Re: BUG: unable to handle kernel paging request in __switch_to

[Dmitry Vyukov]

On Fri, Dec 15, 2017 at 10:13 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
> On Fri, Dec 15, 2017 at 10:07 AM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote:
>> On Thu, Dec 14, 2017 at 10:39 PM, Linus Torvalds
>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>> On Thu, Dec 14, 2017 at 1:27 PM, Andy Lutomirski <luto@xxxxxxxxxx> wrote:
>>>> On Thu, Dec 14, 2017 at 11:28 AM, Linus Torvalds
>>>> <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>>>>> I don't think that's the case. "int3" is entirely synchronous, and
>>>>> doesn't have the same odd issues as a breakpoint trap (which honors RF
>>>>> etc). It's literally just a one-byte shorthand for "int $3".
>>>>
>>>> The SDM says precisely the same thing about INT N, so, whichever way
>>>> you dice it, int3 is a benign exception.
>>>
>>> That just means that it doesn't double-fault when it takes the page fault.
>>>
>>> Which we already know, because we see a page fault, not a double fault.
>>>
>>>> 0xfffffffffffffff8 is *exactly* where the fault would be if the
>>>> microcoded push of SS faulted if the IST contained zeros.
>>>
>>> Yes, I suspect it's the stack that is buggered for some reason.
>>>
>>>>> Plus I think the instruction that gets overwritten is just a 5-byte
>>>>> nop isn't it? So it really shouldn't take a fault without the "int3"
>>>>> overwriting.
>>>>
>>>> Unless it was being overwritten the other way and the oops hit while
>>>> tracing was being turned *off*.
>>>
>>> Doesn't really matter. The two forms of that instruction are "5-byte
>>> nop" and "unconditional branch".
>>>
>>> Neither of them will write to anything - the only page fault they
>>> could take is for instruction fetch.
>>>
>>> So it really must be the "int3" that fails. Unless we're looking at
>>> some odd CPU errata, which sounds very very unlikely.
>>
>> FTR the commit is:
>>
>> commit d127129e85a020879f334154300ddd3f7ec21c1e (HEAD, tag: next-20171129)
>> Author: Stephen Rothwell <sfr@xxxxxxxxxxxxxxxx>
>> Date:   Wed Nov 29 14:09:56 2017 +1100
>>     Add linux-next specific files for 20171129
>>
>> You can get it from
>> git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next-history.git
>> Compiler is this: https://storage.googleapis.com/syzkaller/gcc-7.tar.gz
>> Config was attached.
>>
>> I've built this exact kernel and here is __switch_to disasm:
>> https://gist.githubusercontent.com/dvyukov/8137559f7da08fbe32f9018972a4498c/raw/0ef2abf723b117f0d0f0306fd50e216d50c5cecb/gistfile1.txt
>>
>> __switch_to+0x95b seems to point to (?):
>>
>> ffffffff81252f6b: 0f 1f 44 00 00        nopl   0x0(%rax,%rax,1)
>>
>> which is branch target alignment nop.
>>
>> We have a bunch of semi-similar non-sense crashes on syzbot:
>>
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/zGz7AVtMBV0/X_-CPbjNAgAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/9nMSJo9jmGs/tkRYgZ-XAwAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/04-q4OZrerA/XfYdNnWXAwAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/6iC6rPtAHKQ/UiZ4fnWXAwAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/2zSDbzRIH_k/SLCMqmeXAwAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/uEsjx8VISco/Mwu_pbGWAwAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/kZ6Z7UQLbCQ/JHpjTGeXAwAJ
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/UjYsJxiGxwU/mponQq2XAwAJ
>>
>> Lots of them are on 0xfffffffffffffff8 address.
>>
>> I have some suspicion towards KVM. Potentially a nested KVM messed
>> host processor state (CRn or page tables) so that then we get these
>> weird crashes.
>>
>> One question: how would triple-fault look like? I am asking because we
>> have hundreds of cases where kernel just starts silently rebooting
>> while running some unprivileged syscalls:
