[PATCH v3 4/4] x86/cpu: Enable modifying bug flags with {clear,set}cpuid

Brendan Jackman
2025-02-18T18:59:56Z

Sometimes it can be very useful to run CPU vulnerability mitigations on
systems where they aren't known to mitigate any real-world
vulnerabilities. This can be handy for mundane reasons like debugging
HW-agnostic logic on whatever machine is to hand, but also for research
reasons: while some mitigations are focused on individual vulns and
uarches, others are fairly general, and it's strategically useful to
have an idea how they'd perform on systems where they aren't currently
needed.

As evidence for this being useful, a flag specifically for Retbleed was
added in commit 5c9a92dec323 ("x86/bugs: Add retbleed=force").

Since CPU bugs are tracked using the same basic mechanism as features,
and there are already parameters for manipulating them by hand, extend
that mechanism to support bug as well as capabilities.

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---

Notes:
With this patch and setcpuid=srso, a QEMU guest running
on an Intel host will boot with Safe-RET enabled.

arch/x86/include/asm/cpufeature.h | 1 +
arch/x86/kernel/cpu/common.c | 16 ++++++++++++----
2 files changed, 13 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/cpufeature.h b/arch/x86/include/asm/cpufeature.h
index f23942108b85..e955da397e0e 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -45,6 +45,7 @@ extern const char * const x86_power_flags[32];
* X86_BUG_<name> - NCAPINTS*32.
*/
extern const char * const x86_bug_flags[NBUGINTS*32];
+#define x86_bug_flag(flag) x86_bug_flags[flag]

#define test_cpu_cap(c, bit) \
arch_test_bit(bit, (unsigned long *)((c)->x86_capability))
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 245807f9351d..08d984ba0347 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1494,7 +1494,8 @@ static inline void parse_set_clear_cpuid(char *arg, bool set)

/*
* Handle naked numbers first for feature flags which don't
- * have names.
+ * have names. It doesn't make sense for a bug not to have a
+ * name so don't handle bug flags here.
*/
if (!kstrtouint(opt, 10, &bit)) {
if (bit < NCAPINTS * 32) {
@@ -1518,11 +1519,18 @@ static inline void parse_set_clear_cpuid(char *arg, bool set)
continue;
}

- for (bit = 0; bit < 32 * NCAPINTS; bit++) {
- if (!x86_cap_flags[bit])
+ for (bit = 0; bit < 32 * (NCAPINTS + NBUGINTS); bit++) {
+ const char *flag;
+
+ if (bit < 32 * NCAPINTS)
+ flag = x86_cap_flags[bit];
+ else
+ flag = x86_bug_flags[bit - (32 * NCAPINTS)];
+
+ if (!flag)
continue;

- if (strcmp(x86_cap_flags[bit], opt))
+ if (strcmp(flag, opt))