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

From: Pavel Machek
Date: Tue Dec 15 2015 - 04:40:31 EST

On Mon 2015-12-14 14:25:10, Andy Lutomirski wrote:
> On Mon, Dec 14, 2015 at 1:24 PM, Arjan van de Ven <arjan@xxxxxxxxxxxxxxx> 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).

I tried applying:

[PATCH 1/2] x86_32/mm: Set NX in __supported_pte_mask before enabling

but I still get

[ 2.685402] ------------[ cut here ]------------
[ 2.688649] WARNING: CPU: 0 PID: 1 at
arch/x86/mm/dump_pagetables.c:225 note_
[ 2.691897] x86/mm: Found insecure W+X mapping at address
[ 2.695090] Modules linked in:

Best regards,

(cesky, pictures)
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