Re: [PATCH v2] x86/mm: warn on W+x mappings

[Ard Biesheuvel]

On 21 October 2015 at 14:49, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
>>
>> * Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> wrote:
>>
>> > > > Right, we could do that, but then we wouldn't be able to support
>> > > > creation/updating variables at runtime, such as when you install a
>> > > > distribution for the first time, or want to boot a new kernel filename
>> > > > directly from the firmware without a boot loader (and need to modify the
>> > > > BootXXXX variables).
>> > >
>> > > Do we know the precise position and address range of these variables?
>> > >
>> > > We could map them writable (but not executable), and the rest executable (but
>> > > not writable).
>> >
>> > The variables are stored in NVRAM, which we don't map into the kernel virtual
>> > address space. [...]
>>
>> Just curious: is there firmware that memory maps those variables privately?
>>
>> > [...] We have to initiate the transaction of writing to the variables by
>> > executing EFI runtime services.
>> >
>> > We obviously have buffers that we pass to the BIOS that contain variable data,
>> > but these should be NX anyway because they're regular kernel allocations.
>> >
>> > > That raises the question whether the same physical page ever mixes variables
>> > > and actual code - but the hope would be that it's suffiently page granular for
>> > > this to work.
>> >
>> > I don't think that would ever happen.
>>
>> Ok, that's promising, so how about this then to solve the security weakness the
>> new warning unearthed: map the whole EFI range as 'r-x (NX)', but detect writes
>> from the page fault handler and transparently allow them to flip over the range
>> to 'rw-'.
>
> So I meant to say 'page' instead of 'range'.
>
> I.e. this dynamic mechanism would flip pages over to 'rw-', as write faults occur
> from EFI code that writes to them.
>
> We don't need to know which regions are writable data, and which regions are
> executable-code/readonly-data.
>
> The following aspect would guarantee safety:
>
>> Note that for security reasons we don't allow a subsequent flipping back to NX
>> if there's an NX fault on the same page, i.e. this new mechanism is a monotonic
>> one-way process that should dynamically 'map out' data pages versus executable
>> pages.
>>
>> It should also be pretty robust, assuming we can take page faults while EFI code
>> is executing and is trying to modify EFI data: is that the case?
>
> and this is why I asked whether boundaries between 'Code' and 'Writable data'
> sections are page granular - which they do appear to be. (i.e. there are no
> singular pages that are both writable data and code at once.)
>

No, sadly they are not. Only in specific cases (which have to do with
the new UEFIv2.5 memory protection feature that got this discussion
started in the first place) can we assume that UEFI runtime pages are
mappable either RW- or R-X but not RWX, and in those cases, we have
the permissions bits that tell us unambiguously which pages are text
and which are data. For the remaining cases, which is the vast
majority, no such assumptions can be made, and since the UEFI runtime
regions are typically populated with a bunch of PE/COFF images (each
of which consists of text + data), inferring where the boundaries are
between them does not seem tractable (for instance, to only map
'boundary' pages RWX)

-- 
Ard.
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