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Date: Thu, 16 May 2024 11:55:01 -0700
From: Kees Cook <keescook@chromium.org>
To: Ard Biesheuvel <ardb+git@google.com>
Cc: linux-efi@vger.kernel.org, linux-kernel@vger.kernel.org, x86@kernel.org,
	Ard Biesheuvel <ardb@kernel.org>, Ben Chaney <bchaney@akamai.com>,
	stable@vger.kernel.org
Subject: Re: [PATCH] x86/efistub: Omit physical KASLR when memory
 reservations exist
Message-ID: <202405161154.01864575AD@keescook>
References: <20240516090541.4164270-2-ardb+git@google.com>
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On Thu, May 16, 2024 at 11:05:42AM +0200, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb@kernel.org>
> 
> 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@akamai.com>
> Fixes: a1b87d54f4e4 ("x86/efistub: Avoid legacy decompressor when doing EFI boot")
> Cc: <stable@vger.kernel.org> # v6.1+
> Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

Yup, all good by me:

Reviewed-by: Kees Cook <keescook@chromium.org>

-- 
Kees Cook