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

Kees Cook
2015-11-08T06:58:52Z

On Fri, Nov 6, 2015 at 11:39 PM, Ard Biesheuvel
<ard.biesheuvel@linaro.org> wrote:
> 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.

I'm sad that UEFI was designed without even the most basic of memory
protections in mind. UEFI _itself_ should be setting up protective
page mappings. :(

For a boot firmware, it seems to me that safe page table layout would
be a top priority bug. The "reporting issues" page for TianoCore
doesn't actually seem to link to the "Project Tracker":
https://github.com/tianocore/tianocore.github.io/wiki/Reporting-Issues

Does anyone know how to get this correctly reported so future UEFI
releases don't suffer from this?

-Kees

>
>>> > 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

--
Kees Cook
Chrome OS Security