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    SubjectRe: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot
    On February 27, 2023 2:29:43 PM PST, Rick Edgecombe <rick.p.edgecombe@intel.com> wrote:
    >When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
    >shadow stack memory. So for shadow stack memory this bit combination is
    >valid, but when Dirty=1,Write=1 (conventionally writable) memory is being
    >write protected, the kernel has been taught to transition the Dirty=1
    >bit to SavedDirty=1, to avoid inadvertently creating shadow stack
    >memory. It does this inside pte_wrprotect() because it knows the PTE is
    >not intended to be a writable shadow stack entry, it is supposed to be
    >write protected.
    >
    >However, when a PTE is created by a raw prot using mk_pte(), mk_pte()
    >can't know whether to adjust Dirty=1 to SavedDirty=1. It can't
    >distinguish between the caller intending to create a shadow stack PTE or
    >needing the SavedDirty shift.
    >
    >The kernel has been updated to not do this, and so Write=0,Dirty=1
    >memory should only be created by the pte_mkfoo() helpers. Add a warning
    >to make sure no new mk_pte() start doing this.
    >
    >Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    >Tested-by: John Allen <john.allen@amd.com>
    >Tested-by: Kees Cook <keescook@chromium.org>
    >Acked-by: Mike Rapoport (IBM) <rppt@kernel.org>
    >Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

    Reviewed-by: Kees Cook <keescook@chromium.org>


    --
    Kees Cook

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