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    SubjectRe: [RFC PATCH v9 07/27] Add guard pages around a Shadow Stack.
    On Wed, Feb 05, 2020 at 10:19:15AM -0800, Yu-cheng Yu wrote:
    > INCSSPD/INCSSPQ instruction is used to unwind a Shadow Stack (SHSTK). It
    > performs 'pop and discard' of the first and last element from SHSTK in the
    > range specified in the operand. The maximum value of the operand is 255,
    > and the maximum moving distance of the SHSTK pointer is 255 * 4 for
    > INCSSPD, 255 * 8 for INCSSPQ.
    >
    > Since SHSTK has a fixed size, creating a guard page above prevents
    > INCSSP/RET from moving beyond. Likewise, creating a guard page below
    > prevents CALL from underflowing the SHSTK.

    This commit log doesn't really explain why the code changes below are
    needed? stack_guard_gap is configurable at boot, etc. This appears to be
    limiting it? I don't follow this change...

    -Kees

    >
    > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@intel.com>
    > ---
    > include/linux/mm.h | 20 ++++++++++++++++----
    > 1 file changed, 16 insertions(+), 4 deletions(-)
    >
    > diff --git a/include/linux/mm.h b/include/linux/mm.h
    > index b5145fbe102e..75de07674649 100644
    > --- a/include/linux/mm.h
    > +++ b/include/linux/mm.h
    > @@ -2464,9 +2464,15 @@ static inline struct vm_area_struct * find_vma_intersection(struct mm_struct * m
    > static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
    > {
    > unsigned long vm_start = vma->vm_start;
    > + unsigned long gap = 0;
    >
    > - if (vma->vm_flags & VM_GROWSDOWN) {
    > - vm_start -= stack_guard_gap;
    > + if (vma->vm_flags & VM_GROWSDOWN)
    > + gap = stack_guard_gap;
    > + else if (vma->vm_flags & VM_SHSTK)
    > + gap = PAGE_SIZE;
    > +
    > + if (gap != 0) {
    > + vm_start -= gap;
    > if (vm_start > vma->vm_start)
    > vm_start = 0;
    > }
    > @@ -2476,9 +2482,15 @@ static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
    > static inline unsigned long vm_end_gap(struct vm_area_struct *vma)
    > {
    > unsigned long vm_end = vma->vm_end;
    > + unsigned long gap = 0;
    > +
    > + if (vma->vm_flags & VM_GROWSUP)
    > + gap = stack_guard_gap;
    > + else if (vma->vm_flags & VM_SHSTK)
    > + gap = PAGE_SIZE;
    >
    > - if (vma->vm_flags & VM_GROWSUP) {
    > - vm_end += stack_guard_gap;
    > + if (gap != 0) {
    > + vm_end += gap;
    > if (vm_end < vma->vm_end)
    > vm_end = -PAGE_SIZE;
    > }
    > --
    > 2.21.0
    >

    --
    Kees Cook

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