On Tue, Jan 17, 2017 at 12:34 PM, Dmitry Vyukov <dvyukov@xxxxxxxxxx> wrote: > On Fri, Jan 13, 2017 at 6:47 PM, Radim Krčmář <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. It's this guy that passes NULL as exception to emulator_write_emulated: static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt) { struct kvm_vcpu *vcpu = emul_to_vcpu(ctxt); char instruction[3]; unsigned long rip = kvm_rip_read(vcpu); kvm_x86_ops->patch_hypercall(vcpu, instruction); return emulator_write_emulated(ctxt, rip, instruction, 3, NULL); } Mailed fix. There is one more place that probably needs exception handling: handle_vmread /* _system ok, as nested_vmx_check_permission verified cpl=0 */ kvm_write_guest_virt_system(&vcpu->arch.emulate_ctxt, gva, &field_value, (is_long_mode(vcpu) ? 8 : 4), NULL); } nested_vmx_succeed(vcpu); return kvm_skip_emulated_instruction(vcpu); } If the write fails, it pretends that it succeed. SDM says: #PF(fault-code) If a page fault occurs in accessing a memory destination operand. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
