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Date:   Tue, 28 Jul 2020 11:49:55 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Arvind Sankar <nivedita@alum.mit.edu>
Cc:     x86@kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 4/8] x86/kaslr: Initialize mem_limit to the real
 maximum address
Message-ID: <202007281133.739AAFD2AF@keescook>
References: <20200727215047.3341098-1-nivedita@alum.mit.edu>
 <20200727230801.3468620-5-nivedita@alum.mit.edu>
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On Mon, Jul 27, 2020 at 07:07:57PM -0400, Arvind Sankar wrote:
> On 64-bit, the kernel must be placed below MAXMEM (64TiB with 4-level
> paging or 4PiB with 5-level paging). This is currently not enforced by
> KASLR, which thus implicitly relies on physical memory being limited to
> less than 64TiB.
> 
> On 32-bit, the limit is KERNEL_IMAGE_SIZE (512MiB). This is enforced by
> special checks in __process_mem_region.
> 
> Initialize mem_limit to the maximum (depending on architecture), instead
> of ULLONG_MAX, and make sure the command-line arguments can only
> decrease it. This makes the enforcement explicit on 64-bit, and
> eliminates the 32-bit specific checks to keep the kernel below 512M.
> 
> Check upfront to make sure the minimum address is below the limit before
> doing any work.
> 
> Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>

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

-- 
Kees Cook