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    Subject[PATCH 5.8 119/186] x86/entry: Fix AC assertion
    Date
    From: Peter Zijlstra <peterz@infradead.org>

    commit 662a0221893a3d58aa72719671844264306f6e4b upstream.

    The WARN added in commit 3c73b81a9164 ("x86/entry, selftests: Further
    improve user entry sanity checks") unconditionally triggers on a IVB
    machine because it does not support SMAP.

    For !SMAP hardware the CLAC/STAC instructions are patched out and thus if
    userspace sets AC, it is still have set after entry.

    Fixes: 3c73b81a9164 ("x86/entry, selftests: Further improve user entry sanity checks")
    Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Tested-by: Daniel Thompson <daniel.thompson@linaro.org>
    Acked-by: Andy Lutomirski <luto@kernel.org>
    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/r/20200902133200.666781610@infradead.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

    ---
    arch/x86/entry/common.c | 12 ++++++++++--
    1 file changed, 10 insertions(+), 2 deletions(-)

    --- a/arch/x86/entry/common.c
    +++ b/arch/x86/entry/common.c
    @@ -55,8 +55,16 @@ static noinstr void check_user_regs(stru
    * state, not the interrupt state as imagined by Xen.
    */
    unsigned long flags = native_save_fl();
    - WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
    - X86_EFLAGS_NT));
    + unsigned long mask = X86_EFLAGS_DF | X86_EFLAGS_NT;
    +
    + /*
    + * For !SMAP hardware we patch out CLAC on entry.
    + */
    + if (boot_cpu_has(X86_FEATURE_SMAP) ||
    + (IS_ENABLED(CONFIG_64_BIT) && boot_cpu_has(X86_FEATURE_XENPV)))
    + mask |= X86_EFLAGS_AC;
    +
    + WARN_ON_ONCE(flags & mask);

    /* We think we came from user mode. Make sure pt_regs agrees. */
    WARN_ON_ONCE(!user_mode(regs));

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