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x86/srso: Add IBPB_BRTYPE support

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Upstream commit: 79113e4060aba744787a81edb9014f2865193854

Add support for the synthetic CPUID flag which "if this bit is 1,
it indicates that MSR 49h (PRED_CMD) bit 0 (IBPB) flushes all branch
type predictions from the CPU branch predictor."

This flag is there so that this capability in guests can be detected
easily (otherwise one would have to track microcode revisions which is
impossible for guests).

It is also needed only for Zen3 and -4. The other two (Zen1 and -2)
always flush branch type predictions by default.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Borislav Petkov (AMD)
2023-07-18 11:13:40 +02:00
committed by Greg Kroah-Hartman
parent 3f9b7101be
commit df76a59feb
2 changed files with 13 additions and 1 deletions
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12
arch/x86/kernel/cpu/bugs.c
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@@ -2286,10 +2286,20 @@ static void __init srso_select_mitigation(void)
if (!boot_cpu_has_bug(X86_BUG_SRSO) || cpu_mitigations_off())
return;
has_microcode = cpu_has_ibpb_brtype_microcode();
/*
* The first check is for the kernel running as a guest in order
* for guests to verify whether IBPB is a viable mitigation.
*/
has_microcode = boot_cpu_has(X86_FEATURE_IBPB_BRTYPE) || cpu_has_ibpb_brtype_microcode();
if (!has_microcode) {
pr_warn("IBPB-extending microcode not applied!\n");
pr_warn(SRSO_NOTICE);
} else {
/*
* Enable the synthetic (even if in a real CPUID leaf)
* flag for guests.
*/
setup_force_cpu_cap(X86_FEATURE_IBPB_BRTYPE);
}
switch (srso_cmd) {