linux-kernel - Re: [patch 1/8] selftests/x86: Test signal frame XSTATE header corruption handling

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Date:   Wed, 2 Jun 2021 14:38:11 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [patch 1/8] selftests/x86: Test signal frame XSTATE header
 corruption handling

On Wed, Jun 02, 2021 at 11:55:44AM +0200, Thomas Gleixner wrote:
> From: Andy Lutomirski <luto@...nel.org>
> 
> This is very heavily based on some code from Thomas Gleixner.  On a system
> without XSAVES, it triggers the WARN_ON():
> 
>     Bad FPU state detected at copy_kernel_to_fpregs+0x2f/0x40, reinitializing FPU registers.

That triggers

[  149.497274] corrupt_xstate_[1627] bad frame in rt_sigreturn frame:00000000dad08ab1 ip:7f031449ffe1 sp:7ffd0c5c59f0 orax:ffffffffffffffff in libpthread-2.31.so[7f0314493000+10000]

on an AMD laptop here.

> +static inline void __cpuid(unsigned int *eax, unsigned int *ebx,
> +			   unsigned int *ecx, unsigned int *edx)
> +{
> +	asm volatile(
> +		"cpuid;"
> +		: "=a" (*eax),
> +		  "=b" (*ebx),
> +		  "=c" (*ecx),
> +		  "=d" (*edx)
> +		: "0" (*eax), "2" (*ecx));
> +}
> +
> +static inline int xsave_enabled(void)
> +{
> +	unsigned int eax, ebx, ecx, edx;
> +
> +	eax = 0x1;
> +	ecx = 0x0;
> +	__cpuid(&eax, &ebx, &ecx, &edx);
> +
> +	/* Is CR4.OSXSAVE enabled ? */
> +	return ecx & (1U << 27);
> +}

One fine day someone should sit down and unify all those auxillary
functions used in the selftests into a lib...

> +
> +static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
> +		       int flags)
> +{
> +	struct sigaction sa;
> +
> +	memset(&sa, 0, sizeof(sa));
> +	sa.sa_sigaction = handler;
> +	sa.sa_flags = SA_SIGINFO | flags;
> +	sigemptyset(&sa.sa_mask);
> +	if (sigaction(sig, &sa, 0))
> +		err(1, "sigaction");
> +}
> +
> +static void sigusr1(int sig, siginfo_t *info, void *uc_void)
> +{
> +	ucontext_t *uc = uc_void;
> +	uint8_t *fpstate = (uint8_t *)uc->uc_mcontext.fpregs;
> +	uint64_t *xfeatures = (uint64_t *)(fpstate + 512);
> +
> +	printf("\tWreckage XSTATE header\n");
> +	/* Wreckage the first reserved byte in the header */
> +	*(xfeatures + 2) = 0xfffffff;
> +}
> +
> +static void sigsegv(int sig, siginfo_t *info, void *uc_void)
> +{
> +	printf("\tGot SIGSEGV\n");
> +}
> +
> +int main()

ERROR: Bad function definition - int main() should probably be int main(void)

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette