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    Subject[PATCH 4.4 205/266] x86/speculation: Disable STIBP when enhanced IBRS is in use
    Date
    From: Tim Chen <tim.c.chen@linux.intel.com>

    commit 34bce7c9690b1d897686aac89604ba7adc365556 upstream.

    If enhanced IBRS is active, STIBP is redundant for mitigating Spectre v2
    user space exploits from hyperthread sibling.

    Disable STIBP when enhanced IBRS is used.

    Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Reviewed-by: Ingo Molnar <mingo@kernel.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Andy Lutomirski <luto@kernel.org>
    Cc: Linus Torvalds <torvalds@linux-foundation.org>
    Cc: Jiri Kosina <jkosina@suse.cz>
    Cc: Tom Lendacky <thomas.lendacky@amd.com>
    Cc: Josh Poimboeuf <jpoimboe@redhat.com>
    Cc: Andrea Arcangeli <aarcange@redhat.com>
    Cc: David Woodhouse <dwmw@amazon.co.uk>
    Cc: Andi Kleen <ak@linux.intel.com>
    Cc: Dave Hansen <dave.hansen@intel.com>
    Cc: Casey Schaufler <casey.schaufler@intel.com>
    Cc: Asit Mallick <asit.k.mallick@intel.com>
    Cc: Arjan van de Ven <arjan@linux.intel.com>
    Cc: Jon Masters <jcm@redhat.com>
    Cc: Waiman Long <longman9394@gmail.com>
    Cc: Greg KH <gregkh@linuxfoundation.org>
    Cc: Dave Stewart <david.c.stewart@intel.com>
    Cc: Kees Cook <keescook@chromium.org>
    Link: https://lkml.kernel.org/r/20181125185003.966801480@linutronix.de
    Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/kernel/cpu/bugs.c | 7 +++++++
    1 file changed, 7 insertions(+)

    --- a/arch/x86/kernel/cpu/bugs.c
    +++ b/arch/x86/kernel/cpu/bugs.c
    @@ -317,6 +317,10 @@ static bool stibp_needed(void)
    if (spectre_v2_enabled == SPECTRE_V2_NONE)
    return false;

    + /* Enhanced IBRS makes using STIBP unnecessary. */
    + if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
    + return false;
    +
    if (!boot_cpu_has(X86_FEATURE_STIBP))
    return false;

    @@ -761,6 +765,9 @@ static void __init l1tf_select_mitigatio

    static char *stibp_state(void)
    {
    + if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
    + return "";
    +
    if (x86_spec_ctrl_base & SPEC_CTRL_STIBP)
    return ", STIBP";
    else

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