Merge tag 'gds-for-linus-2023-08-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip

Pull x86/gds fixes from Dave Hansen:
 "Mitigate Gather Data Sampling issue:

   - Add Base GDS mitigation

   - Support GDS_NO under KVM

   - Fix a documentation typo"

* tag 'gds-for-linus-2023-08-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
  Documentation/x86: Fix backwards on/off logic about YMM support
  KVM: Add GDS_NO support to KVM
  x86/speculation: Add Kconfig option for GDS
  x86/speculation: Add force option to GDS mitigation
  x86/speculation: Add Gather Data Sampling mitigation

4 files changed, 151 insertions, 19 deletions

diff --git a/Documentation/ABI/testing/sysfs-devices-system-cpu b/Documentation/ABI/testing/sysfs-devices-system-cpu
index ecd585ca2d50..77942eedf4f6 100644
--- a/Documentation/ABI/testing/sysfs-devices-system-cpu
+++ b/Documentation/ABI/testing/sysfs-devices-system-cpu
@@ -513,17 +513,18 @@ Description:	information about CPUs heterogeneity.
 		cpu_capacity: capacity of cpuX.
 
 What:		/sys/devices/system/cpu/vulnerabilities
+		/sys/devices/system/cpu/vulnerabilities/gather_data_sampling
+		/sys/devices/system/cpu/vulnerabilities/itlb_multihit
+		/sys/devices/system/cpu/vulnerabilities/l1tf
+		/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/retbleed
+		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass
 		/sys/devices/system/cpu/vulnerabilities/spectre_v1
 		/sys/devices/system/cpu/vulnerabilities/spectre_v2
-		/sys/devices/system/cpu/vulnerabilities/spec_store_bypass
-		/sys/devices/system/cpu/vulnerabilities/l1tf
-		/sys/devices/system/cpu/vulnerabilities/mds
 		/sys/devices/system/cpu/vulnerabilities/srbds
 		/sys/devices/system/cpu/vulnerabilities/tsx_async_abort
-		/sys/devices/system/cpu/vulnerabilities/itlb_multihit
-		/sys/devices/system/cpu/vulnerabilities/mmio_stale_data
-		/sys/devices/system/cpu/vulnerabilities/retbleed
 Date:		January 2018
 Contact:	Linux kernel mailing list <linux-kernel@vger.kernel.org>
 Description:	Information about CPU vulnerabilities
diff --git a/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst b/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst
new file mode 100644
index 000000000000..264bfa937f7d
--- /dev/null
+++ b/Documentation/admin-guide/hw-vuln/gather_data_sampling.rst
@@ -0,0 +1,109 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+GDS - Gather Data Sampling
+==========================
+
+Gather Data Sampling is a hardware vulnerability which allows unprivileged
+speculative access to data which was previously stored in vector registers.
+
+Problem
+-------
+When a gather instruction performs loads from memory, different data elements
+are merged into the destination vector register. However, when a gather
+instruction that is transiently executed encounters a fault, stale data from
+architectural or internal vector registers may get transiently forwarded to the
+destination vector register instead. This will allow a malicious attacker to
+infer stale data using typical side channel techniques like cache timing
+attacks. GDS is a purely sampling-based attack.
+
+The attacker uses gather instructions to infer the stale vector register data.
+The victim does not need to do anything special other than use the vector
+registers. The victim does not need to use gather instructions to be
+vulnerable.
+
+Because the buffers are shared between Hyper-Threads cross Hyper-Thread attacks
+are possible.
+
+Attack scenarios
+----------------
+Without mitigation, GDS can infer stale data across virtually all
+permission boundaries:
+
+	Non-enclaves can infer SGX enclave data
+	Userspace can infer kernel data
+	Guests can infer data from hosts
+	Guest can infer guest from other guests
+	Users can infer data from other users
+
+Because of this, it is important to ensure that the mitigation stays enabled in
+lower-privilege contexts like guests and when running outside SGX enclaves.
+
+The hardware enforces the mitigation for SGX. Likewise, VMMs should  ensure
+that guests are not allowed to disable the GDS mitigation. If a host erred and
+allowed this, a guest could theoretically disable GDS mitigation, mount an
+attack, and re-enable it.
+
+Mitigation mechanism
+--------------------
+This issue is mitigated in microcode. The microcode defines the following new
+bits:
+
+ ================================   ===   ============================
+ IA32_ARCH_CAPABILITIES[GDS_CTRL]   R/O   Enumerates GDS vulnerability
+                                          and mitigation support.
+ IA32_ARCH_CAPABILITIES[GDS_NO]     R/O   Processor is not vulnerable.
+ IA32_MCU_OPT_CTRL[GDS_MITG_DIS]    R/W   Disables the mitigation
+                                          0 by default.
+ IA32_MCU_OPT_CTRL[GDS_MITG_LOCK]   R/W   Locks GDS_MITG_DIS=0. Writes
+                                          to GDS_MITG_DIS are ignored
+                                          Can't be cleared once set.
+ ================================   ===   ============================
+
+GDS can also be mitigated on systems that don't have updated microcode by
+disabling AVX. This can be done by setting gather_data_sampling="force" or
+"clearcpuid=avx" on the kernel command-line.
+
+If used, these options will disable AVX use by turning off XSAVE YMM support.
+However, the processor will still enumerate AVX support.  Userspace that
+does not follow proper AVX enumeration to check both AVX *and* XSAVE YMM
+support will break.
+
+Mitigation control on the kernel command line
+---------------------------------------------
+The mitigation can be disabled by setting "gather_data_sampling=off" or
+"mitigations=off" on the kernel command line. Not specifying either will default
+to the mitigation being enabled. Specifying "gather_data_sampling=force" will
+use the microcode mitigation when available or disable AVX on affected systems
+where the microcode hasn't been updated to include the mitigation.
+
+GDS System Information
+------------------------
+The kernel provides vulnerability status information through sysfs. For
+GDS this can be accessed by the following sysfs file:
+
+/sys/devices/system/cpu/vulnerabilities/gather_data_sampling
+
+The possible values contained in this file are:
+
+ ============================== =============================================
+ Not affected                   Processor not vulnerable.
+ Vulnerable                     Processor vulnerable and mitigation disabled.
+ Vulnerable: No microcode       Processor vulnerable and microcode is missing
+                                mitigation.
+ Mitigation: AVX disabled,
+ no microcode                   Processor is vulnerable and microcode is missing
+                                mitigation. AVX disabled as mitigation.
+ Mitigation: Microcode          Processor is vulnerable and mitigation is in
+                                effect.
+ Mitigation: Microcode (locked) Processor is vulnerable and mitigation is in
+                                effect and cannot be disabled.
+ Unknown: Dependent on
+ hypervisor status              Running on a virtual guest processor that is
+                                affected but with no way to know if host
+                                processor is mitigated or vulnerable.
+ ============================== =============================================
+
+GDS Default mitigation
+----------------------
+The updated microcode will enable the mitigation by default. The kernel's
+default action is to leave the mitigation enabled.
diff --git a/Documentation/admin-guide/hw-vuln/index.rst b/Documentation/admin-guide/hw-vuln/index.rst
index ff4d3fa2a75c..a7d37e124831 100644
--- a/Documentation/admin-guide/hw-vuln/index.rst
+++ b/Documentation/admin-guide/hw-vuln/index.rst
@@ -20,3 +20,4 @@ are configurable at compile, boot or run time.
    processor_mmio_stale_data.rst
    cross-thread-rsb.rst
    srso
+   gather_data_sampling.rst
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index f5ec3dade58e..722b6eca2e93 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1623,6 +1623,26 @@
 			Format: off | on
 			default: on
 
