Merge tag 'ibti-hisory-for-linus-2025-05-06' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 IBTI mitigation from Dave Hansen:
 "Mitigate Intra-mode Branch History Injection via classic BFP programs

  This adds the branch history clearing mitigation to cBPF programs for
  x86. Intra-mode BHI attacks via cBPF a.k.a IBTI-History was reported
  by researchers at VUSec.

  For hardware that doesn't support BHI_DIS_S, the recommended
  mitigation is to run the short software sequence followed by the IBHF
  instruction after cBPF execution. On hardware that does support
  BHI_DIS_S, enable BHI_DIS_S and execute the IBHF after cBPF execution.

  The Indirect Branch History Fence (IBHF) is a new instruction that
  prevents indirect branch target predictions after the barrier from
  using branch history from before the barrier while BHI_DIS_S is
  enabled. On older systems this will map to a NOP. It is recommended to
  add this fence at the end of the cBPF program to support VM migration.
  This instruction is required on newer parts with BHI_NO to fully
  mitigate against these attacks.

  The current code disables the mitigation for anything running with the
  SYS_ADMIN capability bit set. The intention was not to waste time
  mitigating a process that has access to anything it wants anyway"

* tag 'ibti-hisory-for-linus-2025-05-06' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  x86/bhi: Do not set BHI_DIS_S in 32-bit mode
  x86/bpf: Add IBHF call at end of classic BPF
  x86/bpf: Call branch history clearing sequence on exit

2 files changed, 9 insertions, 6 deletions

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 362602b705cc..f219f0f4f2d1 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1697,11 +1697,11 @@ static void __init bhi_select_mitigation(void)
 			return;
 	}
 
-	/* Mitigate in hardware if supported */
-	if (spec_ctrl_bhi_dis())
+	if (!IS_ENABLED(CONFIG_X86_64))
 		return;
 
-	if (!IS_ENABLED(CONFIG_X86_64))
+	/* Mitigate in hardware if supported */
+	if (spec_ctrl_bhi_dis())
 		return;
 
 	if (bhi_mitigation == BHI_MITIGATION_VMEXIT_ONLY) {
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 12126adbc3a9..5ab13d9241c0 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1439,9 +1439,12 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	if (vulnerable_to_rfds(x86_arch_cap_msr))
 		setup_force_cpu_bug(X86_BUG_RFDS);
 
-	/* When virtualized, eIBRS could be hidden, assume vulnerable */
-	if (!(x86_arch_cap_msr & ARCH_CAP_BHI_NO) &&
-	    !cpu_matches(cpu_vuln_whitelist, NO_BHI) &&
+	/*
+	 * Intel parts with eIBRS are vulnerable to BHI attacks. Parts with
+	 * BHI_NO still need to use the BHI mitigation to prevent Intra-mode
+	 * attacks.  When virtualized, eIBRS could be hidden, assume vulnerable.
+	 */
+	if (!cpu_matches(cpu_vuln_whitelist, NO_BHI) &&
 	    (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) ||
 	     boot_cpu_has(X86_FEATURE_HYPERVISOR)))
 		setup_force_cpu_bug(X86_BUG_BHI);