lkml.org 
[lkml]   [2024]   [Apr]   [11]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH v2] x86/boot/64: Clear CR4.PGE to disable global 1:1 mappings

* 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
> consistent value with PGE cleared.
>
> So let's reinstate a simplified version of the original logic to put CR4
> into a known state, and preserve the PAE, MCE and LA57 bits, none of
> which can be modified freely at this point (PAE and LA57 cannot be
> changed while running in long mode, and MCE cannot be cleared when
> running under some hypervisors)
>
> Cc: Conrad Grobler <grobler@google.com>
> Cc: Kevin Loughlin <kevinloughlin@google.com>
> Fixes: f97b67a773cd84b ("x86/decompressor: Only call the trampoline when ...")
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
> ---
> v2: Bring back the original logic rather than only clearing PGE. Note
> that this means that the load of CR4 cannot be elided since MCE and
> LA57 cannot be set unconditionally.
>
> arch/x86/boot/compressed/head_64.S | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index bf4a10a5794f..d0f2c591f730 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -398,6 +398,11 @@ SYM_CODE_START(startup_64)
> call sev_enable
> #endif
>
> + /* Preserve only the CR4 bits that must be preserved, and clear the rest */
> + movq %cr4, %rax
> + andl $(X86_CR4_PAE | X86_CR4_MCE | X86_CR4_LA57), %eax
> + movq %rax, %cr4

Yeah, this is still better IMO than the original patch.

Note that you reused the original title, which isn't entirely accurate
anymore:

x86/boot/64: Clear CR4.PGE to disable global 1:1 mappings

I updated it to:

x86/boot/64: Clear most of CR4 in startup_64(), except PAE, MCE and LA57

Thanks,

Ingo

\
 
 \ /
  Last update: 2024-05-27 16:35    [W:0.065 / U:0.532 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site