x86/bhi: Do not set BHI_DIS_S in 32-bit mode

With the possibility of intra-mode BHI via cBPF, complete mitigation for
BHI is to use IBHF (history fence) instruction with BHI_DIS_S set. Since
this new instruction is only available in 64-bit mode, setting BHI_DIS_S in
32-bit mode is only a partial mitigation.

Do not set BHI_DIS_S in 32-bit mode so as to avoid reporting misleading
mitigated status. With this change IBHF won't be used in 32-bit mode, also
remove the CONFIG_X86_64 check from emit_spectre_bhb_barrier().

Suggested-by: Josh Poimboeuf <[email protected]>
Signed-off-by: Pawan Gupta <[email protected]>
Signed-off-by: Dave Hansen <[email protected]>
Reviewed-by: Josh Poimboeuf <[email protected]>
Reviewed-by: Alexandre Chartre <[email protected]>

Author: pa1gupta

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) {

arch/x86/net/bpf_jit_comp.c

@@ -1527,8 +1527,7 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
 	/* Insert IBHF instruction */
 	if ((cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_LOOP) &&
 	     cpu_feature_enabled(X86_FEATURE_HYPERVISOR)) ||
-	    (cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_HW) &&
-	     IS_ENABLED(CONFIG_X86_64))) {
+	    cpu_feature_enabled(X86_FEATURE_CLEAR_BHB_HW)) {
 		/*
 		 * Add an Indirect Branch History Fence (IBHF). IBHF acts as a
 		 * fence preventing branch history from before the fence from
@@ -1538,6 +1537,8 @@ static int emit_spectre_bhb_barrier(u8 **pprog, u8 *ip,
 		 * hardware that doesn't need or support it.  The REP and REX.W
 		 * prefixes are required by the microcode, and they also ensure
 		 * that the NOP is unlikely to be used in existing code.
+		 *
+		 * IBHF is not a valid instruction in 32-bit mode.
 		 */
 		EMIT5(0xF3, 0x48, 0x0F, 0x1E, 0xF8); /* ibhf */
 	}