[PATCH v2 1/6] x86/cpu: Allow feature bit names from /proc/cpuinfo in clearcpuid=

Borislav Petkov
2022-01-27T11:56:40Z

From: Borislav Petkov <bp@suse.de>

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:

[ 0.000000] Clearing CPUID bits: de 13:24 smca bmi1 3dnow

Signed-off-by: Borislav Petkov <bp@suse.de>
---
.../admin-guide/kernel-parameters.txt | 11 +++-
arch/x86/include/asm/cpufeature.h | 7 ++-
arch/x86/kernel/cpu/common.c | 57 +++++++++++++++----
3 files changed, 58 insertions(+), 17 deletions(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f5a27f067db9..b67d0cf27997 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -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
diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index 1261842d006c..66d3e3b1d24d 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -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

/*
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 7b8382c11788..aaac18d800a4 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1237,8 +1237,8 @@ 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"))
@@ -1266,21 +1266,54 @@ 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) {
+ unsigned int bit;
+
+ opt = strsep(&argptr, ",");

- if (bit >= 0 && bit < NCAPINTS * 32) {
- pr_cont(" " X86_CAP_FMT, x86_cap_flag(bit));
+ /*
+ * Handle naked numbers first for feature flags which don't
+ * have names.
+ */
+ if (!kstrtouint(opt, 10, &bit)) {
+ if (bit < NCAPINTS * 32) {
+
+ /* empty-string, i.e., ""-defined feature flags */
+ if (!x86_cap_flags[bit])
+ pr_cont(" " X86_CAP_FMT_NUM, x86_cap_flag_num(bit));
+ else
+ 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++;
+ break;
}
- } while (res == 2);
+#endif
+ }
pr_cont("\n");
+
+ if (taint)
+ add_taint(TAINT_CPU_OUT_OF_SPEC, LOCKDEP_STILL_OK);
}

/*
--
2.29.2