Merge tag 'rfds-for-linus-2024-03-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86 RFDS mitigation from Dave Hansen:
 "RFDS is a CPU vulnerability that may allow a malicious userspace to
  infer stale register values from kernel space. Kernel registers can
  have all kinds of secrets in them so the mitigation is basically to
  wait until the kernel is about to return to userspace and has user
  values in the registers. At that point there is little chance of
  kernel secrets ending up in the registers and the microarchitectural
  state can be cleared.

  This leverages some recent robustness fixes for the existing MDS
  vulnerability. Both MDS and RFDS use the VERW instruction for
  mitigation"

* tag 'rfds-for-linus-2024-03-11' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  KVM/x86: Export RFDS_NO and RFDS_CLEAR to guests
  x86/rfds: Mitigate Register File Data Sampling (RFDS)
  Documentation/hw-vuln: Add documentation for RFDS
  x86/mmio: Disable KVM mitigation when X86_FEATURE_CLEAR_CPU_BUF is set

Author: torvalds

Documentation/ABI/testing/sysfs-devices-system-cpu

@@ -516,6 +516,7 @@ What:		/sys/devices/system/cpu/vulnerabilities
 		/sys/devices/system/cpu/vulnerabilities/mds
 		/sys/devices/system/cpu/vulnerabilities/meltdown
 		/sys/devices/system/cpu/vulnerabilities/mmio_stale_data
+		/sys/devices/system/cpu/vulnerabilities/reg_file_data_sampling
 		/sys/devices/system/cpu/vulnerabilities/retbleed
 		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass
 		/sys/devices/system/cpu/vulnerabilities/spectre_v1

Documentation/admin-guide/hw-vuln/index.rst

@@ -21,3 +21,4 @@ are configurable at compile, boot or run time.
    cross-thread-rsb
    srso
    gather_data_sampling
+   reg-file-data-sampling

Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst

@@ -0,0 +1,104 @@
+==================================
+Register File Data Sampling (RFDS)
+==================================
+
+Register File Data Sampling (RFDS) is a microarchitectural vulnerability that
+only affects Intel Atom parts(also branded as E-cores). RFDS may allow
+a malicious actor to infer data values previously used in floating point
+registers, vector registers, or integer registers. RFDS does not provide the
+ability to choose which data is inferred. CVE-2023-28746 is assigned to RFDS.
+
+Affected Processors
+===================
+Below is the list of affected Intel processors [#f1]_:
+
+   ===================  ============
+   Common name          Family_Model
+   ===================  ============
+   ATOM_GOLDMONT           06_5CH
+   ATOM_GOLDMONT_D         06_5FH
+   ATOM_GOLDMONT_PLUS      06_7AH
+   ATOM_TREMONT_D          06_86H
+   ATOM_TREMONT            06_96H
+   ALDERLAKE               06_97H
+   ALDERLAKE_L             06_9AH
+   ATOM_TREMONT_L          06_9CH
+   RAPTORLAKE              06_B7H
+   RAPTORLAKE_P            06_BAH
+   ATOM_GRACEMONT          06_BEH
+   RAPTORLAKE_S            06_BFH
+   ===================  ============
+
+As an exception to this table, Intel Xeon E family parts ALDERLAKE(06_97H) and
+RAPTORLAKE(06_B7H) codenamed Catlow are not affected. They are reported as
+vulnerable in Linux because they share the same family/model with an affected
+part. Unlike their affected counterparts, they do not enumerate RFDS_CLEAR or
+CPUID.HYBRID. This information could be used to distinguish between the
+affected and unaffected parts, but it is deemed not worth adding complexity as
+the reporting is fixed automatically when these parts enumerate RFDS_NO.
+
+Mitigation
+==========
+Intel released a microcode update that enables software to clear sensitive
+information using the VERW instruction. Like MDS, RFDS deploys the same
+mitigation strategy to force the CPU to clear the affected buffers before an
+attacker can extract the secrets. This is achieved by using the otherwise
+unused and obsolete VERW instruction in combination with a microcode update.
+The microcode clears the affected CPU buffers when the VERW instruction is
+executed.
+
+Mitigation points
+-----------------
+VERW is executed by the kernel before returning to user space, and by KVM
+before VMentry. None of the affected cores support SMT, so VERW is not required
+at C-state transitions.
+
+New bits in IA32_ARCH_CAPABILITIES
+----------------------------------
+Newer processors and microcode update on existing affected processors added new
+bits to IA32_ARCH_CAPABILITIES MSR. These bits can be used to enumerate
+vulnerability and mitigation capability:
+
+- Bit 27 - RFDS_NO - When set, processor is not affected by RFDS.
+- Bit 28 - RFDS_CLEAR - When set, processor is affected by RFDS, and has the
+  microcode that clears the affected buffers on VERW execution.
+
+Mitigation control on the kernel command line
+---------------------------------------------
+The kernel command line allows to control RFDS mitigation at boot time with the
+parameter "reg_file_data_sampling=". The valid arguments are:
+
+  ==========  =================================================================
+  on          If the CPU is vulnerable, enable mitigation; CPU buffer clearing
+              on exit to userspace and before entering a VM.
+  off         Disables mitigation.
+  ==========  =================================================================
+
+Mitigation default is selected by CONFIG_MITIGATION_RFDS.
+
+Mitigation status information
+-----------------------------
+The Linux kernel provides a sysfs interface to enumerate the current
+vulnerability status of the system: whether the system is vulnerable, and
+which mitigations are active. The relevant sysfs file is:
+
+	/sys/devices/system/cpu/vulnerabilities/reg_file_data_sampling
+
+The possible values in this file are:
+
+  .. list-table::
+
+     * - 'Not affected'
+       - The processor is not vulnerable
+     * - 'Vulnerable'
+       - The processor is vulnerable, but no mitigation enabled
+     * - 'Vulnerable: No microcode'
+       - The processor is vulnerable but microcode is not updated.
+     * - 'Mitigation: Clear Register File'
+       - The processor is vulnerable and the CPU buffer clearing mitigation is
+	 enabled.
+
+References
+----------
+.. [#f1] Affected Processors
+   https://www.intel.com/content/www/us/en/developer/topic-technology/software-security-guidance/processors-affected-consolidated-product-cpu-model.html

Documentation/admin-guide/kernel-parameters.txt

@@ -1136,6 +1136,26 @@
 			The filter can be disabled or changed to another
 			driver later using sysfs.
 
+	reg_file_data_sampling=
+			[X86] Controls mitigation for Register File Data
+			Sampling (RFDS) vulnerability. RFDS is a CPU
+			vulnerability which may allow userspace to infer
+			kernel data values previously stored in floating point
+			registers, vector registers, or integer registers.
+			RFDS only affects Intel Atom processors.
+
+			on:	Turns ON the mitigation.
+			off:	Turns OFF the mitigation.
+
+			This parameter overrides the compile time default set
+			by CONFIG_MITIGATION_RFDS. Mitigation cannot be
+			disabled when other VERW based mitigations (like MDS)
+			are enabled. In order to disable RFDS mitigation all
+			VERW based mitigations need to be disabled.
+
+			For details see:
+			Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst
+
 	driver_async_probe=  [KNL]
 			List of driver names to be probed asynchronously. *
 			matches with all driver names. If * is specified, the
@@ -3394,6 +3414,7 @@
 					       nospectre_bhb [ARM64]
 					       nospectre_v1 [X86,PPC]
 					       nospectre_v2 [X86,PPC,S390,ARM64]
+					       reg_file_data_sampling=off [X86]
 					       retbleed=off [X86]
 					       spec_rstack_overflow=off [X86]
 					       spec_store_bypass_disable=off [X86,PPC]

arch/x86/Kconfig

@@ -2623,6 +2623,17 @@ config MITIGATION_GDS_FORCE
 
 	  If in doubt, say N.
 
