Processor MMIO Stale Data Vulnerabilities¶
Processor MMIO Stale Data Vulnerabilities are a class of memory-mapped I/O (MMIO) vulnerabilities that can expose data. The sequences of operations for exposing data range from simple to very complex. Because most of the vulnerabilities require the attacker to have access to MMIO, many environments are not affected. System environments using virtualization where MMIO access is provided to untrusted guests may need mitigation. These vulnerabilities are not transient execution attacks. However, these vulnerabilities may propagate stale data into core fill buffers where the data can subsequently be inferred by an unmitigated transient execution attack. Mitigation for these vulnerabilities includes a combination of microcode update and software changes, depending on the platform and usage model. Some of these mitigations are similar to those used to mitigate Microarchitectural Data Sampling (MDS) or those used to mitigate Special Register Buffer Data Sampling (SRBDS).
Propagators are operations that result in stale data being copied or moved from one microarchitectural buffer or register to another. Processor MMIO Stale Data Vulnerabilities are operations that may result in stale data being directly read into an architectural, software-visible state or sampled from a buffer or register.
Fill Buffer Stale Data Propagator (FBSDP)¶
Stale data may propagate from fill buffers (FB) into the non-coherent portion of the uncore on some non-coherent writes. Fill buffer propagation by itself does not make stale data architecturally visible. Stale data must be propagated to a location where it is subject to reading or sampling.
Sideband Stale Data Propagator (SSDP)¶
The sideband stale data propagator (SSDP) is limited to the client (including Intel Xeon server E3) uncore implementation. The sideband response buffer is shared by all client cores. For non-coherent reads that go to sideband destinations, the uncore logic returns 64 bytes of data to the core, including both requested data and unrequested stale data, from a transaction buffer and the sideband response buffer. As a result, stale data from the sideband response and transaction buffers may now reside in a core fill buffer.
Primary Stale Data Propagator (PSDP)¶
The primary stale data propagator (PSDP) is limited to the client (including Intel Xeon server E3) uncore implementation. Similar to the sideband response buffer, the primary response buffer is shared by all client cores. For some processors, MMIO primary reads will return 64 bytes of data to the core fill buffer including both requested data and unrequested stale data. This is similar to the sideband stale data propagator.
Device Register Partial Write (DRPW) (CVE-2022-21166)¶
Some endpoint MMIO registers incorrectly handle writes that are smaller than the register size. Instead of aborting the write or only copying the correct subset of bytes (for example, 2 bytes for a 2-byte write), more bytes than specified by the write transaction may be written to the register. On processors affected by FBSDP, this may expose stale data from the fill buffers of the core that created the write transaction.
Not all the CPUs are affected by all the variants. For instance, most processors for the server market (excluding Intel Xeon E3 processors) are impacted by only Device Register Partial Write (DRPW).
Below is the list of affected Intel processors 1:
9 to 12
9 to 13
If a CPU is in the affected processor list, but not affected by a variant, it is indicated by new bits in MSR IA32_ARCH_CAPABILITIES. As described in a later section, mitigation largely remains the same for all the variants, i.e. to clear the CPU fill buffers via VERW instruction.
New bits in MSRs¶
Newer processors and microcode update on existing affected processors added new bits to IA32_ARCH_CAPABILITIES MSR. These bits can be used to enumerate specific variants of Processor MMIO Stale Data vulnerabilities and mitigation capability.
- Bit 13 - SBDR_SSDP_NO - When set, processor is not affected by either the
Shared Buffers Data Read (SBDR) vulnerability or the sideband stale data propagator (SSDP).
- Bit 14 - FBSDP_NO - When set, processor is not affected by the Fill Buffer
Stale Data Propagator (FBSDP).
- Bit 15 - PSDP_NO - When set, processor is not affected by Primary Stale Data
- Bit 17 - FB_CLEAR - When set, VERW instruction will overwrite CPU fill buffer
values as part of MD_CLEAR operations. Processors that do not enumerate MDS_NO (meaning they are affected by MDS) but that do enumerate support for both L1D_FLUSH and MD_CLEAR implicitly enumerate FB_CLEAR as part of their MD_CLEAR support.
- Bit 18 - FB_CLEAR_CTRL - Processor supports read and write to MSR
IA32_MCU_OPT_CTRL[FB_CLEAR_DIS]. On such processors, the FB_CLEAR_DIS bit can be set to cause the VERW instruction to not perform the FB_CLEAR action. Not all processors that support FB_CLEAR will support FB_CLEAR_CTRL.
Bit 3 - FB_CLEAR_DIS - When set, VERW instruction does not perform the FB_CLEAR action. This may be useful to reduce the performance impact of FB_CLEAR in cases where system software deems it warranted (for example, when performance is more critical, or the untrusted software has no MMIO access). Note that FB_CLEAR_DIS has no impact on enumeration (for example, it does not change FB_CLEAR or MD_CLEAR enumeration) and it may not be supported on all processors that enumerate FB_CLEAR.
Like MDS, all variants of Processor MMIO Stale Data vulnerabilities have 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.
Kernel reuses the MDS function to invoke the buffer clearing:
On MDS affected CPUs, the kernel already invokes CPU buffer clear on kernel/userspace, hypervisor/guest and C-state (idle) transitions. No additional mitigation is needed on such CPUs.
For CPUs not affected by MDS or TAA, mitigation is needed only for the attacker with MMIO capability. Therefore, VERW is not required for kernel/userspace. For virtualization case, VERW is only needed at VMENTER for a guest with MMIO capability.
Return to user space¶
Same mitigation as MDS when affected by MDS/TAA, otherwise no mitigation needed.
Control register writes by CPU during C-state transition can propagate data from fill buffer to uncore buffers. Execute VERW before C-state transition to clear CPU fill buffers.
Guest entry point¶
Same mitigation as MDS when processor is also affected by MDS/TAA, otherwise execute VERW at VMENTER only for MMIO capable guests. On CPUs not affected by MDS/TAA, guest without MMIO access cannot extract secrets using Processor MMIO Stale Data vulnerabilities, so there is no need to execute VERW for such guests.
Mitigation control on the kernel command line¶
The kernel command line allows to control the Processor MMIO Stale Data mitigations at boot time with the option “mmio_stale_data=”. The valid arguments for this option are:
If the CPU is vulnerable, enable mitigation; CPU buffer clearing on exit to userspace and when entering a VM. Idle transitions are protected as well. It does not automatically disable SMT.
Same as full, with SMT disabled on vulnerable CPUs. This is the complete mitigation.
Disables mitigation completely.
If the CPU is affected and mmio_stale_data=off is not supplied on the kernel command line, then the kernel selects the appropriate mitigation.
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:
The possible values in this file are:
The processor is not vulnerable
The processor is vulnerable, but no mitigation enabled
‘Vulnerable: Clear CPU buffers attempted, no microcode’
The processor is vulnerable, but microcode is not updated. The mitigation is enabled on a best effort basis.
‘Mitigation: Clear CPU buffers’
The processor is vulnerable and the CPU buffer clearing mitigation is enabled.
If the processor is vulnerable then the following information is appended to the above information:
SMT is enabled
SMT is disabled
‘SMT Host state unknown’
Kernel runs in a VM, Host SMT state unknown