linux-kernel - [tip: x86/cpu] x86/cpu: Enable modifying CPU bug flags with '{clear,set}puid='

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Message-ID: <174073793204.10177.9171183498085789285.tip-bot2@tip-bot2>
Date: Fri, 28 Feb 2025 10:18:51 -0000
From: "tip-bot2 for Brendan Jackman" <tip-bot2@...utronix.de>
To: linux-tip-commits@...r.kernel.org
Cc: Brendan Jackman <jackmanb@...gle.com>, Ingo Molnar <mingo@...nel.org>,
 x86@...nel.org, linux-kernel@...r.kernel.org
Subject: [tip: x86/cpu] x86/cpu: Enable modifying CPU bug flags with
 '{clear,set}puid='

The following commit has been merged into the x86/cpu branch of tip:

Commit-ID:     ab68d2e36532806b8f86ff2f60861dbb8443f0be
Gitweb:        https://git.kernel.org/tip/ab68d2e36532806b8f86ff2f60861dbb8443f0be
Author:        Brendan Jackman <jackmanb@...gle.com>
AuthorDate:    Fri, 20 Dec 2024 15:18:33 
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Fri, 28 Feb 2025 10:57:50 +01:00

x86/cpu: Enable modifying CPU bug flags with '{clear,set}puid='

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:

  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@...gle.com>
Signed-off-by: Ingo Molnar <mingo@...nel.org>
Link: https://lore.kernel.org/r/20241220-force-cpu-bug-v2-3-7dc71bce742a@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 de1ad09..e5fc003 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 ff483c9..0f32b6f 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_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);