Re: [PATCH 4.4 00/37] 4.4.110-stable review

[Thomas Gleixner]

On Thu, 11 Jan 2018, Steven Sistare wrote:
> On 1/11/2018 5:30 PM, Thomas Gleixner wrote:
> > On Thu, 11 Jan 2018, Thomas Gleixner wrote:
> >> On Thu, 11 Jan 2018, Linus Torvalds wrote:
> >>
> >>> On Thu, Jan 11, 2018 at 12:37 PM, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
> >>>>
> >>>> 67a9108ed431 ("x86/efi: Build our own page table structures")
> >>>>
> >>>> got rid of EFI depending on real_mode_header->trampoline_pgd
> >>>
> >>> So I think it only got rid of by default - the codepath is still
> >>> there, the allocation is still there, it's just that it's not actually
> >>> used unless somebody does that "efi=old_mmap" thing.
> >>
> >> Yes, the trampoline_pgd is still around, but I can't figure out how it
> >> would be used after boot. Confused, digging more.
> > 
> > So coming back to the same commit. From the changelog:
> > 
> >     This is caused by mapping EFI regions with RWX permissions.
> >     There isn't much we can do to restrict the permissions for these
> >     regions due to the way the firmware toolchains mix code and
> >     data, but we can at least isolate these mappings so that they do
> >     not appear in the regular kernel page tables.
> >     
> >     In commit d2f7cbe7b26a ("x86/efi: Runtime services virtual
> >     mapping") we started using 'trampoline_pgd' to map the EFI
> >     regions because there was an existing identity mapping there
> >     which we use during the SetVirtualAddressMap() call and for
> >     broken firmware that accesses those addresses.
> > 
> > So this very commit gets rid of the (ab)use of trampoline_pgd and allocates
> > efi_pgd, which we made use the proper size.
> > 
> > trampoline_pgd is since then only used to get into long mode in
> > realmode/rm/trampoline_64.S and for reboot in machine_real_restart().
> > 
> > The runtime services stuff does not use it in kernel versions >= 4.6
> > 
> > Thanks,
> > 
> > 	tglx
> 
> Yes, and addressing Linus' concern about EFI_OLD_MEMMAP, those paths are 
> independent of it.  When EFI_OLD_MMAP is enabled, the efi pgd is not 
> used, and the bug will not bite.

We have a fix queued in tip/x86/pti which addresses a missing NX clear, but
that's a different story.

Thanks,

	tglx