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 <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Pawan Gupta <[email protected]>
---
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
On Thu, Mar 28, 2024 at 06:53:43PM -0700, Chang S. Bae wrote:
> 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 <[email protected]>
> Cc: Dave Hansen <[email protected]>
> Cc: Pawan Gupta <[email protected]>
> ---
> 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;
X86_FEATURE_CLEAR_CPU_BUF is also set by other VERW based mitigations
like MDS. The feature flag does not guarantee that the microcode
required to mitigate RFDS is loaded.
A more robust check would be:
if (rfds_mitigation == RFDS_MITIGATION_VERW)
return true;
And it would be apt to move this function to arch/x86/kernel/cpu/bugs.c
> +
> + 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;
> }
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 <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Pawan Gupta <[email protected]>
---
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