linux-kernel - Re: [PATCH v2 1/4] riscv: implement user_access

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Message-ID: <e4a4c688-b78c-468b-8196-68d2df980167@tenstorrent.com>
Date: Thu, 13 Feb 2025 11:07:03 -0600
From: Cyril Bur <cyrilbur@...storrent.com>
To: Ben Dooks <ben.dooks@...ethink.co.uk>,
 Charlie Jenkins <charlie@...osinc.com>
Cc: palmer@...belt.com, aou@...s.berkeley.edu, paul.walmsley@...ive.com,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 Jisheng Zhang <jszhang@...nel.org>
Subject: Re: [PATCH v2 1/4] riscv: implement user_access_begin and families



On 6/2/2025 1:08 am, Ben Dooks wrote:
> 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@...nel.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@...nel.org>
>>> Signed-off-by: Cyril Bur <cyrilbur@...storrent.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.

Oh! Could you please link the patch? I haven't seen it and can't seem to 
find it now.

Thanks.

> 
>