[PATCH v9 11/14] x86/cpu/keylocker: Check Register File Data Sampling mitigation

[Chang S. Bae]

The Register File Data Sampling vulnerability may allow malicious
userspace programs to infer stale kernel register data, potentially
exposing sensitive key values, including AES keys.

To address this vulnerability, a microcode update needs to be applied to
the CPU, which modifies the VERW instruction to flush the affected CPU
buffers.

The kernel already has a facility to flush CPU buffers before returning
to userspace, which is indicated by the X86_FEATURE_CLEAR_CPU_BUF flag.

Ensure the mitigation before enabling Key Locker. Do not enable the
feature on CPUs affected by the vulnerability but lacks mitigation.

Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
---
Change from v8:
* Add as a new patch.

Note that the code change follows the mitigation guidance [1]:
  "Software loading Key Locker keys using LOADIWKEY should execute a VERW
   to clear registers before transitioning to untrusted code to prevent
   later software from inferring the loaded key."

[1] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/register-file-data-sampling.html
---
 arch/x86/kernel/keylocker.c | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/arch/x86/kernel/keylocker.c b/arch/x86/kernel/keylocker.c
index d4f3aa65ea8a..6e805c4da76d 100644
--- a/arch/x86/kernel/keylocker.c
+++ b/arch/x86/kernel/keylocker.c
@@ -135,12 +135,29 @@ static bool __init have_gds_mitigation(void)
 	return false;
 }
 
+/*
+ * IA32_ARCH_CAPABILITIES MSR is retrieved during the setting of
+ * X86_BUG_RFDS. Ensure that the mitigation is applied to flush CPU
+ * buffers by checking the flag.
+ */
+static bool __init have_rfds_mitigation(void)
+{
+	if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
+		return true;
+
+	pr_warn("x86/keylocker: Susceptible to the RFDS vulnerability.\n");
+	return false;
+}
+
 /* Check if Key Locker is secure enough to be used. */
 static bool __init secure_keylocker(void)
 {
 	if (boot_cpu_has_bug(X86_BUG_GDS) && !have_gds_mitigation())
 		return false;
 
+	if (boot_cpu_has_bug(X86_BUG_RFDS) && !have_rfds_mitigation())
+		return false;
+
 	return true;
 }
 
-- 
2.34.1