x86/bugs: Select the BHI mitigation based on the attack vector properly

The BHI mitigation selection is inconsistent with the documentation
(attack_vector_controls.rst), which lists BHI under both the
user->kernel and guest->host attack vectors. bhi_select_mitigation()
only enables it when CPU_MITIGATE_GUEST_HOST is set; otherwise it
selects BHI_MITIGATION_OFF, even if CPU_MITIGATE_USER_KERNEL is set.

As a result, a kernel compiled with KVM disabled gets BHI automatically
disabled, since CPU_MITIGATE_GUEST_HOST defaults to off without KVM.
This is a bug, given that CPU_MITIGATE_USER_KERNEL still needs it: on
affected Intel CPUs these kernels lose BHI_DIS_S or the BHB clearing
loop by default. Booting with mitigations=auto,no_guest_host has the
same effect.

Select BHI_MITIGATION_ON whenever user->kernel is mitigated, and keep
BHI_MITIGATION_VMEXIT_ONLY for the guest->host-only case.

  [ bp: Clarify commit title. ]

Fixes: ddcd4d3cb37c ("x86/bugs: Add attack vector controls for BHI")
Signed-off-by: Breno Leitao <leitao@debian.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: David Kaplan <david.kaplan@amd.com>
Cc: <stable@kernel.org>
Link: https://patch.msgid.link/20261029-bhi-attack-vector-v1-1-29cfa697efb6@debian.org

0 files changed, 0 insertions, 0 deletions