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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:

  5c9a92dec3 ("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>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20241220-force-cpu-bug-v2-3-7dc71bce742a@google.com
pull/1183/head
Brendan Jackman 2024-12-20 15:18:33 +00:00 committed by Ingo Molnar
parent 814165e9fd
commit ab68d2e365
2 changed files with 13 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

1
arch/x86/include/asm/cpufeature.h
View File

@ -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))

16
arch/x86/kernel/cpu/common.c
View File

@ -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);