On Fri, Nov 6, 2015 at 11:39 PM, Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx> wrote: > On 7 November 2015 at 08:09, Ingo Molnar <mingo@xxxxxxxxxx> wrote: >> >> * Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx> 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 -- To unsubscribe from this list: send the line "unsubscribe linux-efi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