+config MITIGATION_RFDS
+	bool "RFDS Mitigation"
+	depends on CPU_SUP_INTEL
+	default y
+	help
+	  Enable mitigation for Register File Data Sampling (RFDS) by default.
+	  RFDS is a hardware vulnerability which affects Intel Atom CPUs. It
+	  allows unprivileged speculative access to stale data previously
+	  stored in floating point, vector and integer registers.
+	  See also <file:Documentation/admin-guide/hw-vuln/reg-file-data-sampling.rst>
+
 endif
 
 config ARCH_HAS_ADD_PAGES

arch/x86/include/asm/cpufeatures.h

@@ -506,4 +506,5 @@
 /* BUG word 2 */
 #define X86_BUG_SRSO			X86_BUG(1*32 + 0) /* AMD SRSO bug */
 #define X86_BUG_DIV0			X86_BUG(1*32 + 1) /* AMD DIV0 speculation bug */
+#define X86_BUG_RFDS			X86_BUG(1*32 + 2) /* CPU is vulnerable to Register File Data Sampling */
 #endif /* _ASM_X86_CPUFEATURES_H */

arch/x86/include/asm/msr-index.h

@@ -176,6 +176,14 @@
 						 * CPU is not vulnerable to Gather
 						 * Data Sampling (GDS).
 						 */
+#define ARCH_CAP_RFDS_NO		BIT(27)	/*
+						 * Not susceptible to Register
+						 * File Data Sampling.
+						 */
+#define ARCH_CAP_RFDS_CLEAR		BIT(28)	/*
+						 * VERW clears CPU Register
+						 * File.
+						 */
 
 #define ARCH_CAP_XAPIC_DISABLE		BIT(21)	/*
 						 * IA32_XAPIC_DISABLE_STATUS MSR

arch/x86/kernel/cpu/bugs.c

@@ -422,6 +422,13 @@ static void __init mmio_select_mitigation(void)
 	if (boot_cpu_has_bug(X86_BUG_MDS) || (boot_cpu_has_bug(X86_BUG_TAA) &&
 					      boot_cpu_has(X86_FEATURE_RTM)))
 		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+
+	/*
+	 * X86_FEATURE_CLEAR_CPU_BUF could be enabled by other VERW based
+	 * mitigations, disable KVM-only mitigation in that case.
+	 */
+	if (boot_cpu_has(X86_FEATURE_CLEAR_CPU_BUF))
+		static_branch_disable(&mmio_stale_data_clear);
 	else
 		static_branch_enable(&mmio_stale_data_clear);
 
@@ -473,6 +480,57 @@ static int __init mmio_stale_data_parse_cmdline(char *str)
 }
 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 =
+	IS_ENABLED(CONFIG_MITIGATION_RFDS) ? RFDS_MITIGATION_VERW : RFDS_MITIGATION_OFF;
+
+static const char * const rfds_strings[] = {
+	[RFDS_MITIGATION_OFF]			= "Vulnerable",
+	[RFDS_MITIGATION_VERW]			= "Mitigation: Clear Register File",
+	[RFDS_MITIGATION_UCODE_NEEDED]		= "Vulnerable: No microcode",
+};
+
+static void __init rfds_select_mitigation(void)
+{
+	if (!boot_cpu_has_bug(X86_BUG_RFDS) || cpu_mitigations_off()) {
+		rfds_mitigation = RFDS_MITIGATION_OFF;
+		return;
+	}
+	if (rfds_mitigation == RFDS_MITIGATION_OFF)
+		return;
+
+	if (x86_read_arch_cap_msr() & ARCH_CAP_RFDS_CLEAR)
+		setup_force_cpu_cap(X86_FEATURE_CLEAR_CPU_BUF);
+	else
+		rfds_mitigation = RFDS_MITIGATION_UCODE_NEEDED;
+}
+
+static __init int rfds_parse_cmdline(char *str)
+{
+	if (!str)
+		return -EINVAL;
+
+	if (!boot_cpu_has_bug(X86_BUG_RFDS))
+		return 0;
+
+	if (!strcmp(str, "off"))
+		rfds_mitigation = RFDS_MITIGATION_OFF;
+	else if (!strcmp(str, "on"))
+		rfds_mitigation = RFDS_MITIGATION_VERW;
+
+	return 0;
+}
+early_param("reg_file_data_sampling", rfds_parse_cmdline);
+
 #undef pr_fmt
 #define pr_fmt(fmt)     "" fmt
 
