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From:   Xin Li <xin3.li@intel.com>
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        shan.kang@intel.com
Subject: [PATCH v6 24/33] x86/fred: disallow the swapgs instruction when FRED is enabled
Date:   Mon, 27 Mar 2023 00:58:29 -0700
Message-Id: <20230327075838.5403-25-xin3.li@intel.com>
In-Reply-To: <20230327075838.5403-1-xin3.li@intel.com>
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From: "H. Peter Anvin (Intel)" <hpa@zytor.com>

The FRED architecture establishes the full supervisor/user through:
1) FRED event delivery swaps the value of the GS base address and
   that of the IA32_KERNEL_GS_BASE MSR.
2) ERETU swaps the value of the GS base address and that of the
   IA32_KERNEL_GS_BASE MSR.
Thus, the swapgs instruction is disallowed when FRED is enabled,
otherwise it causes #UD.

Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
Tested-by: Shan Kang <shan.kang@intel.com>
Signed-off-by: Xin Li <xin3.li@intel.com>
---
 arch/x86/kernel/process_64.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index a1aa74864c8b..2bea86073646 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -165,7 +165,8 @@ static noinstr unsigned long __rdgsbase_inactive(void)
 
 	lockdep_assert_irqs_disabled();
 
-	if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    !cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		gsbase = rdgsbase();
 		native_swapgs();
@@ -190,7 +191,8 @@ static noinstr void __wrgsbase_inactive(unsigned long gsbase)
 {
 	lockdep_assert_irqs_disabled();
 
-	if (!cpu_feature_enabled(X86_FEATURE_XENPV)) {
+	if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+	    !cpu_feature_enabled(X86_FEATURE_XENPV)) {
 		native_swapgs();
 		wrgsbase(gsbase);
 		native_swapgs();
-- 
2.34.1