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x86/bugs: Skip RSB fill at VMEXIT

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commit 0fad287864 upstream.

entry_ibpb() is designed to follow Intel's IBPB specification regardless
of CPU. This includes invalidating RSB entries.

Hence, if IBPB on VMEXIT has been selected, entry_ibpb() as part of the
RET untraining in the VMEXIT path will take care of all BTB and RSB
clearing so there's no need to explicitly fill the RSB anymore.

  [ bp: Massage commit message. ]

Suggested-by: Borislav Petkov <bp@alien8.de>
Signed-off-by: Johannes Wikner <kwikner@ethz.ch>
Cc: <stable@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
This commit is contained in:
Johannes Wikner 2024-10-08 12:36:30 +02:00 committed by Greg Kroah-Hartman
parent 61211f2da0
commit c42a343158
1 changed files with 15 additions and 0 deletions
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15
arch/x86/kernel/cpu/bugs.c
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@ -1115,6 +1115,14 @@ do_cmd_auto:
setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
mitigate_smt = true;
/*
* There is no need for RSB filling: entry_ibpb() ensures
* all predictions, including the RSB, are invalidated,
* regardless of IBPB implementation.
*/
setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
break;
case RETBLEED_MITIGATION_STUFF:
@ -2622,6 +2630,13 @@ static void __init srso_select_mitigation(void)
if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
/*
* There is no need for RSB filling: entry_ibpb() ensures
* all predictions, including the RSB, are invalidated,
* regardless of IBPB implementation.
*/
setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
}
} else {
pr_err("WARNING: kernel not compiled with CPU_SRSO.\n");