Re: 4.4-rc5: ugly warn on: 5 W+X pages found

Andy Lutomirski
2015-12-14T22:25:33Z

On Mon, Dec 14, 2015 at 1:24 PM, Arjan van de Ven <arjan@linux.intel.com> wrote:
>
>> That's weird. The only API to do that seems to be manually setting
>> kmap_prot to _PAGE_KERNEL_EXEC, and nothing does that. (Why is
>> kmap_prot a variable on x86 at all? It has exactly one writer, and
>> that's the code that initializes it in the first place. Shouldn't we
>> #define kmap_prot _PAGE_KERNEL?
>
>
> iirc it changes based on runtime detection of NX capability
>

Maybe it did, but if it still does, I can't find the code.

What *does* change is __supported_pte_mask. If we're willing to make
disable_nx work a little less well, we could try to initialize
__supported_pte_mask from the very beginning. (We currently seem to
detect and enable NX even before we enable paging.) I suspect that
Pavel is seeing a kmap mapping left over from so early that it didn't
have NX set (killed by massage_pgprot).

Borislav, could we do that? (Why do we have disable_nx at all? I
suspect it was for debugging a long, long time ago.)

Alternatively, we could go through and set NX everywhere after we
decide we have NX, but that seems rather error-prone.

--Andy

--
Andy Lutomirski
AMA Capital Management, LLC