Re: [patch V3 6/6] selftests/x86: Test signal frame XSTATE header corruption handling

[David Edmondson <dme@dme.org>]

On Tuesday, 2021-06-08 at 16:36:23 +02, Thomas Gleixner wrote:

> From: Andy Lutomirski <luto@kernel.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.
>
> Signed-off-by: Andy Lutomirski <luto@kernel.org>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> V2: main() -> main(void) - Borislav
> ---
>  tools/testing/selftests/x86/Makefile                |    3 
>  tools/testing/selftests/x86/corrupt_xstate_header.c |  114 ++++++++++++++++++++
>  2 files changed, 116 insertions(+), 1 deletion(-)
>  create mode 100644 tools/testing/selftests/x86/corrupt_xstate_header.c
>
> --- a/tools/testing/selftests/x86/Makefile
> +++ b/tools/testing/selftests/x86/Makefile
> @@ -17,7 +17,8 @@ TARGETS_C_BOTHBITS := single_step_syscal
>  TARGETS_C_32BIT_ONLY := entry_from_vm86 test_syscall_vdso unwind_vdso \
>  			test_FCMOV test_FCOMI test_FISTTP \
>  			vdso_restorer
> -TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering
> +TARGETS_C_64BIT_ONLY := fsgsbase sysret_rip syscall_numbering \
> +			corrupt_xstate_header
>  # Some selftests require 32bit support enabled also on 64bit systems
>  TARGETS_C_32BIT_NEEDED := ldt_gdt ptrace_syscall
>  
> --- /dev/null
> +++ b/tools/testing/selftests/x86/corrupt_xstate_header.c
> @@ -0,0 +1,114 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Corrupt the XSTATE header in a signal frame
> + *
> + * Based on analysis and a test case from Thomas Gleixner.
> + */
> +
> +#define _GNU_SOURCE
> +
> +#include <stdlib.h>
> +#include <stdio.h>
> +#include <string.h>
> +#include <sched.h>
> +#include <signal.h>
> +#include <err.h>
> +#include <unistd.h>
> +#include <stdint.h>
> +#include <sys/wait.h>
> +
> +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);
> +}
> +
> +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" is an odd word to use here, as it usually refers to the
objects that are the result of a wreck. Maybe "wreck" or (to match the
test name) "corrupt"?

> +	/* Wreckage the first reserved byte in the header */
> +	*(xfeatures + 2) = 0xfffffff;

This trashes more than a byte, which doesn't seem significant, but it
would be good to have the comment and code match.

> +}
> +
> +static void sigsegv(int sig, siginfo_t *info, void *uc_void)
> +{
> +	printf("\tGot SIGSEGV\n");
> +}
> +
> +int main(void)
> +{
> +	cpu_set_t set;
> +
> +	sethandler(SIGUSR1, sigusr1, 0);
> +	sethandler(SIGSEGV, sigsegv, 0);
> +
> +	if (!xsave_enabled()) {
> +		printf("[SKIP] CR4.OSXSAVE disabled.\n");
> +		return 0;
> +	}
> +
> +	CPU_ZERO(&set);
> +	CPU_SET(0, &set);
> +
> +	/*
> +	 * Enforce that the child runs on the same CPU
> +	 * which in turn forces a schedule.
> +	 */
> +	sched_setaffinity(getpid(), sizeof(set), &set);
> +
> +	printf("[RUN]\tSend ourselves a signal\n");
> +	raise(SIGUSR1);
> +
> +	printf("[OK]\tBack from the signal.  Now schedule.\n");
> +	pid_t child = fork();
> +	if (child < 0)
> +		err(1, "fork");
> +	if (child == 0)
> +		return 0;
> +	if (child)
> +		waitpid(child, NULL, 0);
> +	printf("[OK]\tBack in the main thread.\n");
> +
> +	/*
> +	 * We could try to confirm that extended state is still preserved
> +	 * when we schedule.  For now, the only indication of failure is
> +	 * a warning in the kernel logs.
> +	 */
> +
> +	return 0;
> +}

dme.
-- 
It's alright, we told you what to dream.