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

[Ram Pai <linuxram@us.ibm.com>]

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)) <--------
		return ARCH_DEFAULT_PKEY;

	/*
	 * 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