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    Subject[PATCH v3 11/37] x86/mm: Update pte_modify for _PAGE_COW
    Date
    From: Yu-cheng Yu <yu-cheng.yu@intel.com>

    The Write=0,Dirty=1 PTE has been used to indicate copy-on-write pages.
    However, newer x86 processors also regard a Write=0,Dirty=1 PTE as a
    shadow stack page. In order to separate the two, the software-defined
    _PAGE_DIRTY is changed to _PAGE_COW for the copy-on-write case, and
    pte_*() are updated to do this.

    pte_modify() takes a "raw" pgprot_t which was not necessarily created
    with any of the existing PTE bit helpers. That means that it can return a
    pte_t with Write=0,Dirty=1, a shadow stack PTE, when it did not intend to
    create one.

    However pte_modify() changes a PTE to 'newprot', but it doesn't use the
    pte_*(). Modify it to also move _PAGE_DIRTY to _PAGE_COW. Apply the same
    changes to pmd_modify().

    Tested-by: Pengfei Xu <pengfei.xu@intel.com>
    Tested-by: John Allen <john.allen@amd.com>
    Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    Co-developed-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>

    ---

    v2:
    - Update commit log with text and suggestions from (Dave Hansen)
    - Drop fixup_dirty_pte() in favor of clearing the HW dirty bit along
    with the _PAGE_CHG_MASK masking, then calling pte_mkdirty() (Dave
    Hansen)

    arch/x86/include/asm/pgtable.h | 41 +++++++++++++++++++++++++++++-----
    1 file changed, 35 insertions(+), 6 deletions(-)

    diff --git a/arch/x86/include/asm/pgtable.h b/arch/x86/include/asm/pgtable.h
    index c284bb6f62a5..81f388a5a5ab 100644
    --- a/arch/x86/include/asm/pgtable.h
    +++ b/arch/x86/include/asm/pgtable.h
    @@ -791,26 +791,55 @@ static inline u64 flip_protnone_guard(u64 oldval, u64 val, u64 mask);

    static inline pte_t pte_modify(pte_t pte, pgprot_t newprot)
    {
    + pteval_t _page_chg_mask_no_dirty = _PAGE_CHG_MASK & ~_PAGE_DIRTY;
    pteval_t val = pte_val(pte), oldval = val;
    + pte_t pte_result;

    /*
    * Chop off the NX bit (if present), and add the NX portion of
    * the newprot (if present):
    */
    - val &= _PAGE_CHG_MASK;
    - val |= check_pgprot(newprot) & ~_PAGE_CHG_MASK;
    + val &= _page_chg_mask_no_dirty;
    + val |= check_pgprot(newprot) & ~_page_chg_mask_no_dirty;
    val = flip_protnone_guard(oldval, val, PTE_PFN_MASK);
    - return __pte(val);
    +
    + pte_result = __pte(val);
    +
    + /*
    + * Dirty bit is not preserved above so it can be done
    + * in a special way for the shadow stack case, where it
    + * needs to set _PAGE_COW. pte_mkdirty() will do this in
    + * the case of shadow stack.
    + */
    + if (pte_dirty(pte))
    + pte_result = pte_mkdirty(pte_result);
    +
    + return pte_result;
    }

    static inline pmd_t pmd_modify(pmd_t pmd, pgprot_t newprot)
    {
    + pteval_t _hpage_chg_mask_no_dirty = _HPAGE_CHG_MASK & ~_PAGE_DIRTY;
    pmdval_t val = pmd_val(pmd), oldval = val;
    + pmd_t pmd_result;

    - val &= _HPAGE_CHG_MASK;
    - val |= check_pgprot(newprot) & ~_HPAGE_CHG_MASK;
    + val &= _hpage_chg_mask_no_dirty;
    + val |= check_pgprot(newprot) & ~_hpage_chg_mask_no_dirty;
    val = flip_protnone_guard(oldval, val, PHYSICAL_PMD_PAGE_MASK);
    - return __pmd(val);
    +
    +
    + pmd_result = __pmd(val);
    +
    + /*
    + * Dirty bit is not preserved above so it can be done
    + * specially for the shadow stack case. It needs to move
    + * the HW dirty bit to the software COW bit. Set in the
    + * result if it was set in the original value.
    + */
    + if (pmd_dirty(pmd))
    + pmd_result = pmd_mkdirty(pmd_result);
    +
    + return pmd_result;
    }

    /*
    --
    2.17.1
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