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 3:46 PM, 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.
> > 
> > Looking around, there's at least one quirk for the SGI UV1 system that
> > enables EFI_OLD_MMAP automatically. There might be others that I
> > missed, but I think that's it.
> > 
> > So it *can* trigger without "efi=old_mmap", but not on any normal machines.
> > 
> > And as Pavel points out, even when the bug is active, it's pretty hard
> > to actually trigger.
> > 
> > But yeah, there may be other EFI patches that I didn't notice that
> > changed things in other ways too.
> > 
> >                Linus
> 
> The bug is not present in the latest upstream kernel because the efi_pgd is
> correctly aligned:
> 
>   arch/x86/platform/efi/efi_64.c
>     int __init efi_alloc_page_tables(void)
>       efi_pgd = (pgd_t *)__get_free_pages(gfp_mask, PGD_ALLOCATION_ORDER);

Yes, I came exactly to the same conclusion, but I didn't want to call Linus
a moron before I triple checked that trampoline_pgd is still there, but
only every used to get out of the realmode swamp at bpot.

Thanks,

	tglx