Re: kvm: WARNING in x86_emulate_insn

[Dmitry Vyukov]

On Fri, Jan 13, 2017 at 6:47 PM, Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx> wrote:
> 2017-01-12 14:55+0100, Dmitry Vyukov:
>> Hello,
>>
>> I've got the following WARNING in x86_emulate_insn while running
>> syzkaller fuzzer:
>>
>> WARNING: CPU: 2 PID: 18646 at arch/x86/kvm/emulate.c:5558
>> x86_emulate_insn+0x16a5/0x4090 arch/x86/kvm/emulate.c:5572
>> Modules linked in:
>> CPU: 2 PID: 18646 Comm: syz-executor Not tainted 4.10.0-rc3+ #155
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>>  __dump_stack lib/dump_stack.c:15 [inline]
>>  dump_stack+0x292/0x3a2 lib/dump_stack.c:51
>>  __warn+0x19f/0x1e0 kernel/panic.c:547
>>  warn_slowpath_null+0x2c/0x40 kernel/panic.c:582
>>  x86_emulate_insn+0x16a5/0x4090 arch/x86/kvm/emulate.c:5572
>>  x86_emulate_instruction+0x403/0x1cc0 arch/x86/kvm/x86.c:5618
>>  emulate_instruction arch/x86/include/asm/kvm_host.h:1127 [inline]
>>  handle_exception+0x594/0xfd0 arch/x86/kvm/vmx.c:5762
>>  vmx_handle_exit+0x2b7/0x38b0 arch/x86/kvm/vmx.c:8625
>>  vcpu_enter_guest arch/x86/kvm/x86.c:6888 [inline]
>>  vcpu_run arch/x86/kvm/x86.c:6947 [inline]
>>  kvm_arch_vcpu_ioctl_run+0xf3d/0x4660 arch/x86/kvm/x86.c:7105
>>  kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
>>  vfs_ioctl fs/ioctl.c:43 [inline]
>>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
>>  SYSC_ioctl fs/ioctl.c:698 [inline]
>>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>>  entry_SYSCALL_64_fastpath+0x1f/0xc2
>> RIP: 0033:0x445329
>> RSP: 002b:00007f9e6e22fb58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
>> RAX: ffffffffffffffda RBX: 0000000000000018 RCX: 0000000000445329
>> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000018
>> RBP: 00000000006deb40 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000700150
>> R13: 0000000000000000 R14: 00007f9e6e2309c0 R15: 00007f9e6e230700
>> ---[ end trace 6b54f749506b620c ]---
>> ------------[ cut here ]------------
>> WARNING: CPU: 2 PID: 18646 at arch/x86/kvm/x86.c:366
>> exception_type+0x73/0x80 arch/x86/kvm/x86.c:366
>> Modules linked in:
>> CPU: 2 PID: 18646 Comm: syz-executor Tainted: G        W       4.10.0-rc3+ #155
>> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
>> Call Trace:
>>  __dump_stack lib/dump_stack.c:15 [inline]
>>  dump_stack+0x292/0x3a2 lib/dump_stack.c:51
>>  __warn+0x19f/0x1e0 kernel/panic.c:547
>>  warn_slowpath_null+0x2c/0x40 kernel/panic.c:582
>>  exception_type+0x73/0x80 arch/x86/kvm/x86.c:366
>>  x86_emulate_instruction+0x1356/0x1cc0 arch/x86/kvm/x86.c:5664
>>  emulate_instruction arch/x86/include/asm/kvm_host.h:1127 [inline]
>>  handle_exception+0x594/0xfd0 arch/x86/kvm/vmx.c:5762
>>  vmx_handle_exit+0x2b7/0x38b0 arch/x86/kvm/vmx.c:8625
>>  vcpu_enter_guest arch/x86/kvm/x86.c:6888 [inline]
>>  vcpu_run arch/x86/kvm/x86.c:6947 [inline]
>>  kvm_arch_vcpu_ioctl_run+0xf3d/0x4660 arch/x86/kvm/x86.c:7105
>>  kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
>>  vfs_ioctl fs/ioctl.c:43 [inline]
>>  do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
>>  SYSC_ioctl fs/ioctl.c:698 [inline]
>>  SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
>>  entry_SYSCALL_64_fastpath+0x1f/0xc2
>> RIP: 0033:0x445329
>> RSP: 002b:00007f9e6e22fb58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
>> RAX: ffffffffffffffda RBX: 0000000000000018 RCX: 0000000000445329
>> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000018
>> RBP: 00000000006deb40 R08: 0000000000000000 R09: 0000000000000000
>> R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000700150
>> R13: 0000000000000000 R14: 00007f9e6e2309c0 R15: 00007f9e6e230700
>> ---[ end trace 6b54f749506b620d ]---
>>
>> On commit ba836a6f5ab1243ff5e08a941a2d1de8b31244e1.
>>
>> Unfortunately I can't reproduce it with a C program.
>> It reproduces with the following syzkaller program within a minute, though:
>> https://gist.githubusercontent.com/dvyukov/d09118fb9d986a9385487d80a1b50680/raw/884c68d22c3a80778ae596a6c5daf7467ea41b68/gistfile1.txt
>> It can be executed following these instructions:
>> https://github.com/google/syzkaller/wiki/How-to-execute-syzkaller-programs
>> I run syz-execprog as:
>> ./syz-execprog -repeat=0 -procs=8 -sandbox=none gistfile1.txt
>>
>> Note that syz_kvm_setup_cpu is a pseudo syscall that setups vcpu into
>> a complex state:
>> https://github.com/google/syzkaller/blob/master/executor/common_kvm_amd64.h#L271
>>
>> My bet would be on some race where VM memory is overwritten
>> concurrently, and it affects either guest execution or
>> emulate_instruction in a bad way...
>
> Yeah, all functions that return X86EMUL_PROPAGATE_FAULT seem to set
> exception.vector to something sane.  The only easy way to get a bad value there
> is when x86_emulate_instruction() clears it to -1U, but I don't see how a race
> would play out.
>
> Anyway, I can't reproduce on bare metal [got another warning, see below].
> Will try after rebuilding a guest kernel.


I've dumped exception before the warning and they all look the same:

[  211.608578] vector=255 error_code_valid=0 error_code=0
nested_page_fault=0 address=          (null)
[  211.609793] ------------[ cut here ]------------
[  211.612438] WARNING: CPU: 3 PID: 1876 at
arch/x86/kvm/emulate.c:5611 x86_emulate_insn+0x17bf/0x4240

Looks like no parts of it were written after initialization.

I've also added just in case:

 static int emulate_exception(struct x86_emulate_ctxt *ctxt, int vec,
                             u32 error, bool valid)
 {
-       WARN_ON(vec > 0x1f);
+       WARN_ON(vec > 0x1f || vec < 0);

And it did _not_ fire.