Re: [PATCH] x86: efi: avoid BUILD_BUG_ON() for non-constant p4d_index

[Arvind Sankar <nivedita@alum.mit.edu>]

On Mon, Jan 18, 2021 at 09:24:09PM +0100, Borislav Petkov wrote:
> > > > As a matter of fact, it seems like the four assertions could be combined
> > > > into:
> > > >       BUILD_BUG_ON((EFI_VA_END & P4D_MASK) != (MODULES_END & P4D_MASK));
> > > >       BUILD_BUG_ON((EFI_VA_START & P4D_MASK) != (EFI_VA_END & P4D_MASK));
> > > > instead of separately asserting they're the same PGD entry and the same
> > > > P4D entry.
> > > >
> > > > Thanks.
> > >
> > > I actually don't quite get the MODULES_END check -- Ard, do you know
> > > what that's for?
> > >
> > 
> > Maybe Boris remembers? He wrote the original code for the 'new' EFI
> > page table layout.
> 
> That was added by Kirill for 5-level pgtables:
> 
>   e981316f5604 ("x86/efi: Add 5-level paging support")

That just duplicates the existing pgd_index() check for the p4d_index()
as well. It looks like the original commit adding
efi_sync_low_kernel_mappings() used to copy upto the PGD entry including
MODULES_END:
  d2f7cbe7b26a7 ("x86/efi: Runtime services virtual mapping")
and then Matt changed that when creating efi_mm:
  67a9108ed4313 ("x86/efi: Build our own page table structures")
to use EFI_VA_END instead but have a check that EFI_VA_END is in the
same entry as MODULES_END.

AFAICT, MODULES_END is only relevant as being something that happens to
be in the top 512GiB, and -1ul would be clearer.

> 
>  Documentation/x86/x86_64/mm.rst should explain the pagetable layout:
> 
>    ffffff8000000000 | -512    GB | ffffffeeffffffff |  444 GB | ... unused hole
>    ffffffef00000000 |  -68    GB | fffffffeffffffff |   64 GB | EFI region mapping space
>    ffffffff00000000 |   -4    GB | ffffffff7fffffff |    2 GB | ... unused hole
>    ffffffff80000000 |   -2    GB | ffffffff9fffffff |  512 MB | kernel text mapping, mapped to physical address 0
>    ffffffff80000000 |-2048    MB |                  |         |
>    ffffffffa0000000 |-1536    MB | fffffffffeffffff | 1520 MB | module mapping space
>    ffffffffff000000 |  -16    MB |                  |         |
>       FIXADDR_START | ~-11    MB | ffffffffff5fffff | ~0.5 MB | kernel-internal fixmap range, variable size and offset
> 
> That thing which starts at -512 GB above is the last PGD on the
> pagetable. In it, between -4G and -68G there are 64G which are the EFI
> region mapping space for runtime services.
> 
> Frankly I'm not sure what this thing is testing because the EFI VA range
> is hardcoded and I can't imagine it being somewhere else *except* in the
> last PGD.

It's just so that someone doesn't just change the #define's for
EFI_VA_END/START and think that it will work, I guess.

Another reasonable option, for example, would be to reserve an entire
PGD entry, allowing everything but the PGD level to be shared, and
adding the EFI PGD to the pgd_list and getting rid of
efi_sync_low_kernel_mappings() altogether. There aren't that many PGD
entries still unused though, so this is probably not worth it.

> 
> Lemme add Kirill for clarification.
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette