tree caf7b704a03abc781716e319be5a257b7458c796
parent 6b5ca3235d0b2ee0728b27431e9c285a91583f4c
author Pawan Gupta <pawan.kumar.gupta@linux.intel.com> 1755192042 -0700
committer Miri Amarilio <mirilio@google.com> 1758755029 -0700

x86/vmscape: Add conditional IBPB mitigation

Commit 2f8f173413f1cbf52660d04df92d0069c4306d25 upstream.

VMSCAPE is a vulnerability that exploits insufficient branch predictor
isolation between a guest and a userspace hypervisor (like QEMU). Existing
mitigations already protect kernel/KVM from a malicious guest. Userspace
can additionally be protected by flushing the branch predictors after a
VMexit.

Since it is the userspace that consumes the poisoned branch predictors,
conditionally issue an IBPB after a VMexit and before returning to
userspace. Workloads that frequently switch between hypervisor and
userspace will incur the most overhead from the new IBPB.

This new IBPB is not integrated with the existing IBPB sites. For
instance, a task can use the existing speculation control prctl() to
get an IBPB at context switch time. With this implementation, the
IBPB is doubled up: one at context switch and another before running
userspace.

The intent is to integrate and optimize these cases post-embargo.

[ dhansen: elaborate on suboptimal IBPB solution ]

BUG=b/444641606
TEST=presubmit,validation
RELEASE_NOTE=Fixed CVE-2025-40300 in the Linux kernel.

Suggested-by: Dave Hansen <dave.hansen@linux.intel.com>
Change-Id: Ib47c2e1b43ef7672db66955c98e49bbebbe854b0
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/112002
Reviewed-by: Kevin Berry <kpberry@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>