linux-kernel - Re: [PATCH 2/2] riscv: mm: Update mmap_rnd_bits

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Date:   Sat, 30 Sep 2023 10:02:35 +0100
From:   Conor Dooley <conor@...nel.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Kees Cook <keescook@...omium.org>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-riscv@...ts.infradead.org, llvm@...ts.linux.dev,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] riscv: mm: Update mmap_rnd_bits_max

On Fri, Sep 29, 2023 at 03:52:22PM -0700, Sami Tolvanen wrote:
> On Fri, Sep 29, 2023 at 2:54 PM Kees Cook <keescook@...omium.org> wrote:
> >
> > On Fri, Sep 29, 2023 at 09:11:58PM +0000, Sami Tolvanen wrote:
> > > ARCH_MMAP_RND_BITS_MAX is based on Sv39, which leaves a few
> > > potential bits of mmap randomness on the table if we end up enabling
> > > 4/5-level paging. Update mmap_rnd_bits_max to take the final address
> > > space size into account. This increases mmap_rnd_bits_max from 24 to
> > > 33 with Sv48/57.
> > >
> > > Signed-off-by: Sami Tolvanen <samitolvanen@...gle.com>
> >
> > I like this. Is RISCV the only arch where the paging level can be chosen
> > at boot time?
> 
> I haven't seen this elsewhere, but I also haven't looked at all the
> other architectures that closely. arm64 does something interesting
> with ARM64_VA_BITS_52, but I think we can still handle that in
> Kconfig.

AFAIU, x86-64 can do this also:

	no4lvl		[RISCV] Disable 4-level and 5-level paging modes. Forces
			kernel to use 3-level paging instead.

	no5lvl		[X86-64,RISCV] Disable 5-level paging mode. Forces
			kernel to use 4-level paging instead.

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