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    Subject[PATCH 4.9 120/151] x86/speculation: Fix incorrect MDS/TAA mitigation status
    Date
    From: Waiman Long <longman@redhat.com>

    commit 64870ed1b12e235cfca3f6c6da75b542c973ff78 upstream.

    For MDS vulnerable processors with TSX support, enabling either MDS or
    TAA mitigations will enable the use of VERW to flush internal processor
    buffers at the right code path. IOW, they are either both mitigated
    or both not. However, if the command line options are inconsistent,
    the vulnerabilites sysfs files may not report the mitigation status
    correctly.

    For example, with only the "mds=off" option:

    vulnerabilities/mds:Vulnerable; SMT vulnerable
    vulnerabilities/tsx_async_abort:Mitigation: Clear CPU buffers; SMT vulnerable

    The mds vulnerabilities file has wrong status in this case. Similarly,
    the taa vulnerability file will be wrong with mds mitigation on, but
    taa off.

    Change taa_select_mitigation() to sync up the two mitigation status
    and have them turned off if both "mds=off" and "tsx_async_abort=off"
    are present.

    Update documentation to emphasize the fact that both "mds=off" and
    "tsx_async_abort=off" have to be specified together for processors that
    are affected by both TAA and MDS to be effective.

    [ bp: Massage and add kernel-parameters.txt change too. ]

    Fixes: 1b42f017415b ("x86/speculation/taa: Add mitigation for TSX Async Abort")
    Signed-off-by: Waiman Long <longman@redhat.com>
    Signed-off-by: Borislav Petkov <bp@suse.de>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Cc: "H. Peter Anvin" <hpa@zytor.com>
    Cc: Ingo Molnar <mingo@redhat.com>
    Cc: Jiri Kosina <jkosina@suse.cz>
    Cc: Jonathan Corbet <corbet@lwn.net>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: linux-doc@vger.kernel.org
    Cc: Mark Gross <mgross@linux.intel.com>
    Cc: <stable@vger.kernel.org>
    Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Tim Chen <tim.c.chen@linux.intel.com>
    Cc: Tony Luck <tony.luck@intel.com>
    Cc: Tyler Hicks <tyhicks@canonical.com>
    Cc: x86-ml <x86@kernel.org>
    Link: https://lkml.kernel.org/r/20191115161445.30809-2-longman@redhat.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    Documentation/hw-vuln/mds.rst | 7 +++++--
    Documentation/hw-vuln/tsx_async_abort.rst | 5 ++++-
    Documentation/kernel-parameters.txt | 11 +++++++++++
    arch/x86/kernel/cpu/bugs.c | 17 +++++++++++++++--
    4 files changed, 35 insertions(+), 5 deletions(-)

    --- a/Documentation/hw-vuln/mds.rst
    +++ b/Documentation/hw-vuln/mds.rst
    @@ -265,8 +265,11 @@ time with the option "mds=". The valid a

    ============ =============================================================

    -Not specifying this option is equivalent to "mds=full".
    -
    +Not specifying this option is equivalent to "mds=full". For processors
    +that are affected by both TAA (TSX Asynchronous Abort) and MDS,
    +specifying just "mds=off" without an accompanying "tsx_async_abort=off"
    +will have no effect as the same mitigation is used for both
    +vulnerabilities.

    Mitigation selection guide
    --------------------------
    --- a/Documentation/hw-vuln/tsx_async_abort.rst
    +++ b/Documentation/hw-vuln/tsx_async_abort.rst
    @@ -174,7 +174,10 @@ the option "tsx_async_abort=". The valid
    CPU is not vulnerable to cross-thread TAA attacks.
    ============ =============================================================

    -Not specifying this option is equivalent to "tsx_async_abort=full".
    +Not specifying this option is equivalent to "tsx_async_abort=full". For
    +processors that are affected by both TAA and MDS, specifying just
    +"tsx_async_abort=off" without an accompanying "mds=off" will have no
    +effect as the same mitigation is used for both vulnerabilities.

    The kernel command line also allows to control the TSX feature using the
    parameter "tsx=" on CPUs which support TSX control. MSR_IA32_TSX_CTRL is used
    --- a/Documentation/kernel-parameters.txt
    +++ b/Documentation/kernel-parameters.txt
    @@ -2365,6 +2365,12 @@ bytes respectively. Such letter suffixes
    SMT on vulnerable CPUs
    off - Unconditionally disable MDS mitigation

    + On TAA-affected machines, mds=off can be prevented by
    + an active TAA mitigation as both vulnerabilities are
    + mitigated with the same mechanism so in order to disable
    + this mitigation, you need to specify tsx_async_abort=off
    + too.
    +
    Not specifying this option is equivalent to
    mds=full.

    @@ -4599,6 +4605,11 @@ bytes respectively. Such letter suffixes
    vulnerable to cross-thread TAA attacks.
    off - Unconditionally disable TAA mitigation

    + On MDS-affected machines, tsx_async_abort=off can be
    + prevented by an active MDS mitigation as both vulnerabilities
    + are mitigated with the same mechanism so in order to disable
    + this mitigation, you need to specify mds=off too.
    +
    Not specifying this option is equivalent to
    tsx_async_abort=full. On CPUs which are MDS affected
    and deploy MDS mitigation, TAA mitigation is not
    --- a/arch/x86/kernel/cpu/bugs.c
    +++ b/arch/x86/kernel/cpu/bugs.c
    @@ -303,8 +303,12 @@ static void __init taa_select_mitigation
    return;
    }

    - /* TAA mitigation is turned off on the cmdline (tsx_async_abort=off) */
    - if (taa_mitigation == TAA_MITIGATION_OFF)
    + /*
    + * TAA mitigation via VERW is turned off if both
    + * tsx_async_abort=off and mds=off are specified.
    + */
    + if (taa_mitigation == TAA_MITIGATION_OFF &&
    + mds_mitigation == MDS_MITIGATION_OFF)
    goto out;

    if (boot_cpu_has(X86_FEATURE_MD_CLEAR))
    @@ -338,6 +342,15 @@ static void __init taa_select_mitigation
    if (taa_nosmt || cpu_mitigations_auto_nosmt())
    cpu_smt_disable(false);

    + /*
    + * Update MDS mitigation, if necessary, as the mds_user_clear is
    + * now enabled for TAA mitigation.
    + */
    + if (mds_mitigation == MDS_MITIGATION_OFF &&
    + boot_cpu_has_bug(X86_BUG_MDS)) {
    + mds_mitigation = MDS_MITIGATION_FULL;
    + mds_select_mitigation();
    + }
    out:
    pr_info("%s\n", taa_strings[taa_mitigation]);
    }

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