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

[Nathan Chancellor]

On Wed, Jan 20, 2021 at 10:33:43AM +0100, Ard Biesheuvel wrote:
> On Mon, 18 Jan 2021 at 22:42, Arvind Sankar <nivedita@xxxxxxxxxxxx> wrote:
> >
> > 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.
> >
> 
> The churn doesn't seem to be worth it, tbh.
> 
> So could we get rid of the complexity here, and only build_bug() on
> the start address of the EFI region being outside the topmost p4d?
> That should make the PGD test redundant as well.

Was there ever a resolution to this conversation or a patch sent? I am
still seeing the build failure that Arnd initially sent the patch for.
x86_64 all{mod,yes}config with clang are going to ship broken in 5.11.

Cheers,
Nathan