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/[e-mail] ]

Signed-off-by: Borislav Petkov <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/kernel-parameters.txt

···
 			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

···
 	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

···
 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"))
···
 		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) {
-			pr_cont(" " X86_CAP_FMT, x86_cap_flag(bit));
-			setup_clear_cpu_cap(bit);
+		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;
 		}
-	} while (res == 2);
+
+#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
+	}
 	pr_cont("\n");
+
+	if (taint)
+		add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
 }
 
 /*