[RFC PATCH v2 25/32] x86/fred: disallow the swapgs instruction when FRED is enabled

From: Xin Li
Date: Fri Jan 06 2023 - 04:23:18 EST


From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>

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 cauess #UD.

Signed-off-by: H. Peter Anvin (Intel) <hpa@xxxxxxxxx>
Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
---
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 bfe6179b7a17..5b6cfd2ca630 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 (!static_cpu_has(X86_FEATURE_XENPV)) {
+ if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+ !static_cpu_has(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 (!static_cpu_has(X86_FEATURE_XENPV)) {
+ if (!cpu_feature_enabled(X86_FEATURE_FRED) &&
+ !static_cpu_has(X86_FEATURE_XENPV)) {
native_swapgs();
wrgsbase(gsbase);
native_swapgs();
--
2.34.1