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Date: Sat, 7 Nov 2015 08:03:39 +0100 From: Ingo Molnar <mingo@...nel.org> To: Andy Lutomirski <luto@...capital.net> Cc: Linus Torvalds <torvalds@...ux-foundation.org>, Stephen Smalley <sds@...ho.nsa.gov>, Matt Fleming <matt.fleming@...el.com>, Dave Jones <davej@...emonkey.org.uk>, Linux Kernel Mailing List <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, "H. Peter Anvin" <hpa@...or.com>, Borislav Petkov <bp@...en8.de>, Andrew Morton <akpm@...ux-foundation.org>, Andy Lutomirski <luto@...nel.org>, Denys Vlasenko <dvlasenk@...hat.com>, Kees Cook <keescook@...omium.org> Subject: Re: [GIT PULL] x86/mm changes for v4.4 * Andy Lutomirski <luto@...capital.net> wrote: > On Thu, Nov 5, 2015 at 10:55 PM, Ingo Molnar <mingo@...nel.org> wrote: > > > > * Linus Torvalds <torvalds@...ux-foundation.org> wrote: > > > >> On Wed, Nov 4, 2015 at 6:17 PM, Dave Jones <davej@...emonkey.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...) > > I think it was Borislav who pointed out that this idea, which might > have been mine, is a bit silly. Why not just skip mapping the EFI > stuff in the init_pgd entirely and only map it in the EFI pgd? > > We'll have RWX stuff in the EFI pgd, but so what? If we're exposing > anything that runs with the EFI pgd loaded to untrusted input, I think > we've already lost. That's certainly true, I was simply confused about the life time of these mappings: I assumed they have to stay around. If they are meant to be and are partly temporary today already, we should go the whole mile and make that really so, because _today_ the mappings are permanent, so this is a real problem ... Thanks, Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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