>> https://groups.google.com/forum/#!msg/syzkaller-bugs/w8dkVNrgzrc/4mLJLOAbCgAJ
>> Can these be triple faults? Reproducer for that one also seems to be
>> related to KVM.
>
>
>
> Well, actually replying log for this crash and for
> https://groups.google.com/forum/#!msg/syzkaller-bugs/zGz7AVtMBV0/X_-CPbjNAgAJ
> with:
>
> ./syz-execprog -procs=10 -sandbox=namespace -repeat=0 raw.txt
> (you can find exact instructions on how to do this here
> https://github.com/google/syzkaller/blob/master/docs/executing_syzkaller_programs.md)
>
> I've got:
>
>
> [  121.553588] binder: 3856:3857 ioctl 40046205 0 returned -22
> [  121.557656] binder: 3856:3857 ERROR: BC_REGISTER_LOOPER called
> without request
> [  121.559744] binder: 3857 RLIMIT_NICE not set
> [  121.586339] binder: 3857 RLIMIT_NICE not set
> [  121.591764] binder: 3856:3857 unknown command 1400526783
> [  121.593226] binder: 3856:3857 ioctl c0306201 20002fd0 returned -22
> [  121.598292] binder: 3857 RLIMIT_NICE not set
> [  121.600827] binder: 3856:3857 ioctl c018620b 20000fe8 returned -14
> [  121.618284] binder: 3856:3857 BC_FREE_BUFFER uffffffffffffffff no match
> [  121.622181] binder: 3856:3857 got reply transaction with no transaction stack
> [  121.626345] binder: 3856:3857 transaction failed 29201/-71, size
> 72-56 line 2747
> [  121.628912] binder: 3856:3857 ioctl c0306201 20005fd0 returned -14
> [  121.635620] binder: unexpected work type, 4, not freed
> [  121.639753] binder: undelivered TRANSACTION_COMPLETE
> [  121.645213] binder: undelivered TRANSACTION_ERROR: 29201
> [  121.654860] binder: 3856:3857 BC_FREE_BUFFER u00000000ffffffff no match
> [  121.667216] *** Guest State ***
> [  121.667728] CR0: actual=0x0000000000000030,
> shadow=0x0000000060000010, gh_mask=fffffffffffffff7
> early console in extract_kernel
> input_data: 0x0000000005f13276
> input_len: 0x0000000001e7fa4c
> output: 0x0000000001000000
> output_len: 0x0000000005c85958
> kernel_total_size: 0x0000000006db2000
>
> Decompressing Linux... Parsing ELF... done.
> Booting the kernel.
> [    0.000000] Linux version 4.15.0-rc1-next-20171129
> (dvyukov@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx) (gcc version 7.1.1 20170620
> (GCC)) #1 SMP Fri Dec 15 09:25:01 CET 2017
> [    0.000000] Command line: kvm-intel.nested=1
> kvm-intel.unrestricted_guest=1 kvm-intel.ept=1
> kvm-intel.flexpriority=1 kvm-intel.vpid=1
> kvm-intel.emulate_invalid_guest_state=1 kvm-intel.eptad=1
> kvm-intel.enable_shadow_vmcs=1 kvm-intel.pml=1
> kvm-intel.enable_apicv=1 console=ttyS0 root=/dev/sda
> earlyprintk=serial slub_debug=UZ vsyscall=native rodata=n oops=panic
> panic_on_warn=1 panic=86400
> [    0.000000] x86/fpu: Supporting XSAVE feature 0x001: 'x87 floating
> point registers'
> [    0.000000] x86/fpu: Supporting XSAVE feature 0x002: 'SSE registers'
> [    0.000000] x86/fpu: Supporting XSAVE feature 0x004: 'AVX registers'
> [    0.000000] x86/fpu: xstate_offset[2]:  576, xstate_sizes[2]:  256
> [    0.000000] x86/fpu: Enabled xstate features 0x7, context size is
> 832 bytes, using 'standard' format.
> [    0.000000] e820: BIOS-provided physical RAM map:
> ...


Well, the crash was minimized down to:

// autogenerated by syzkaller (http://github.com/google/syzkaller)
#define _GNU_SOURCE
#include <sys/syscall.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <linux/kvm.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>

int main()
{
  int fd = open("/dev/kvm", 0x80102ul);
  int vm = ioctl(fd, KVM_CREATE_VM, 0);
  int  cpu = ioctl(vm, KVM_CREATE_VCPU, 4);
  ioctl(cpu, KVM_RUN, 0);
  return 0;
}

And, yes, this in fact triggers instant reboot of kernel (running in qemu).
Am I missing something here?

+kvm maintainers, you can see full thread here:
https://groups.google.com/forum/#!topic/syzkaller-bugs/_oveOKGm3jw