• Pawan Gupta's avatar
    x86/speculation/taa: Add mitigation for TSX Async Abort · 1b42f017
    Pawan Gupta authored
    TSX Async Abort (TAA) is a side channel vulnerability to the internal
    buffers in some Intel processors similar to Microachitectural Data
    Sampling (MDS). In this case, certain loads may speculatively pass
    invalid data to dependent operations when an asynchronous abort
    condition is pending in a TSX transaction.
    
    This includes loads with no fault or assist condition. Such loads may
    speculatively expose stale data from the uarch data structures as in
    MDS. Scope of exposure is within the same-thread and cross-thread. This
    issue affects all current processors that support TSX, but do not have
    ARCH_CAP_TAA_NO (bit 8) set in MSR_IA32_ARCH_CAPABILITIES.
    
    On CPUs which have their IA32_ARCH_CAPABILITIES MSR bit MDS_NO=0,
    CPUID.MD_CLEAR=1 and the MDS mitigation is clearing the CPU buffers
    using VERW or L1D_FLUSH, there is no additional mitigation needed for
    TAA. On affected CPUs with MDS_NO=1 this issue can be mitigated by
    disabling the Transactional Synchronization Extensions (TSX) feature.
    
    A new MSR IA32_TSX_CTRL in future and current processors after a
    microcode update can be used to control the TSX feature. There are two
    bits in that MSR:
    
    * TSX_CTRL_RTM_DISABLE disables the TSX sub-feature Restricted
    Transactional Memory (RTM).
    
    * TSX_CTRL_CPUID_CLEAR clears the RTM enumeration in CPUID. The other
    TSX sub-feature, Hardware Lock Elision (HLE), is unconditionally
    disabled with updated microcode but still enumerated as present by
    CPUID(EAX=7).EBX{bit4}.
    
    The second mitigation approach is similar to MDS which is clearing the
    affected CPU buffers on return to user space and when entering a guest.
    Relevant microcode update is required for the mitigation to work.  More
    details on this approach can be found here:
    
      https://www.kernel.org/doc/html/latest/admin-guide/hw-vuln/mds.html
    
    The TSX feature can be controlled by the "tsx" command line parameter.
    If it is force-enabled then "Clear CPU buffers" (MDS mitigation) is
    deployed. The effective mitigation state can be read from sysfs.
    
     [ bp:
       - massage + comments cleanup
       - s/TAA_MITIGATION_TSX_DISABLE/TAA_MITIGATION_TSX_DISABLED/g - Josh.
       - remove partial TAA mitigation in update_mds_branch_idle() - Josh.
       - s/tsx_async_abort_cmdline/tsx_async_abort_parse_cmdline/g
     ]
    Signed-off-by: default avatarPawan Gupta <pawan.kumar.gupta@linux.intel.com>
    Signed-off-by: default avatarBorislav Petkov <bp@suse.de>
    Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
    Reviewed-by: default avatarJosh Poimboeuf <jpoimboe@redhat.com>
    1b42f017
processor.h 25 KB