Re: [PATCH v2 1/4] riscv: implement user_access_begin and families

Ben Dooks
2025-02-05T15:09:04Z

On 17/01/2025 23:21, Charlie Jenkins wrote:
> On Mon, Nov 18, 2024 at 11:01:09PM +0000, Cyril Bur wrote:
>> From: Jisheng Zhang <jszhang@kernel.org>
>>
>> Currently, when a function like strncpy_from_user() is called,
>> the userspace access protection is disabled and enabled
>> for every word read.
>>
>> By implementing user_access_begin and families, the protection
>> is disabled at the beginning of the copy and enabled at the end.
>>
>> The __inttype macro is borrowed from x86 implementation.
>>
>> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
>> Signed-off-by: Cyril Bur <cyrilbur@tenstorrent.com>

If we're doing this, then saving the STATUS.SUM flag is going to
be more important than before. We had this discussion when the
initial user-access with syzbot stress testing turned up.

We partially fixed this by rewriting the ordering in the __put_user
function to stop the 'x' argument being evaluated inside the area
where SUM is enabled, but this is going to make the window of
opportunity of a thread switch much bigger and the bug will just
come back and bite harder.

If you want I can look at re-doing my original patch and resubmitting.

--
Ben Dooks http://www.codethink.co.uk/
Senior Engineer Codethink - Providing Genius