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

Ingo Molnar
2015-11-07T07:09:22Z

* Matt Fleming <matt-mF/unelCI9GS6iBeEJttW/XRex20P6io@public.gmane.org> 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.

> > 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