linux-kernel - Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

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Date:   Wed, 10 Jun 2020 23:34:21 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     "David P. Reed" <dpreed@...pplum.com>, dpreed@...pplum.com
Cc:     Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        Allison Randal <allison@...utok.net>,
        Enrico Weigelt <info@...ux.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kate Stewart <kstewart@...uxfoundation.org>,
        "Peter Zijlstra \(Intel\)" <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Martin Molnar <martin.molnar.programming@...il.com>,
        Andy Lutomirski <luto@...nel.org>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        Jann Horn <jannh@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] Fix undefined operation VMXOFF during reboot and crash

"David P. Reed" <dpreed@...pplum.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