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x86/cpu/kvm: SRSO: Fix possible missing IBPB on VM-Exit

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In [1] the meaning of the synthetic IBPB flags has been redefined for a
better separation of concerns:
 - ENTRY_IBPB     -- issue IBPB on entry only
 - IBPB_ON_VMEXIT -- issue IBPB on VM-Exit only
and the Retbleed mitigations have been updated to match this new
semantics.

Commit [2] was merged shortly before [1], and their interaction was not
handled properly. This resulted in IBPB not being triggered on VM-Exit
in all SRSO mitigation configs requesting an IBPB there.

Specifically, an IBPB on VM-Exit is triggered only when
X86_FEATURE_IBPB_ON_VMEXIT is set. However:

 - X86_FEATURE_IBPB_ON_VMEXIT is not set for "spec_rstack_overflow=ibpb",
   because before [1] having X86_FEATURE_ENTRY_IBPB was enough. Hence,
   an IBPB is triggered on entry but the expected IBPB on VM-exit is
   not.

 - X86_FEATURE_IBPB_ON_VMEXIT is not set also when
   "spec_rstack_overflow=ibpb-vmexit" if X86_FEATURE_ENTRY_IBPB is
   already set.

   That's because before [1] this was effectively redundant. Hence, e.g.
   a "retbleed=ibpb spec_rstack_overflow=bpb-vmexit" config mistakenly
   reports the machine still vulnerable to SRSO, despite an IBPB being
   triggered both on entry and VM-Exit, because of the Retbleed selected
   mitigation config.

 - UNTRAIN_RET_VM won't still actually do anything unless
   CONFIG_MITIGATION_IBPB_ENTRY is set.

For "spec_rstack_overflow=ibpb", enable IBPB on both entry and VM-Exit
and clear X86_FEATURE_RSB_VMEXIT which is made superfluous by
X86_FEATURE_IBPB_ON_VMEXIT. This effectively makes this mitigation
option similar to the one for 'retbleed=ibpb', thus re-order the code
for the RETBLEED_MITIGATION_IBPB option to be less confusing by having
all features enabling before the disabling of the not needed ones.

For "spec_rstack_overflow=ibpb-vmexit", guard this mitigation setting
with CONFIG_MITIGATION_IBPB_ENTRY to ensure UNTRAIN_RET_VM sequence is
effectively compiled in. Drop instead the CONFIG_MITIGATION_SRSO guard,
since none of the SRSO compile cruft is required in this configuration.
Also, check only that the required microcode is present to effectively
enabled the IBPB on VM-Exit.

Finally, update the KConfig description for CONFIG_MITIGATION_IBPB_ENTRY
to list also all SRSO config settings enabled by this guard.

Fixes: 864bcaa38e ("x86/cpu/kvm: Provide UNTRAIN_RET_VM") [1]
Fixes: d893832d0e ("x86/srso: Add IBPB on VMEXIT") [2]
Reported-by: Yosry Ahmed <yosryahmed@google.com>
Signed-off-by: Patrick Bellasi <derkling@google.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Patrick Bellasi 2025-02-05 14:04:41 +00:00 • committed by Linus Torvalds
parent febbc555cf
commit 318e8c339c
2 changed files with 16 additions and 8 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

3
arch/x86/Kconfig
View file

@ -2599,7 +2599,8 @@ config MITIGATION_IBPB_ENTRY
depends on CPU_SUP_AMD && X86_64
default y
help
Compile the kernel with support for the retbleed=ibpb mitigation.
Compile the kernel with support for the retbleed=ibpb and
spec_rstack_overflow={ibpb,ibpb-vmexit} mitigations.
config MITIGATION_IBRS_ENTRY
bool "Enable IBRS on kernel entry"

21
arch/x86/kernel/cpu/bugs.c
View file

@ -1115,6 +1115,8 @@ do_cmd_auto:
case RETBLEED_MITIGATION_IBPB:
setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
mitigate_smt = true;
/*
* IBPB on entry already obviates the need for
@ -1124,9 +1126,6 @@ do_cmd_auto:
setup_clear_cpu_cap(X86_FEATURE_UNRET);
setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
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,
@ -2646,6 +2645,7 @@ static void __init srso_select_mitigation(void)
if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
srso_mitigation = SRSO_MITIGATION_IBPB;
/*
@ -2655,6 +2655,13 @@ static void __init srso_select_mitigation(void)
*/
setup_clear_cpu_cap(X86_FEATURE_UNRET);
setup_clear_cpu_cap(X86_FEATURE_RETHUNK);
/*
* 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 MITIGATION_IBPB_ENTRY.\n");
@ -2663,8 +2670,8 @@ static void __init srso_select_mitigation(void)
ibpb_on_vmexit:
case SRSO_CMD_IBPB_ON_VMEXIT:
if (IS_ENABLED(CONFIG_MITIGATION_SRSO)) {
if (!boot_cpu_has(X86_FEATURE_ENTRY_IBPB) && has_microcode) {
if (IS_ENABLED(CONFIG_MITIGATION_IBPB_ENTRY)) {
if (has_microcode) {
setup_force_cpu_cap(X86_FEATURE_IBPB_ON_VMEXIT);
srso_mitigation = SRSO_MITIGATION_IBPB_ON_VMEXIT;
@ -2676,8 +2683,8 @@ ibpb_on_vmexit:
setup_clear_cpu_cap(X86_FEATURE_RSB_VMEXIT);
}
} else {
pr_err("WARNING: kernel not compiled with MITIGATION_SRSO.\n");
}
pr_err("WARNING: kernel not compiled with MITIGATION_IBPB_ENTRY.\n");
}
break;
default:
break;