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Commit 5700f743 authored by Borislav Petkov's avatar Borislav Petkov Committed by H. Peter Anvin
Browse files

x86: Sanity-check static_cpu_has usage 



static_cpu_has may be used only after alternatives have run. Before that
it always returns false if constant folding with __builtin_constant_p()
doesn't happen. And you don't want that.

This patch is the result of me debugging an issue where I overzealously
put static_cpu_has in code which executed before alternatives have run
and had to spend some time with scratching head and cursing at the
monitor.

So add a jump to a warning which screams loudly when we use this
function too early. The alternatives patch that check away in
conjunction with patching the rest of the kernel image.

[ hpa: factored this into its own configuration option.  If we want to
  have an overarching option, it should be an option which selects
  other options, not as a group option in the source code. ]

Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1370772454-6106-4-git-send-email-bp@alien8.de


Signed-off-by: default avatarH. Peter Anvin <hpa@linux.intel.com>
parent c3b83598

arch/x86/Kconfig.debug

+10 −0
Original line number Diff line number Diff line
@@ -304,4 +304,14 @@ config DEBUG_NMI_SELFTEST 


	  If unsure, say N.



config X86_DEBUG_STATIC_CPU_HAS

	bool "Debug alternatives"

	depends on DEBUG_KERNEL

	---help---

	  This option causes additional code to be generated which

	  fails if static_cpu_has() is used before alternatives have

	  run.



	  If unsure, say N.



endmenu

arch/x86/include/asm/cpufeature.h

+28 −2
Original line number Diff line number Diff line
@@ -356,15 +356,35 @@ extern const char * const x86_power_flags[32]; 
#endif /* CONFIG_X86_64 */



#if __GNUC__ >= 4

extern void warn_pre_alternatives(void);



/*

 * Static testing of CPU features.  Used the same as boot_cpu_has().

 * These are only valid after alternatives have run, but will statically

 * patch the target code for additional performance.

 *

 */

static __always_inline __pure bool __static_cpu_has(u16 bit)

{

#if __GNUC__ > 4 || __GNUC_MINOR__ >= 5



#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS

		/*

		 * Catch too early usage of this before alternatives

		 * have run.

		 */

		asm goto("1: jmp %l[t_warn]\n"

			 "2:\n"

			 ".section .altinstructions,\"a\"\n"

			 " .long 1b - .\n"

			 " .long 0\n"		/* no replacement */

			 " .word %P0\n"		/* 1: do replace */

			 " .byte 2b - 1b\n"	/* source len */

			 " .byte 0\n"		/* replacement len */

			 ".previous\n"

			 /* skipping size check since replacement size = 0 */

			 : : "i" (X86_FEATURE_ALWAYS) : : t_warn);

#endif



		asm goto("1: jmp %l[t_no]\n"

			 "2:\n"

			 ".section .altinstructions,\"a\"\n"
@@ -379,7 +399,13 @@ static __always_inline __pure bool __static_cpu_has(u16 bit) 
		return true;

	t_no:

		return false;

#else



#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS

	t_warn:

		warn_pre_alternatives();

		return false;

#endif

#else /* GCC_VERSION >= 40500 */

		u8 flag;

		/* Open-coded due to __stringify() in ALTERNATIVE() */

		asm volatile("1: movb $0,%0\n"

arch/x86/kernel/cpu/common.c

+8 −0
Original line number Diff line number Diff line
@@ -1364,3 +1364,11 @@ void __cpuinit cpu_init(void) 
	fpu_init();

}

#endif



#ifdef CONFIG_X86_DEBUG_STATIC_CPU_HAS

void warn_pre_alternatives(void)

{

	WARN(1, "You're using static_cpu_has before alternatives have run!\n");

}

EXPORT_SYMBOL_GPL(warn_pre_alternatives);

#endif