linux-kernel - Re: [PATCH 8/8] x86/kaslr: Don't use 64-bit mem

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Date:   Tue, 28 Jul 2020 13:05:12 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     Kees Cook <keescook@...omium.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] x86/kaslr: Don't use 64-bit mem_vector for 32-bit
 kernel


* Arvind Sankar <nivedita@...m.mit.edu> wrote:

> Commit
>   f28442497b5c ("x86/boot: Fix KASLR and memmap= collision")
> converted mem_vector type to use 64-bit on the 32-bit kernel as well,
> based on Thomas's review [0]. However:
> - the code still doesn't consistently use 64-bit types. For instance,
>   mem_avoid_overlap uses 32-bit types when checking for overlaps.  This
>   is actually okay, as the passed in memory range will have been clipped
>   to below 4G, but it is difficult to reason about the safety of the
>   code.
> - KASLR on 32-bit can't use memory above 4G anyway, so it's pointless
>   to keep track of ranges above 4G.
> 
> Switch the type back to unsigned long, and use a helper function to clip
> regions to 4G on 32-bit, when storing mem_avoid, immovable_mem, EFI,
> E820 and command-line memory regions.
> 
> [0] https://lore.kernel.org/linux-nvdimm/alpine.DEB.2.20.1701111246400.3579@nanos/

So why not just standardize all the calculations around u64, and for 
32-bit kernels add in a debug check that double checks that it all 
remained within the first 32 bits?

It's much easier to argue about this if we don't have to worry about 
32-bit overflows and if the types are simple and bitness invariant.

Yes, technically it's slightly bloated on 32-bit kernels, but 
maintainability of 32-bit kernels is the primary concern now ...

Thanks,

	Ingo