x86/amd: Use IBPB for firmware calls

On AMD IBRS does not prevent Retbleed; as such use IBPB before a
firmware call to flush the branch history state.

And because in order to do an EFI call, the kernel maps a whole lot of
the kernel page table into the EFI page table, do an IBPB just in case
in order to prevent the scenario of poisoning the BTB and causing an EFI
call using the unprotected RET there.

  [ bp: Massage. ]

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lore.kernel.org/r/20220715194550.793957-1-cascardo@canonical.com

Author: Peter Zijlstra

arch/x86/include/asm/cpufeatures.h

@@ -302,6 +302,7 @@
 #define X86_FEATURE_RETPOLINE_LFENCE	(11*32+13) /* "" Use LFENCE for Spectre variant 2 */
 #define X86_FEATURE_RETHUNK		(11*32+14) /* "" Use REturn THUNK */
 #define X86_FEATURE_UNRET		(11*32+15) /* "" AMD BTB untrain return */
+#define X86_FEATURE_USE_IBPB_FW		(11*32+16) /* "" Use IBPB during runtime firmware calls */
 
 /* Intel-defined CPU features, CPUID level 0x00000007:1 (EAX), word 12 */
 #define X86_FEATURE_AVX_VNNI		(12*32+ 4) /* AVX VNNI instructions */

arch/x86/include/asm/nospec-branch.h

@@ -297,6 +297,8 @@ do {									\
 	alternative_msr_write(MSR_IA32_SPEC_CTRL,			\
 			      spec_ctrl_current() | SPEC_CTRL_IBRS,	\
 			      X86_FEATURE_USE_IBRS_FW);			\
+	alternative_msr_write(MSR_IA32_PRED_CMD, PRED_CMD_IBPB,		\
+			      X86_FEATURE_USE_IBPB_FW);			\
 } while (0)
 
 #define firmware_restrict_branch_speculation_end()			\

arch/x86/kernel/cpu/bugs.c

@@ -1516,7 +1516,16 @@ static void __init spectre_v2_select_mitigation(void)
 	 * the CPU supports Enhanced IBRS, kernel might un-intentionally not
 	 * enable IBRS around firmware calls.
 	 */
-	if (boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode)) {
+	if (boot_cpu_has_bug(X86_BUG_RETBLEED) &&
+	    (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
+	     boot_cpu_data.x86_vendor == X86_VENDOR_HYGON)) {
+
+		if (retbleed_cmd != RETBLEED_CMD_IBPB) {
+			setup_force_cpu_cap(X86_FEATURE_USE_IBPB_FW);
+			pr_info("Enabling Speculation Barrier for firmware calls\n");
+		}
+
+	} else if (boot_cpu_has(X86_FEATURE_IBRS) && !spectre_v2_in_ibrs_mode(mode)) {
 		setup_force_cpu_cap(X86_FEATURE_USE_IBRS_FW);
 		pr_info("Enabling Restricted Speculation for firmware calls\n");
 	}