Re: [PATCH] x86/efistub: Omit physical KASLR when memory reservations exist

[Kees Cook]

On Thu, May 16, 2024 at 11:05:42AM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@xxxxxxxxxx>
> 
> The legacy decompressor has elaborate logic to ensure that the
> randomized physical placement of the decompressed kernel image does not
> conflict with any memory reservations, including ones specified on the
> command line using mem=, memmap=, efi_fake_mem= or hugepages=, which are
> taken into account by the kernel proper at a later stage.
> 
> When booting in EFI mode, it is the firmware's job to ensure that the
> chosen range does not conflict with any memory reservations that it
> knows about, and this is trivially achieved by using the firmware's
> memory allocation APIs.
> 
> That leaves reservations specified on the command line, though, which
> the firmware knows nothing about, as these regions have no other special
> significance to the platform. Since commit
> 
>   a1b87d54f4e4 ("x86/efistub: Avoid legacy decompressor when doing EFI boot")
> 
> these reservations are not taken into account when randomizing the
> physical placement, which may result in conflicts where the memory
> cannot be reserved by the kernel proper because its own executable image
> resides there.
> 
> To avoid having to duplicate or reuse the existing complicated logic,
> disable physical KASLR entirely when such overrides are specified. These
> are mostly diagnostic tools or niche features, and physical KASLR (as
> opposed to virtual KASLR, which is much more important as it affects the
> memory addresses observed by code executing in the kernel) is something
> we can live without.
> 
> Closes: https://lkml.kernel.org/r/FA5F6719-8824-4B04-803E-82990E65E627%40akamai.com
> Reported-by: Ben Chaney <bchaney@xxxxxxxxxx>
> Fixes: a1b87d54f4e4 ("x86/efistub: Avoid legacy decompressor when doing EFI boot")
> Cc: <stable@xxxxxxxxxxxxxxx> # v6.1+
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>

Yup, all good by me:

Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>

-- 
Kees Cook