[PATCH 5.4 07/18] x86/speculation: Use generic retpoline by default on AMD

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Kim Phillips <kim.phillips@amd.com>

commit 244d00b5dd4755f8df892c86cab35fb2cfd4f14b upstream.

AMD retpoline may be susceptible to speculation. The speculation
execution window for an incorrect indirect branch prediction using
LFENCE/JMP sequence may potentially be large enough to allow
exploitation using Spectre V2.

By default, don't use retpoline,lfence on AMD.  Instead, use the
generic retpoline.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kernel/cpu/bugs.c |    9 ---------
 1 file changed, 9 deletions(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -898,15 +898,6 @@ static enum spectre_v2_mitigation __init
 		return SPECTRE_V2_NONE;
 	}
 
-	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD ||
-	    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
-		if (!boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
-			pr_err("LFENCE not serializing, switching to generic retpoline\n");
-			return SPECTRE_V2_RETPOLINE;
-		}
-		return SPECTRE_V2_LFENCE;
-	}
-
 	return SPECTRE_V2_RETPOLINE;
 }