Messages in this thread | | | From | Thomas Gleixner <> | Subject | Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash | Date | Wed, 10 Jun 2020 23:34:21 +0200 |
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"David P. Reed" <dpreed@deepplum.com> writes: > +/* > + * Fix any unwanted undefined operation fault due to VMXOFF instruction that > + * is needed to ensure that CPU is not in VMX root operation at time of > + * a reboot/panic CPU reset. There is no safe and reliable way to know > + * if a processor is in VMX root operation, other than to skip the > + * VMXOFF. It is safe to just skip any VMXOFF that might generate this > + * exception, when VMX operation is enabled in CR4. In the extremely > + * rare case that a VMXOFF is erroneously executed while VMX is enabled, > + * but VMXON has not been executed yet, the undefined opcode fault > + * should not be missed by valid code, though it would be an error. > + * To detect this, we could somehow restrict the instruction address > + * to the specific use during reboot/panic. > + */ > +static int fixup_emergency_vmxoff(struct pt_regs *regs, int trapnr) > +{ > + const static u8 insn_vmxoff[3] = { 0x0f, 0x01, 0xc4 }; > + u8 ud[3]; > + > + if (trapnr != X86_TRAP_UD) > + return 0; > + if (!cpu_vmx_enabled()) > + return 0; > + if (!this_cpu_read(doing_emergency_vmxoff)) > + return 0; > + > + /* undefined instruction must be in kernel and be VMXOFF */ > + if (regs->ip < TASK_SIZE_MAX) > + return 0; > + if (probe_kernel_address((u8 *)regs->ip, ud)) > + return 0; > + if (memcmp(ud, insn_vmxoff, sizeof(insn_vmxoff))) > + return 0; > + > + regs->ip += sizeof(insn_vmxoff); > + return 1;
We have exception fixups to avoid exactly that kind of horrible workarounds all over the place.
static inline int cpu_vmxoff_safe(void) { int err; asm volatile("2: vmxoff; xor %[err],%[err]\n" "1:\n\t" ".section .fixup,\"ax\"\n\t" "3: mov %[fault],%[err] ; jmp 1b\n\t" ".previous\n\t" _ASM_EXTABLE(2b, 3b) : [err] "=a" (err) : [fault] "i" (-EFAULT) : "memory"); return err; }
static inline void __cpu_emergency_vmxoff(void) { if (!cpu_vmx_enabled()) return; if (!cpu_vmxoff_safe()) cr4_clear_bits(X86_CR4_VMXE); }
Problem solved.
Thanks,
tglx
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