Re: [PATCH v3 18/37] mm: Add guard pages around a shadow stack.

["Edgecombe, Rick P" <rick.p.edgecombe@intel.com>]

On Tue, 2022-11-15 at 13:04 +0100, Peter Zijlstra wrote:
> 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.

Sure.

> 
> > +      * 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?

I don't think it can work with weak then.