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    SubjectRe: [PATCH v3 18/37] mm: Add guard pages around a shadow stack.
    On Fri, Nov 04, 2022 at 03:35:45PM -0700, Rick Edgecombe wrote:

    > diff --git a/arch/x86/mm/mmap.c b/arch/x86/mm/mmap.c
    > index c90c20904a60..66da1f3298b0 100644
    > --- a/arch/x86/mm/mmap.c
    > +++ b/arch/x86/mm/mmap.c
    > @@ -248,3 +248,26 @@ bool pfn_modify_allowed(unsigned long pfn, pgprot_t prot)
    > return false;
    > return true;
    > }
    > +
    > +unsigned long stack_guard_start_gap(struct vm_area_struct *vma)
    > +{
    > + if (vma->vm_flags & VM_GROWSDOWN)
    > + return stack_guard_gap;
    > +
    > + /*
    > + * Shadow stack pointer is moved by CALL, RET, and INCSSP(Q/D).

    Can we perhaps write this like: INCSPP[QD] ? The () notation makes it
    look like a function.

    > + * INCSSPQ moves shadow stack pointer up to 255 * 8 = ~2 KB
    > + * (~1KB for INCSSPD) and touches the first and the last element
    > + * in the range, which triggers a page fault if the range is not
    > + * in a shadow stack. Because of this, creating 4-KB guard pages
    > + * around a shadow stack prevents these instructions from going
    > + * beyond.
    > + *
    > + * Creation of VM_SHADOW_STACK is tightly controlled, so a vma
    > + * can't be both VM_GROWSDOWN and VM_SHADOW_STACK
    > + */
    > + if (vma->vm_flags & VM_SHADOW_STACK)
    > + return PAGE_SIZE;
    > +
    > + return 0;
    > +}
    > diff --git a/include/linux/mm.h b/include/linux/mm.h
    > index 5d9536fa860a..0a3f7e2b32df 100644
    > --- a/include/linux/mm.h
    > +++ b/include/linux/mm.h
    > @@ -2832,15 +2832,16 @@ struct vm_area_struct *vma_lookup(struct mm_struct *mm, unsigned long addr)
    > return mtree_load(&mm->mm_mt, addr);
    > }
    >
    > +unsigned long stack_guard_start_gap(struct vm_area_struct *vma);
    > +
    > static inline unsigned long vm_start_gap(struct vm_area_struct *vma)
    > {
    > + unsigned long gap = stack_guard_start_gap(vma);
    > unsigned long vm_start = vma->vm_start;
    >
    > - if (vma->vm_flags & VM_GROWSDOWN) {
    > - vm_start -= stack_guard_gap;
    > - if (vm_start > vma->vm_start)
    > - vm_start = 0;
    > - }
    > + vm_start -= gap;
    > + if (vm_start > vma->vm_start)
    > + vm_start = 0;
    > return vm_start;
    > }
    >
    > diff --git a/mm/mmap.c b/mm/mmap.c
    > index 2def55555e05..f67606fbc464 100644
    > --- a/mm/mmap.c
    > +++ b/mm/mmap.c
    > @@ -281,6 +281,13 @@ SYSCALL_DEFINE1(brk, unsigned long, brk)
    > return origbrk;
    > }
    >
    > +unsigned long __weak stack_guard_start_gap(struct vm_area_struct *vma)
    > +{
    > + if (vma->vm_flags & VM_GROWSDOWN)
    > + return stack_guard_gap;
    > + return 0;
    > +}

    I'm thinking perhaps this wants to be an inline function?

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