Re: [PATCH] x86/boot/64: Clear CR4.PGE to disable global 1:1 mappings

[Ingo Molnar <mingo@kernel.org>]

* Ard Biesheuvel <ardb+git@google.com> wrote:

> From: Ard Biesheuvel <ardb@kernel.org>
> 
> The early 64-bit boot code must be entered with a 1:1 mapping of the
> bootable image, but it cannot operate without a 1:1 mapping of all the
> assets in memory that it accesses, and therefore, it creates such
> mappings for all known assets upfront, and additional ones on demand
> when a page fault happens on a memory address.
> 
> These mappings are created with the global bit G set, as the flags used
> to create page table descriptors are based on __PAGE_KERNEL_LARGE_EXEC
> defined by the core kernel, even though the context where these mappings
> are used is very different.
> 
> This means that the TLB maintenance carried out by the decompressor is
> not sufficient if it is entered with CR4.PGE enabled, which has been
> observed to happen with the stage0 bootloader of project Oak. While this
> is a dubious practice if no global mappings are being used to begin
> with, the decompressor is clearly at fault here for creating global
> mappings and not performing the appropriate TLB maintenance.
> 
> Since commit
> 
>   f97b67a773cd84b ("x86/decompressor: Only call the trampoline when changing paging levels")
> 
> CR4 is no longer modified by the decompressor if no change in the number
> of paging levels is needed. Before that, CR4 would always be set to a
> known value with PGE cleared.

So if we do this for robustness & historical pre-f97b67a773cd84b 
quirk-reliance's sake, I'd prefer if we loaded a known CR4 value again, 
instead of just turning off the PGE bit.

It's probably also a tiny bit faster, as no CR4 read has to be performed.

Thanks,

	Ingo