fact it popped the return address, and then returned to a crazy address.
>>
>> Which sounds like a corrupted stack pointer (not a corrupted stack).

So more analysis would seem to confirm that RSP has been bumped +8
while in ttwu_stat() so when the epilog executed, register restore
was off by 1 qword. However, there's nothing in ttwu_stat() that
results in stack pointer offset by +1 qword from prolog.

Below I highlighted key instructions from try_to_wake_up() => ttwu_stat() and
what presumably was the resultant stack state at each instruction:


call try_to_wake_up   ffffffff810a5585  \
push rbp              ffff8800bb2a7c90   |
push r15              0000000000010e30   |
push r14              0000000000000005   |
push r13              ffff88017ed2a830   |- values from stack trace
push r12              ffff880234e26a08   |
push rbx              ffff88017ee19f00   |
sub  0x10, rsp        000000008146e197   /
                      ffff88023fd40000   => rip @ crash

call raw_spin_lock_irqsave
mov  rax, r13
mov  0x16e80, r15

mov  1, r12d
call ttwu_stat        ffffffff810a535a   => rbp @ crash
push rbp              ffff8800bb2a7c80   => r15 @ crash
push r15              0000000000016e80   => r14 @ crash
push r14              ffff8800bb37e180   => r13 @ crash
push r13              0000000000000046   => r12 @ crash
push r12              0000000000000001   => rbx @ crash
push rbx                    ???
sub  8, rsp                 ???


So in addition to rbp <= ret addr and r15 <= saved rbp, note also

  rbx <= saved r12 (== 1)
  r12 <= saved r13 (rflags == 00046)
  r14 <= saved r15 (== 0x16e80)

which neatly corresponds to the ttwu_stat() epilog if rsp has
been offset by +1 qword.


Regards,
Peter Hurley