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    SubjectRe: [PATCH v7 27/41] x86/mm: Warn if create Write=0,Dirty=1 with raw prot
    On Mon, Feb 27, 2023 at 02:29:43PM -0800, Rick Edgecombe wrote:
    > When user shadow stack is use, Write=0,Dirty=1 is treated by the CPU as
    ^
    in

    > 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.

    Might wanna add the note from below here:

    "... start doing this, like, for example, set_memory_rox() did."

    > 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>
    >
    > ---
    > v6:
    > - New patch (Note, this has already been a useful warning, it caught the
    > newly added set_memory_rox() doing this)

    Thx.

    --
    Regards/Gruss,
    Boris.

    https://people.kernel.org/tglx/notes-about-netiquette

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