@@ -498,11 +556,19 @@ static void __init md_clear_update_mitigation(void)
 		taa_mitigation = TAA_MITIGATION_VERW;
 		taa_select_mitigation();
 	}
-	if (mmio_mitigation == MMIO_MITIGATION_OFF &&
-	    boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
+	/*
+	 * MMIO_MITIGATION_OFF is not checked here so that mmio_stale_data_clear
+	 * gets updated correctly as per X86_FEATURE_CLEAR_CPU_BUF state.
+	 */
+	if (boot_cpu_has_bug(X86_BUG_MMIO_STALE_DATA)) {
 		mmio_mitigation = MMIO_MITIGATION_VERW;
 		mmio_select_mitigation();
 	}
+	if (rfds_mitigation == RFDS_MITIGATION_OFF &&
+	    boot_cpu_has_bug(X86_BUG_RFDS)) {
+		rfds_mitigation = RFDS_MITIGATION_VERW;
+		rfds_select_mitigation();
+	}
 out:
 	if (boot_cpu_has_bug(X86_BUG_MDS))
 		pr_info("MDS: %s\n", mds_strings[mds_mitigation]);
@@ -512,18 +578,21 @@ static void __init md_clear_update_mitigation(void)
 		pr_info("MMIO Stale Data: %s\n", mmio_strings[mmio_mitigation]);
 	else if (boot_cpu_has_bug(X86_BUG_MMIO_UNKNOWN))
 		pr_info("MMIO Stale Data: Unknown: No mitigations\n");
+	if (boot_cpu_has_bug(X86_BUG_RFDS))
+		pr_info("Register File Data Sampling: %s\n", rfds_strings[rfds_mitigation]);
 }
 
 static void __init md_clear_select_mitigation(void)
 {
 	mds_select_mitigation();
 	taa_select_mitigation();
 	mmio_select_mitigation();
+	rfds_select_mitigation();
 
 	/*
-	 * As MDS, TAA and MMIO Stale Data mitigations are inter-related, update
-	 * and print their mitigation after MDS, TAA and MMIO Stale Data
-	 * mitigation selection is done.
+	 * As these mitigations are inter-related and rely on VERW instruction
+	 * to clear the microarchitural buffers, update and print their status
+	 * after mitigation selection is done for each of these vulnerabilities.
 	 */
 	md_clear_update_mitigation();
 }
@@ -2613,6 +2682,11 @@ static ssize_t mmio_stale_data_show_state(char *buf)
 			  sched_smt_active() ? "vulnerable" : "disabled");
 }
 
+static ssize_t rfds_show_state(char *buf)
+{
+	return sysfs_emit(buf, "%s\n", rfds_strings[rfds_mitigation]);
+}
+
 static char *stibp_state(void)
 {
 	if (spectre_v2_in_eibrs_mode(spectre_v2_enabled) &&
@@ -2772,6 +2846,9 @@ static ssize_t cpu_show_common(struct device *dev, struct device_attribute *attr
 	case X86_BUG_GDS:
 		return gds_show_state(buf);
 
+	case X86_BUG_RFDS:
+		return rfds_show_state(buf);
+
 	default:
 		break;
 	}
@@ -2846,6 +2923,11 @@ ssize_t cpu_show_gds(struct device *dev, struct device_attribute *attr, char *bu
 {
 	return cpu_show_common(dev, attr, buf, X86_BUG_GDS);
 }
+
+ssize_t cpu_show_reg_file_data_sampling(struct device *dev, struct device_attribute *attr, char *buf)
+{
+	return cpu_show_common(dev, attr, buf, X86_BUG_RFDS);
+}
 #endif
 
 void __warn_thunk(void)