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

Peter Zijlstra
2022-11-15T19:48:59Z

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?