Re: [PATCH v2 3/5] x86/mm: check exec permissions on fault

Peter Zijlstra
2021-10-25T11:00:16Z

On Thu, Oct 21, 2021 at 05:21:10AM -0700, Nadav Amit wrote:
> From: Nadav Amit <namit@vmware.com>

> Add a check to prevent access_error() from returning mistakenly that
> page-faults due to instruction fetch are not allowed. Intel SDM does not
> indicate whether "instruction fetch" and "write" in the hardware error
> code are mutual exclusive, so check both before returning whether the
> access is allowed.

Dave, can we get that clarified? It seems a bit naf and leads to
confusing code IMO.

Other than that, the change looks ok to me.

> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index b2eefdefc108..e776130473ce 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1100,10 +1100,17 @@ access_error(unsigned long error_code, struct vm_area_struct *vma)
> (error_code & X86_PF_INSTR), foreign))
> return 1;
>
> - if (error_code & X86_PF_WRITE) {
> + if (error_code & (X86_PF_WRITE | X86_PF_INSTR)) {
> /* write, present and write, not present: */
> - if (unlikely(!(vma->vm_flags & VM_WRITE)))
> + if ((error_code & X86_PF_WRITE) &&
> + unlikely(!(vma->vm_flags & VM_WRITE)))
> return 1;
> +
> + /* exec, present and exec, not present: */
> + if ((error_code & X86_PF_INSTR) &&
> + unlikely(!(vma->vm_flags & VM_EXEC)))
> + return 1;
> +
> return 0;
> }