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Date:   Sat, 30 Sep 2023 14:01:11 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Conor Dooley <conor@kernel.org>
Cc:     Sami Tolvanen <samitolvanen@google.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-riscv@lists.infradead.org, llvm@lists.linux.dev,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 2/2] riscv: mm: Update mmap_rnd_bits_max
Message-ID: <202309301400.4E1AD87@keescook>
References: <20230929211155.3910949-4-samitolvanen@google.com>
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 <202309291452.66ED9B4D83@keescook>
 <CABCJKufxUVoO+yJ+513W5FOFu6u45N=6wZe6a69u+8LU6A_N2Q@mail.gmail.com>
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On Sat, Sep 30, 2023 at 10:02:35AM +0100, Conor Dooley wrote:
> 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@chromium.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@google.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.

Ah-ha! Okay, well, then let's track this idea:
https://github.com/KSPP/linux/issues/346


-- 
Kees Cook