Re: [GIT PULL] x86/mm changes for v4.4

[Ingo Molnar <mingo@kernel.org>]

(resent with Matt's email address fixed.)

* Ingo Molnar <mingo@kernel.org> wrote:

> 
> * Linus Torvalds <torvalds@linux-foundation.org> wrote:
> 
> > On Wed, Nov 4, 2015 at 6:17 PM, Dave Jones <davej@codemonkey.org.uk> wrote:
> > > On Wed, Nov 04, 2015 at 05:31:59PM -0800, Linus Torvalds wrote:
> > >  >
> > >  > I don't have that later debug output at all. Presumably some config difference.
> > >
> > > CONFIG_X86_PTDUMP_CORE iirc.
> > 
> > No, I have that. I suspect CONFIG_EFI_PGT_DUMP instead.
> > 
> > Anyway, as it stands now, I think the CONFIG_DEBUG_WX option should
> > not default to 'y' unless it is made more useful if it actually
> > triggers. Ingo?
> 
> Yeah, agreed absolutely.
> 
> So this is a bit sad because RWX pages are a real problem in practice, especially 
> since the EFI addresses are well predictable, but generating a warning without 
> being able to fix it quickly is counterproductive as well, as it only annoys 
> people and makes them turn off the option. (Which we could do as well to begin 
> with, without the annoyance factor...)
> 
> So the plan would be:
> 
>  1) Make it default-n.
> 
>  2) We should try to further improve the messages to make it easier to determine
>     what's wrong. We _do_ try to output symbolic information in the warning, to 
>     make it easier to find buggy mappings, but these are not standard kernel
>     mappings. So I think we need an e820 mappings based semi-symbolic printout of
>     bad addresses - maybe even correlate it with the MMIO resource tree.
> 
>  3) We should fix the EFI permission problem without relying on the firmware: it 
>     appears we could just mark everything R-X optimistically, and if a write fault 
>     happens (it's pretty rare in fact, only triggers when we write to an EFI 
>     variable and so), we can mark the faulting page RW- on the fly, because it 
>     appears that writable EFI sections, while not enumerated very well in 'old' 
>     firmware, are still supposed to be page granular. (Even 'new' firmware I 
>     wouldn't automatically trust to get the enumeration right...)
> 
>     If that 'supposed to be' turns out to be 'not true' (not unheard of in
>     firmware land), then plan B would be to mark pages that generate write faults 
>     RWX as well, to not break functionality. (This 'mark it RWX' is not something 
>     that exploits would have easy access to, and we could also generate a warning
>     [after the EFI call has finished] if it ever triggers.)
> 
>     Admittedly this approach might not be without its own complications, but it 
>     looks reasonably simple (I don't think we need per EFI call page tables, 
>     etc.), and does not assume much about the firmware being able to enumerate its 
>     permissions properly. Were we to merge EFI support today I'd have insisted on 
>     trying such an approach from day 1 on.
> 
> Thanks,
> 
> 	Ingo