Re: [PATCH 2/2] riscv: mm: Update mmap_rnd_bits_max

Pedro Falcato
2023-10-01T15:20:02Z

On Sun, Oct 1, 2023 at 2:51 AM Kees Cook <keescook@chromium.org> wrote:
>
> 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

(Replying here for visibility, tell me if you want to move this
discussion to github)

AIUI, x86 cannot do this for compat reasons. Even if you enable LA57,
mmap only gives you < 48-bit addresses, for compatibility with things
like JITs, etc that stash information in the upper 16 bits. You need
to pass a > 48-bit mmap hint to get 57-bit addresses.

I imagine riscv does not have this issue yet, due to little
accumulated cruft, but it may be wise to check against popular JITters
for these problems on riscv code.

--
Pedro