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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        Thadeu Lima de Souza Cascardo <cascardo@canonical.com>,
        Borislav Petkov <bp@suse.de>
Subject: [PATCH 5.15 75/89] x86/bugs: Do not enable IBPB-on-entry when IBPB is not supported
Date:   Fri, 22 Jul 2022 11:11:49 +0200
Message-Id: <20220722091137.539782973@linuxfoundation.org>
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From: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

commit 2259da159fbe5dba8ac00b560cf00b6a6537fa18 upstream.

There are some VM configurations which have Skylake model but do not
support IBPB. In those cases, when using retbleed=ibpb, userspace is going
to be killed and kernel is going to panic.

If the CPU does not support IBPB, warn and proceed with the auto option. Also,
do not fallback to IBPB on AMD/Hygon systems if it is not supported.

Fixes: 3ebc17006888 ("x86/bugs: Add retbleed=ibpb")
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
 arch/x86/kernel/cpu/bugs.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -858,7 +858,10 @@ static void __init retbleed_select_mitig
 		break;
 
 	case RETBLEED_CMD_IBPB:
-		if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
+		if (!boot_cpu_has(X86_FEATURE_IBPB)) {
+			pr_err("WARNING: CPU does not support IBPB.\n");
+			goto do_cmd_auto;
+		} else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY)) {
 			retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
 		} else {
 			pr_err("WARNING: kernel not compiled with CPU_IBPB_ENTRY.\n");
@@ -873,7 +876,7 @@ do_cmd_auto:
 		    boot_cpu_data.x86_vendor == X86_VENDOR_HYGON) {
 			if (IS_ENABLED(CONFIG_CPU_UNRET_ENTRY))
 				retbleed_mitigation = RETBLEED_MITIGATION_UNRET;
-			else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY))
+			else if (IS_ENABLED(CONFIG_CPU_IBPB_ENTRY) && boot_cpu_has(X86_FEATURE_IBPB))
 				retbleed_mitigation = RETBLEED_MITIGATION_IBPB;
 		}