[PATCH v2 3/3] x86/cpu: Enable modifying bug flags with {clear,set}puid

[Brendan Jackman <jackmanb@google.com>]

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.

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

Signed-off-by: Brendan Jackman <jackmanb@google.com>
---
 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 0b9611da6c53f19ae6c45d85d1ee191118ad1895..6e17f47ab0521acadb7db38ce5934c4717d457ba 100644
--- a/arch/x86/include/asm/cpufeature.h
+++ b/arch/x86/include/asm/cpufeature.h
@@ -50,6 +50,7 @@ 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 e26cf8789f0e1a27ad126f531e05afee0fdebbb8..d94d7ebff42dadae30f77af1ef675d1a83ba6c3f 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1492,7 +1492,8 @@ 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) {
@@ -1516,11 +1517,18 @@ static inline void parse_set_clear_cpuid(char *arg, bool set)
 			continue;
 		}
 
-		for (bit = 0; bit < 32 * NCAPINTS; bit++) {
-			if (!x86_cap_flag(bit))
+		for (bit = 0; bit < 32 * (NCAPINTS + NBUGINTS); bit++) {
+			const char *flag;
+
+			if (bit < 32 * NCAPINTS)
+				flag = x86_cap_flag(bit);
+			else
+				flag = x86_bug_flag(bit - (32 * NCAPINTS));
+
+			if (!flag)
 				continue;
 
-			if (strcmp(x86_cap_flag(bit), opt))
+			if (strcmp(flag, opt))
 				continue;
 
 			pr_cont(" %s", opt);

-- 
2.47.1.613.gc27f4b7a9f-goog