x86/cpufeatures: Add a IBPB_NO_RET BUG flag

Set this flag if the CPU has an IBPB implementation that does not
invalidate return target predictions. Zen generations < 4 do not flush
the RSB when executing an IBPB and this bug flag denotes that.

  [ bp: Massage. ]
Signed-off-by: Johannes Wikner <kwikner@ethz.ch>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>

arch/x86/include/asm/cpufeatures.h

@@ -524,4 +524,5 @@
 #define X86_BUG_DIV0			X86_BUG(1*32 + 1) /* "div0" AMD DIV0 speculation bug */
 #define X86_BUG_RFDS			X86_BUG(1*32 + 2) /* "rfds" CPU is vulnerable to Register File Data Sampling */
 #define X86_BUG_BHI			X86_BUG(1*32 + 3) /* "bhi" CPU is affected by Branch History Injection */
+#define X86_BUG_IBPB_NO_RET	   	X86_BUG(1*32 + 4) /* "ibpb_no_ret" IBPB omits return target predictions */
 #endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/kernel/cpu/common.c

@@ -1443,6 +1443,9 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	     boot_cpu_has(X86_FEATURE_HYPERVISOR)))
 		setup_force_cpu_bug(X86_BUG_BHI);
 
+	if (cpu_has(c, X86_FEATURE_AMD_IBPB) && !cpu_has(c, X86_FEATURE_AMD_IBPB_RET))
+		setup_force_cpu_bug(X86_BUG_IBPB_NO_RET);
+
 	if (cpu_matches(cpu_vuln_whitelist, NO_MELTDOWN))
 		return;