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    Subject[PATCH v4 21/39] mm/mprotect: Exclude shadow stack from preserve_write
    Date
    From: Yu-cheng Yu <yu-cheng.yu@intel.com>

    The x86 Control-flow Enforcement Technology (CET) feature includes a new
    type of memory called shadow stack. This shadow stack memory has some
    unusual properties, which requires some core mm changes to function
    properly.

    In change_pte_range(), when a PTE is changed for prot_numa, _PAGE_RW is
    preserved to avoid the additional write fault after the NUMA hinting fault.
    However, pte_write() now includes both normal writable and shadow stack
    (Write=0, Dirty=1) PTEs, but the latter does not have _PAGE_RW and has no
    need to preserve it.

    Exclude shadow stack from preserve_write test, and apply the same change to
    change_huge_pmd().

    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>
    Reviewed-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
    Signed-off-by: Rick Edgecombe <rick.p.edgecombe@intel.com>
    ---

    v4:
    - Add "why" to comments in code (Peterz)

    Yu-cheng v25:
    - Move is_shadow_stack_mapping() to a separate line.

    Yu-cheng v24:
    - Change arch_shadow_stack_mapping() to is_shadow_stack_mapping().

    mm/huge_memory.c | 8 ++++++++
    mm/mprotect.c | 8 ++++++++
    2 files changed, 16 insertions(+)

    diff --git a/mm/huge_memory.c b/mm/huge_memory.c
    index 60451e588955..b6294c4ad471 100644
    --- a/mm/huge_memory.c
    +++ b/mm/huge_memory.c
    @@ -1803,6 +1803,14 @@ int change_huge_pmd(struct mmu_gather *tlb, struct vm_area_struct *vma,
    return 0;

    preserve_write = prot_numa && pmd_write(*pmd);
    +
    + /*
    + * Preserve only normal writable huge PMD, but not shadow
    + * stack (RW=0, Dirty=1), so the restoring code doesn't
    + * make the shadow stack memory conventionally writable.
    + */
    + if (vma->vm_flags & VM_SHADOW_STACK)
    + preserve_write = false;
    ret = 1;

    #ifdef CONFIG_ARCH_ENABLE_THP_MIGRATION
    diff --git a/mm/mprotect.c b/mm/mprotect.c
    index de7351631e21..026347f1f1ee 100644
    --- a/mm/mprotect.c
    +++ b/mm/mprotect.c
    @@ -115,6 +115,14 @@ static unsigned long change_pte_range(struct mmu_gather *tlb,
    pte_t ptent;
    bool preserve_write = prot_numa && pte_write(oldpte);

    + /*
    + * Preserve only normal writable PTE, but not shadow
    + * stack (RW=0, Dirty=1), so the restoring code doesn't
    + * make the shadow stack memory conventionally writable.
    + */
    + if (vma->vm_flags & VM_SHADOW_STACK)
    + preserve_write = false;
    +
    /*
    * Avoid trapping faults against the zero or KSM
    * pages. See similar comment in change_huge_pmd.
    --
    2.17.1
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