Re: [PATCH 4/9] x86, pkeys: override pkey when moving away from PROT_EXEC

From: Ram Pai
Date: Mon Apr 30 2018 - 03:51:24 EST

On Thu, Apr 26, 2018 at 10:57:31AM -0700, Dave Hansen wrote:
> On 04/06/2018 06:09 PM, Ram Pai wrote:
> > Well :). my point is add this code and delete the other
> > code that you add later in that function.
> I don't think I'm understanding what your suggestion was. I looked at
> the code and I honestly do not think I can remove any of it.
> For the plain (non-explicit pkey_mprotect()) case, there are exactly
> four paths through __arch_override_mprotect_pkey(), resulting in three
> different results.
> 1. New prot==PROT_EXEC, no pkey-exec support -> do not override
> 2. New prot!=PROT_EXEC, old VMA not PROT_EXEC-> do not override
> 3. New prot==PROT_EXEC, w/ pkey-exec support -> override to exec pkey
> 4. New prot!=PROT_EXEC, old VMA is PROT_EXEC -> override to default
> I don't see any redundancy there, or any code that we can eliminate or
> simplify. It was simpler before, but that's what where bug was.

Your code is fine. But than the following code accomplishes the same
outcome; arguably with a one line change. Its not a big deal. Just
trying to clarify my comment.

int __arch_override_mprotect_pkey(struct vm_area_struct *vma, int prot, int pkey)
* Is this an mprotect_pkey() call? If so, never
* override the value that came from the user.
if (pkey != -1)
return pkey;
* Look for a protection-key-drive execute-only mapping
* which is now being given permissions that are not
* execute-only. Move it back to the default pkey.
if (vma_is_pkey_exec_only(vma) && (prot != PROT_EXEC)) <--------

* The mapping is execute-only. Go try to get the
* execute-only protection key. If we fail to do that,
* fall through as if we do not have execute-only
* support.
if (prot == PROT_EXEC) {
pkey = execute_only_pkey(vma->vm_mm);
if (pkey > 0)
return pkey;
* This is a vanilla, non-pkey mprotect (or we failed to
* setup execute-only), inherit the pkey from the VMA we
* are working on.
return vma_pkey(vma);

Ram Pai