[PATCH 2/3] arm64: efi: Ensure efi_create_mapping() does not map overlapping regions

[Ard Biesheuvel]

On 29 June 2016 at 12:50, Catalin Marinas <catalin.marinas at arm.com> wrote:
> On Wed, Jun 29, 2016 at 12:03:16PM +0200, Ard Biesheuvel wrote:
>> On 29 June 2016 at 11:39, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> > On Tue, Jun 28, 2016 at 06:12:22PM +0200, Ard Biesheuvel wrote:
>> >> On 28 June 2016 at 18:05, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> >> > (Restarting the thread before I forget the entire discussion)
>> >> >
>> >> > On Mon, Jun 06, 2016 at 11:18:14PM +0200, Ard Biesheuvel wrote:
>> >> >> >> > On 31 May 2016 at 17:14, Catalin Marinas <catalin.marinas at arm.com> wrote:
>> >> >> >> > > diff --git a/arch/arm64/kernel/efi.c b/arch/arm64/kernel/efi.c
>> >> >> >> > > index 78f52488f9ff..0d5753c31c7f 100644
>> >> >> >> > > --- a/arch/arm64/kernel/efi.c
>> >> >> >> > > +++ b/arch/arm64/kernel/efi.c
>> >> >> >> > > @@ -62,10 +62,26 @@ struct screen_info screen_info __section(.data);
>> >> >> >> > >  int __init efi_create_mapping(struct mm_struct *mm, efi_memory_desc_t *md)
>> >> >> >> > >  {
>> >> >> >> > >         pteval_t prot_val = create_mapping_protection(md);
>> >> >> >> > > +       phys_addr_t length = md->num_pages << EFI_PAGE_SHIFT;
>> >> >> >> > > +       efi_memory_desc_t *next = md;
>> >> >> >> > >
>> >> >> >> > > -       create_pgd_mapping(mm, md->phys_addr, md->virt_addr,
>> >> >> >> > > -                          md->num_pages << EFI_PAGE_SHIFT,
>> >> >> >> > > -                          __pgprot(prot_val | PTE_NG));
>> >> >> >> > > +       /*
>> >> >> >> > > +        * Search for the next EFI runtime map and check for any overlap with
>> >> >> >> > > +        * the current map when aligned to PAGE_SIZE. In such case, defer
>> >> >> >> > > +        * mapping the end of the current range until the next
>> >> >> >> > > +        * efi_create_mapping() call.
>> >> >> >> > > +        */
>> >> >> >> > > +       for_each_efi_memory_desc_continue(next) {
>> >> >> >> > > +               if (!(next->attribute & EFI_MEMORY_RUNTIME))
>> >> >> >> > > +                       continue;
>> >> >> >> > > +               if (next->phys_addr < PAGE_ALIGN(md->phys_addr + length))
>> >> >> >> > > +                       length -= (md->phys_addr + length) & ~PAGE_MASK;
>> >> > [...]
>> >> >> Another thing I failed to mention is that the new Memory Attributes
>> >> >> table support may map all of the RuntimeServicesCode regions a second
>> >> >> time, but with a higher granularity, using RO for .text and .rodata
>> >> >> and NX for .data and .bss (and the PE/COFF header).
>> >> >
>> >> > Can this not be done in a single go without multiple passes? That's what
>> >> > we did for the core arm64 code, the only one left being EFI run-time
>> >> > mappings.
>> >>
>> >> Well, we probably could, but it is far from trivial.
>> >>
>> >> >> Due to the higher
>> >> >> granularity, regions that were mapped using the contiguous bit the
>> >> >> first time around may be split into smaller regions. Your current code
>> >> >> does not address that case.
>> >> >
>> >> > If the above doesn't work, the only solution would be to permanently map
>> >> > these ranges as individual pages, no large blocks.
>> >>
>> >> That is not unreasonable, since regions >2MB are unusual.
>> >
>> > We'll have the contiguous bit supported at some point and we won't be
>> > able to use it for EFI run-time mappings. But I don't think that's
>> > essential, minor improvement on a non-critical path.
>> >
>> > I'll post some patches to always use PAGE_SIZE granularity for EFI
>> > run-time mappings.
>>
>> Given that contiguous bit mappings only affect the TLB footprint, I'd
>> be more concerned about not using block mappings for EfiMemoryMappedIo
>> regions (since they may cover fairly sizable NOR flashes like the 64
>> MB one QEMU mach-virt exposes).
>
> Good point.
>
>> So I would recommend to only use PAGE_SIZE granularity for
>> EfiRuntimeServicesCode and EfiRuntimeServicesData regions, since those
>> are the only ones that can be expected to appear in the Memory
>> Attributes table, and all other regions will only be mapped a single
>> time.
>
> Is there a possibility that EfiMemoryMappedIo share the same 64K page
> with EfiRuntimeServicesCode? If it does, it won't help much with
> avoiding splitting.

The spec does not allow it, and it would also imply that memory and
!memory share a 64 KB page frame in the hardware, which seems highly
unlikely as well.

> Unless I keep a combination of these series
> (checking the end/start overlap) with a forced page-only mapping for
> EfiRuntimeServicesCode/Data.
>

If we get rid of the splitting, the only 'issue' that remains is that
the page frame shared between two adjacent unaligned regions is mapped
twice (but the current code will always map them with the same
attribute)

So back to my question I posed a couple of posts ago: if the UEFI page
tables were live at this time (which they are not), could it ever be a
problem that a page table entry is rewritten with the exact same value
it had before (but without bbm?) If not, I think we could educate the
debug routines to allow this case (since it needs to read the entry to
check the valid bit anyway, if it needs to be strict about break
before make)