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

From: Matt Fleming
Date: Fri Nov 06 2015 - 07:39:19 EST

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

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

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.

> 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.
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