[PATCH v9a 11/14] x86/cpu/keylocker: Check Register File Data Sampling mitigation

Chang S. Bae
2024-04-07T23:20:37Z

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.

Reference the 'rfds_mitigation' variable to check the mitigation status.
Do not enable Key Locker on CPUs affected by the vulnerability but
lacking mitigation.

Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
Changes from v9:
* Remove the helper function and simplify the code by directly reading
the status variable. (Pawan Gupta)

Note that this code change aligns with mitigation guidance, which
recommends:
"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."

For more information, refer to Intel's guidance on Register File Data
Sampling:
https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/advisory-guidance/register-file-data-sampling.html
---
arch/x86/include/asm/processor.h | 8 ++++++++
arch/x86/kernel/cpu/bugs.c | 8 +-------
arch/x86/kernel/keylocker.c | 3 +++
3 files changed, 12 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index 74eaa3a2b85b..b823163f4786 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -728,6 +728,14 @@ enum mitigation_info {

extern bool gds_ucode_mitigated(enum mitigation_info mitg);

+enum rfds_mitigations {
+ RFDS_MITIGATION_OFF,
+ RFDS_MITIGATION_VERW,
+ RFDS_MITIGATION_UCODE_NEEDED,
+};
+
+extern enum rfds_mitigations rfds_mitigation;
+
/*
* Make previous memory operations globally visible before
* a WRMSR.
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 80f6e70619cb..a2ba1a0ef872 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -483,14 +483,8 @@ early_param("mmio_stale_data", mmio_stale_data_parse_cmdline);
#undef pr_fmt
#define pr_fmt(fmt) "Register File Data Sampling: " fmt

-enum rfds_mitigations {
- RFDS_MITIGATION_OFF,
- RFDS_MITIGATION_VERW,
- RFDS_MITIGATION_UCODE_NEEDED,
-};
-
/* Default mitigation for Register File Data Sampling */
-static enum rfds_mitigations rfds_mitigation __ro_after_init =
+enum rfds_mitigations rfds_mitigation __ro_after_init =
IS_ENABLED(CONFIG_MITIGATION_RFDS) ? RFDS_MITIGATION_VERW : RFDS_MITIGATION_OFF;

static const char * const rfds_strings[] = {
diff --git a/arch/x86/kernel/keylocker.c b/arch/x86/kernel/keylocker.c
index 23cf4a235f11..09876693414c 100644
--- a/arch/x86/kernel/keylocker.c
+++ b/arch/x86/kernel/keylocker.c
@@ -119,6 +119,9 @@ static bool __init secure_keylocker(void)
if (boot_cpu_has_bug(X86_BUG_GDS) && !gds_ucode_mitigated(MITG_LOCKED))
return false;

+ if (boot_cpu_has_bug(X86_BUG_RFDS) && rfds_mitigation != RFDS_MITIGATION_VERW)
+ return false;
+
return true;
}

--
2.40.1