From: Ard Biesheuvel <ard.biesheuvel@linaro.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>,
Linus Torvalds <torvalds@linux-foundation.org>,
Stephen Smalley <sds@tycho.nsa.gov>,
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>,
Kees Cook <keescook@chromium.org>,
"linux-efi@vger.kernel.org" <linux-efi@vger.kernel.org>
Subject: Re: [GIT PULL] x86/mm changes for v4.4
Date: Sat, 7 Nov 2015 08:39:35 +0100 [thread overview]
Message-ID: <CAKv+Gu-s=91p2hVV-8r5AWQwgjD2sbXC86sPhtmq9UyqqcOz4w@mail.gmail.com> (raw)
In-Reply-To: <20151107070922.GC6235@gmail.com>
On 7 November 2015 at 08:09, Ingo Molnar <mingo@kernel.org> wrote:
>
> * Matt Fleming <matt@codeblueprint.co.uk> wrote:
>
>> On Fri, 06 Nov, at 07:55:50AM, Ingo Molnar wrote:
>> >
>> > 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...)
>>
>> Sorry, this isn't true. I misled you with one of my earlier posts on
>> this topic. Let me try and clear things up...
>>
>> Writing to EFI regions has to do with every invocation of the EFI
>> runtime services - it's not limited to when you read/write/delete EFI
>> variables. In fact, EFI variables really have nothing to do with this
>> discussion, they're a completely opaque concept to the OS, we have no
>> idea how the firmware implements them. Everything is done via the EFI
>> boot/runtime services.
>>
>> The firmware itself will attempt to write to EFI regions when we
>> invoke the EFI services because that's where the PE/COFF ".data" and
>> ".bss" sections live along with the heap. There's even some relocation
>> fixups that occur as SetVirtualAddressMap() time so it'll write to
>> ".text" too.
>>
>> Now, the above PE/COFF sections are usually (always?) contained within
>> EFI regions of type EfiRuntimeServicesCode. We know this is true
>> because the firmware folks have told us so, and because stopping that
>> is the motivation behind the new EFI_PROPERTIES_TABLE feature in UEFI
>> V2.5.
>>
>> The data sections within the region are also *not* guaranteed to be
>> page granular because work was required in Tianocore for emitting
>> sections with 4k alignment as part of the EFI_PROPERTIES_TABLE
>> support.
>>
>> Ultimately, what this means is that if you were to attempt to
>> dynamically fixup those regions that required write permission, you'd
>> have to modify the mappings for the majority of the EFI regions
>> anyway. And if you're blindly allowing write permission as a fixup,
>> there's not much security to be had.
>
> I think you misunderstood my suggestion: the 'fixup' would be changing it from R-X
> to RW-, i.e. it would add 'write' permission but remove 'execute' permission.
>
> Note that there would be no 'RWX' permission at any given moment - which is the
> dangerous combination.
>
The problem with that is that /any/ page in the UEFI runtime region
may intersect with both .text and .data of any of the PE/COFF images
that make up the runtime firmware (since the PE/COFF sections are not
necessarily page aligned). Such pages require RWX permissions. The
UEFI memory map does not provide the information to identify those
pages a priori (the entire region containing several PE/COFF images
could be covered by a single entry) so it is hard to guess which pages
should be allowed these RWX permissions.
>> > 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.
>>
>> We already have separate EFI page tables, though with the caveat that
>> we share some of swapper_pg_dir's PGD entries. The best solution would
>> be to stop sharing entries and isolate the EFI mappings from every
>> other page table structure, so that they're only used during the EFI
>> service calls.
>
> Absolutely. Can you try to fix this for v4.3?
>
> Thanks,
>
> Ingo
next prev parent reply other threads:[~2015-11-07 7:39 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2015-11-03 11:16 [GIT PULL] x86/mm changes for v4.4 Ingo Molnar
2015-11-04 19:26 ` Linus Torvalds
2015-11-04 23:39 ` Dave Jones
2015-11-05 1:31 ` Linus Torvalds
2015-11-05 2:17 ` Dave Jones
2015-11-05 21:27 ` Linus Torvalds
2015-11-05 21:33 ` Linus Torvalds
2015-11-06 11:39 ` Matt Fleming
2015-11-07 7:05 ` Ingo Molnar
2015-11-07 10:03 ` Matt Fleming
2015-11-05 22:04 ` Linus Torvalds
2015-11-05 22:27 ` Borislav Petkov
2015-11-06 6:55 ` Ingo Molnar
2015-11-06 7:05 ` Andy Lutomirski
2015-11-06 13:09 ` Matt Fleming
2015-11-06 13:24 ` Borislav Petkov
2015-11-07 7:03 ` Ingo Molnar
2015-11-06 7:44 ` Ingo Molnar
2015-11-06 12:39 ` Matt Fleming
2015-11-07 7:09 ` Ingo Molnar
2015-11-07 7:39 ` Ard Biesheuvel [this message]
2015-11-08 6:58 ` Kees Cook
2015-11-08 7:55 ` Ard Biesheuvel
2015-11-09 21:08 ` Kees Cook
2015-11-10 7:08 ` Ard Biesheuvel
2015-11-10 20:11 ` Kees Cook
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to='CAKv+Gu-s=91p2hVV-8r5AWQwgjD2sbXC86sPhtmq9UyqqcOz4w@mail.gmail.com' \
--to=ard.biesheuvel@linaro.org \
--cc=akpm@linux-foundation.org \
--cc=bp@alien8.de \
--cc=davej@codemonkey.org.uk \
--cc=dvlasenk@redhat.com \
--cc=hpa@zytor.com \
--cc=keescook@chromium.org \
--cc=linux-efi@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=luto@kernel.org \
--cc=matt@codeblueprint.co.uk \
--cc=mingo@kernel.org \
--cc=sds@tycho.nsa.gov \
--cc=tglx@linutronix.de \
--cc=torvalds@linux-foundation.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).