Re: general protection fault in finish_task_switch

[Linus Torvalds <torvalds@linux-foundation.org>]

On Wed, Dec 20, 2017 at 8:03 AM, syzbot
<bot+72c44cd8b0e8a1a64b9c03c4396aea93a16465ef@syzkaller.appspotmail.com>
wrote:
> Hello,
>
> syzkaller hit the following crash on
> 7dc9f647127d6955ffacaf51cb6a627b31dceec2
> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master
>
> kasan: CONFIG_KASAN_INLINE enabled
> kasan: GPF could be caused by NULL-ptr deref or user memory access
> general protection fault: 0000 [#1] SMP KASAN
> Dumping ftrace buffer:
>    (ftrace buffer empty)
> Modules linked in:
> CPU: 0 PID: 4227 Comm: syzkaller244813 Not tainted 4.15.0-rc4-next-20171220+
> #77
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS
> Google 01/01/2011
> RIP: __fire_sched_in_preempt_notifiers kernel/sched/core.c:2534 [inline]

That line 2534 is the call inside the hlist_for_each_entry() loop:

        hlist_for_each_entry(notifier, &curr->preempt_notifiers, link)
                notifier->ops->sched_in(notifier, raw_smp_processor_id());

and the Code: line disassembly is

   0: ff 11                callq  *(%rcx)
   2: 4c 89 f9              mov    %r15,%rcx
   5: 48 c1 e9 03          shr    $0x3,%rcx
   9: 42 80 3c 31 00        cmpb   $0x0,(%rcx,%r14,1)
   e: 0f 85 1b 02 00 00    jne    0x22f
  14: 4d 8b 3f              mov    (%r15),%r15
  17: 4d 85 ff              test   %r15,%r15
  1a: 0f 84 c0 fd ff ff    je     0xfffffffffffffde0
  20: 49 8d 7f 10          lea    0x10(%r15),%rdi
  24: 48 89 f9              mov    %rdi,%rcx
  27: 48 c1 e9 03          shr    $0x3,%rcx
  2b:* 42 80 3c 31 00        cmpb   $0x0,(%rcx,%r14,1) <-- trapping instruction
  30: 74 ae                je     0xffffffffffffffe0
  32: e8 a7 cc 5b 00        callq  0x5bccde
  37: eb a7                jmp    0xffffffffffffffe0
  39: 4c 89 fe              mov    %r15,%rsi
  3c: 4c 89 e7              mov    %r12,%rdi

and while the "callq *(%rcx)" might be just the end part of some
previous instruction, I think it may be right (there is indeed an
indirect call in that function - that very "->sched_in()" call).

So I think the oops happens after the indirect call returns.

I think the second "callq" is

    call    __asan_report_load8_noabort

and the actual trapping instruction is loading the KASAN byte state.

As far as I can tell, the kasan check is trying to check this part of
hlist_for_each_entry():

    movq    (%r15), %r15    # notifier_110->link.next,

and %r15 is dead000000000100, which is LIST_POISON1.

End result: KASAN actually makes these things harder to debug, because
it's trying to "validate" the list poison values before they are used,
and takes a much more complex and indirect fault in the process,
instead of just getting a page-fault on the LIST_POISON1 that would
have made it more obvious.

Oh well.

There is nothing in this that indicates that it's actually related to
KASAN, and it _should_ oops even without KASAN enabled.

But the reproducer does nothing for me. Of course, I didn't actually
run it on linux-next at all, so it is quite possibly related to
scheduler work (or the TLB/pagetable work) that just hasn't hit
mainstream yet.

None of the scheduler people seem to have been on the report, though.
Adding some in.

                Linus