+	gather_data_sampling=
+			[X86,INTEL] Control the Gather Data Sampling (GDS)
+			mitigation.
+
+			Gather Data Sampling is a hardware vulnerability which
+			allows unprivileged speculative access to data which was
+			previously stored in vector registers.
+
+			This issue is mitigated by default in updated microcode.
+			The mitigation may have a performance impact but can be
+			disabled. On systems without the microcode mitigation
+			disabling AVX serves as a mitigation.
+
+			force:	Disable AVX to mitigate systems without
+				microcode mitigation. No effect if the microcode
+				mitigation is present. Known to cause crashes in
+				userspace with buggy AVX enumeration.
+
+			off:	Disable GDS mitigation.
+
 	gcov_persist=	[GCOV] When non-zero (default), profiling data for
 			kernel modules is saved and remains accessible via
 			debugfs, even when the module is unloaded/reloaded.
@@ -3273,24 +3293,25 @@
 				Disable all optional CPU mitigations.  This
 				improves system performance, but it may also
 				expose users to several CPU vulnerabilities.
-				Equivalent to: nopti [X86,PPC]
-					       if nokaslr then kpti=0 [ARM64]
-					       nospectre_v1 [X86,PPC]
-					       nobp=0 [S390]
-					       nospectre_v2 [X86,PPC,S390,ARM64]
-					       spectre_v2_user=off [X86]
-					       spec_store_bypass_disable=off [X86,PPC]
-					       ssbd=force-off [ARM64]
-					       nospectre_bhb [ARM64]
+				Equivalent to: if nokaslr then kpti=0 [ARM64]
+					       gather_data_sampling=off [X86]
+					       kvm.nx_huge_pages=off [X86]
 					       l1tf=off [X86]
 					       mds=off [X86]
-					       tsx_async_abort=off [X86]
-					       kvm.nx_huge_pages=off [X86]
-					       srbds=off [X86,INTEL]
+					       mmio_stale_data=off [X86]
 					       no_entry_flush [PPC]
 					       no_uaccess_flush [PPC]
-					       mmio_stale_data=off [X86]
+					       nobp=0 [S390]
+					       nopti [X86,PPC]
+					       nospectre_bhb [ARM64]
+					       nospectre_v1 [X86,PPC]
+					       nospectre_v2 [X86,PPC,S390,ARM64]
 					       retbleed=off [X86]
+					       spec_store_bypass_disable=off [X86,PPC]
+					       spectre_v2_user=off [X86]
+					       srbds=off [X86,INTEL]
+					       ssbd=force-off [ARM64]
+					       tsx_async_abort=off [X86]
 
 				Exceptions:
 					       This does not have any effect on