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

From: Matt Fleming <matt@codeblueprint.co.uk>
To: Ingo Molnar <mingo@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
	Dave Jones <davej@codemonkey.org.uk>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H. Peter Anvin" <hpa@zytor.com>, Borislav Petkov <bp@alien8.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Lutomirski <luto@kernel.org>,
	Denys Vlasenko <dvlasenk@redhat.com>,
	Stephen Smalley <sds@tycho.nsa.gov>,
	linux-efi@vger.kernel.org
Subject: Re: [GIT PULL] x86/mm changes for v4.4
Date: Sat, 7 Nov 2015 10:03:28 +0000	[thread overview]
Message-ID: <20151107100328.GB2387@codeblueprint.co.uk> (raw)
In-Reply-To: <20151107070554.GB6235@gmail.com>

On Sat, 07 Nov, at 08:05:54AM, Ingo Molnar wrote:
> 
> * Matt Fleming <matt@codeblueprint.co.uk> wrote:
> 
> > On Thu, 05 Nov, at 01:33:10PM, Linus Torvalds wrote:
> > > 
> > > And if this turns out to be due to EFI wanting those permissions, what should 
> > > we do? People have talked about running the EFI callbacks in their own private 
> > > page table setup, which sounds like the right idea, but until that actually 
> > > *happens*....
> > 
> > We have separate page tables today, for a few reasons, but mainly it's
> > so that we can have an identity mapping of memory present in the
> > region usually used by user processes - broken firmware still uses
> > those identity mappings even after the kernel tells it they're
> > invalid.
> > 
> > Note that when I say "separate" I'm talking about trampoline_pgd[]
> > which is also used by the x86 suspend/resume code.
> > 
> > However, turns out that the issue with the current scheme is the fact
> > that trampoline_pgd[] actually shares a couple of PGD entries with
> > swapper_pg_dir as can be seen in setup_real_mode(),
> > 
> > 
> >         trampoline_pgd = (u64 *)__va(real_mode_header->trampoline_pgd);
> >         trampoline_pgd[0] = init_level4_pgt[pgd_index(__PAGE_OFFSET)].pgd;
> >         trampoline_pgd[511] = init_level4_pgt[511].pgd;
> > 
> > 
> > So when we map the EFI regions in efi_map_regions() we're inserting
> > them into swapper_pg_dir also, which is why you're seeing the
> > warnings.
> > 
> > If I remember correctly the rationale for using trampoline_pgd[] was
> > that it already did what we wanted (provided the identity mapping) and
> > would save us the overhead of maintaining more page tables for no good
> > reason. Obviously this entire thread is a good reason.
> > 
> > I suggest we stop using trampoline_pgd[] (since it has a good reason
> > for sharing the kernel mapping PGD entries) and create our own so that
> > we can isolate EFI completely.
> 
> Ok. Could you please make this fix a priority for upcoming EFI changes?

Yep, I'll get on it.