kvm: WARNING in x86_emulate_insn

[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...