Commit 1625c833 authored by Borislav Petkov's avatar Borislav Petkov
Browse files

x86/cpu: Allow feature bit names from /proc/cpuinfo in clearcpuid= 

Having to give the X86_FEATURE array indices in order to disable a
feature bit for testing is not really user-friendly. So accept the
feature bit names too.

Some feature bits don't have names so there the array indices are still
accepted, of course.

Clearing CPUID flags is not something which should be done in production
so taint the kernel too.

An exemplary cmdline would then be something like:

  clearcpuid=de,440,smca,succory,bmi1,3dnow

("succory" is wrong on purpose). And it says:

  [   ... ] Clearing CPUID bits: de 13:24 smca (unknown: succory) bmi1 3dnow

  [ Fix CONFIG_X86_FEATURE_NAMES=n build error as reported by the 0day
    robot: https://lore.kernel.org/r/202203292206.ICsY2RKX-lkp@intel.com

 ]

Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
Reviewed-by: default avatarKees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220127115626.14179-2-bp@alien8.de
parent 31231092

Documentation/admin-guide/kernel-parameters.txt

+8 −3
Original line number Diff line number Diff line
@@ -631,12 +631,17 @@ 
			Defaults to zero when built as a module and to

			10 seconds when built into the kernel.



	clearcpuid=BITNUM[,BITNUM...] [X86]

	clearcpuid=X[,X...] [X86]

			Disable CPUID feature X for the kernel. See

			arch/x86/include/asm/cpufeatures.h for the valid bit

			numbers. Note the Linux specific bits are not necessarily

			stable over kernel options, but the vendor specific

			numbers X. Note the Linux-specific bits are not necessarily

			stable over kernel options, but the vendor-specific

			ones should be.

			X can also be a string as appearing in the flags: line

			in /proc/cpuinfo which does not have the above

			instability issue. However, not all features have names

			in /proc/cpuinfo.

			Note that using this option will taint your kernel.

			Also note that user programs calling CPUID directly

			or using the feature without checking anything

			will still see it. This just prevents it from

arch/x86/include/asm/cpufeature.h

+5 −2
Original line number Diff line number Diff line
@@ -34,14 +34,17 @@ enum cpuid_leafs 
	CPUID_8000_001F_EAX,

};



#define X86_CAP_FMT_NUM "%d:%d"

#define x86_cap_flag_num(flag) ((flag) >> 5), ((flag) & 31)



#ifdef CONFIG_X86_FEATURE_NAMES

extern const char * const x86_cap_flags[NCAPINTS*32];

extern const char * const x86_power_flags[32];

#define X86_CAP_FMT "%s"

#define x86_cap_flag(flag) x86_cap_flags[flag]

#else

#define X86_CAP_FMT "%d:%d"

#define x86_cap_flag(flag) ((flag) >> 5), ((flag) & 31)

#define X86_CAP_FMT X86_CAP_FMT_NUM

#define x86_cap_flag x86_cap_flag_num

#endif



/*

arch/x86/kernel/cpu/common.c

+52 −12
Original line number Diff line number Diff line
@@ -1368,8 +1368,8 @@ static void detect_nopl(void) 
static void __init cpu_parse_early_param(void)

{

	char arg[128];

	char *argptr = arg;

	int arglen, res, bit;

	char *argptr = arg, *opt;

	int arglen, taint = 0;



#ifdef CONFIG_X86_32

	if (cmdline_find_option_bool(boot_command_line, "no387"))
@@ -1397,21 +1397,61 @@ static void __init cpu_parse_early_param(void) 
		return;



	pr_info("Clearing CPUID bits:");

	do {

		res = get_option(&argptr, &bit);

		if (res == 0 || res == 3)

			break;



		/* If the argument was too long, the last bit may be cut off */

		if (res == 1 && arglen >= sizeof(arg))

			break;

	while (argptr) {

		bool found __maybe_unused = false;

		unsigned int bit;



		if (bit >= 0 && bit < NCAPINTS * 32) {

		opt = strsep(&argptr, ",");



		/*

		 * Handle naked numbers first for feature flags which don't

		 * have names.

		 */

		if (!kstrtouint(opt, 10, &bit)) {

			if (bit < NCAPINTS * 32) {



#ifdef CONFIG_X86_FEATURE_NAMES

				/* empty-string, i.e., ""-defined feature flags */

				if (!x86_cap_flags[bit])

					pr_cont(" " X86_CAP_FMT_NUM, x86_cap_flag_num(bit));

				else

#endif

					pr_cont(" " X86_CAP_FMT, x86_cap_flag(bit));



				setup_clear_cpu_cap(bit);

				taint++;

			}

			/*

			 * The assumption is that there are no feature names with only

			 * numbers in the name thus go to the next argument.

			 */

			continue;

		}



#ifdef CONFIG_X86_FEATURE_NAMES

		for (bit = 0; bit < 32 * NCAPINTS; bit++) {

			if (!x86_cap_flag(bit))

				continue;



			if (strcmp(x86_cap_flag(bit), opt))

				continue;



			pr_cont(" %s", opt);

			setup_clear_cpu_cap(bit);

			taint++;

			found = true;

			break;

		}



		if (!found)

			pr_cont(" (unknown: %s)", opt);

#endif

	}

	} while (res == 2);

	pr_cont("\n");



	if (taint)

		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);

}



/*