NAS-123761 / 22.04 / Update to v5.15.128 to fix CVEs

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

@@ -511,17 +511,18 @@ Description: information about CPUs heterogeneity.
  cpu_capacity: capacity of cpu#.
 
 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 <[email protected]>
 Description: Information about CPU vulnerabilities

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.

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

@@ -19,3 +19,5 @@ are configurable at compile, boot or run time.
  l1d_flush.rst
  processor_mmio_stale_data.rst
  cross-thread-rsb.rst
+ gather_data_sampling.rst
+ srso

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

@@ -0,0 +1,133 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Speculative Return Stack Overflow (SRSO)
+========================================
+
+This is a mitigation for the speculative return stack overflow (SRSO)
+vulnerability found on AMD processors. The mechanism is by now the well
+known scenario of poisoning CPU functional units - the Branch Target
+Buffer (BTB) and Return Address Predictor (RAP) in this case - and then
+tricking the elevated privilege domain (the kernel) into leaking
+sensitive data.
+
+AMD CPUs predict RET instructions using a Return Address Predictor (aka
+Return Address Stack/Return Stack Buffer). In some cases, a non-architectural
+CALL instruction (i.e., an instruction predicted to be a CALL but is
+not actually a CALL) can create an entry in the RAP which may be used
+to predict the target of a subsequent RET instruction.
+
+The specific circumstances that lead to this varies by microarchitecture
+but the concern is that an attacker can mis-train the CPU BTB to predict
+non-architectural CALL instructions in kernel space and use this to
+control the speculative target of a subsequent kernel RET, potentially
+leading to information disclosure via a speculative side-channel.
+
+The issue is tracked under CVE-2023-20569.
+
+Affected processors
+-------------------
+
+AMD Zen, generations 1-4. That is, all families 0x17 and 0x19. Older
+processors have not been investigated.
+
+System information and options
+------------------------------
+
+First of all, it is required that the latest microcode be loaded for
+mitigations to be effective.
+
+The sysfs file showing SRSO mitigation status is:
+
+ /sys/devices/system/cpu/vulnerabilities/spec_rstack_overflow
+
+The possible values in this file are:
+
+ - 'Not affected' The processor is not vulnerable
+
+ - 'Vulnerable: no microcode' The processor is vulnerable, no
+ microcode extending IBPB functionality
+ to address the vulnerability has been
+ applied.
+
+ - 'Mitigation: microcode' Extended IBPB functionality microcode
+ patch has been applied. It does not
+ address User->Kernel and Guest->Host
+ transitions protection but it does
+ address User->User and VM->VM attack
+ vectors.
+
+ (spec_rstack_overflow=microcode)
+
+ - 'Mitigation: safe RET' Software-only mitigation. It complements
+ the extended IBPB microcode patch
+ functionality by addressing User->Kernel
+ and Guest->Host transitions protection.
+
+ Selected by default or by
+ spec_rstack_overflow=safe-ret
+
+ - 'Mitigation: IBPB' Similar protection as "safe RET" above
+ but employs an IBPB barrier on privilege
+ domain crossings (User->Kernel,
+ Guest->Host).
+
+ (spec_rstack_overflow=ibpb)
+
+ - 'Mitigation: IBPB on VMEXIT' Mitigation addressing the cloud provider
+ scenario - the Guest->Host transitions
+ only.
+
+ (spec_rstack_overflow=ibpb-vmexit)
+
+In order to exploit vulnerability, an attacker needs to:
+
+ - gain local access on the machine
+
+ - break kASLR
+
+ - find gadgets in the running kernel in order to use them in the exploit
+
+ - potentially create and pin an additional workload on the sibling
+ thread, depending on the microarchitecture (not necessary on fam 0x19)
+
+ - run the exploit
+
+Considering the performance implications of each mitigation type, the
+default one is 'Mitigation: safe RET' which should take care of most
+attack vectors, including the local User->Kernel one.
+
+As always, the user is advised to keep her/his system up-to-date by
+applying software updates regularly.
+
+The default setting will be reevaluated when needed and especially when
+new attack vectors appear.
+
+As one can surmise, 'Mitigation: safe RET' does come at the cost of some
+performance depending on the workload. If one trusts her/his userspace
+and does not want to suffer the performance impact, one can always
+disable the mitigation with spec_rstack_overflow=off.
+
+Similarly, 'Mitigation: IBPB' is another full mitigation type employing
+an indrect branch prediction barrier after having applied the required
+microcode patch for one's system. This mitigation comes also at
+a performance cost.
+
+Mitigation: safe RET
+--------------------
+
+The mitigation works by ensuring all RET instructions speculate to
+a controlled location, similar to how speculation is controlled in the
+retpoline sequence. To accomplish this, the __x86_return_thunk forces
+the CPU to mispredict every function return using a 'safe return'
+sequence.
+
+To ensure the safety of this mitigation, the kernel must ensure that the
+safe return sequence is itself free from attacker interference. In Zen3
+and Zen4, this is accomplished by creating a BTB alias between the
+untraining function srso_alias_untrain_ret() and the safe return
+function srso_alias_safe_ret() which results in evicting a potentially
+poisoned BTB entry and using that safe one for all function returns.
+
+In older Zen1 and Zen2, this is accomplished using a reinterpretation
+technique similar to Retbleed one: srso_untrain_ret() and
+srso_safe_ret().

Documentation/admin-guide/kernel-parameters.txt

@@ -1501,6 +1501,26 @@
  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.
@@ -3034,22 +3054,23 @@
  Disable all optional CPU mitigations. This
  improves system performance, but it may also
  expose users to several CPU vulnerabilities.
- Equivalent to: nopti [X86,PPC]
+ Equivalent to: gather_data_sampling=off [X86]
  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]
+ kvm.nx_huge_pages=off [X86]
  l1tf=off [X86]
  mds=off [X86]
- tsx_async_abort=off [X86]
- kvm.nx_huge_pages=off [X86]
+ 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_v1 [X86,PPC]
+ nospectre_v2 [X86,PPC,S390,ARM64]
  retbleed=off [X86]
+ spec_store_bypass_disable=off [X86,PPC]
+ spectre_v2_user=off [X86]
+ ssbd=force-off [ARM64]
+ tsx_async_abort=off [X86]
 
  Exceptions:
  This does not have any effect on
@@ -5429,6 +5450,17 @@
  Not specifying this option is equivalent to
  spectre_v2_user=auto.
 
+ spec_rstack_overflow=
+ [X86] Control RAS overflow mitigation on AMD Zen CPUs
+
+ off - Disable mitigation
+ microcode - Enable microcode mitigation only
+ safe-ret - Enable sw-only safe RET mitigation (default)
+ ibpb - Enable mitigation by issuing IBPB on
+ kernel entry
+ ibpb-vmexit - Issue IBPB only on VMEXIT
+ (cloud-specific mitigation)
+
  spec_store_bypass_disable=
  [HW] Control Speculative Store Bypass (SSB) Disable mitigation
  (Speculative Store Bypass vulnerability)