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

Alexandre Ghiti
2023-10-02T07:02:53Z

On 01/10/2023 17:19, Pedro Falcato wrote:
> 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.
>

We already encountered those issues and the same solution was recently
merged (restrict to sv48 unless otherwise specified):