linux-kernel - Re: [PATCH v2] Fixing directly deferencing a _

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAGudoHFXH_FDgKRaJvVgQ3W8wD2TC=8yhiNm1NECApnQ-CNAZQ@mail.gmail.com>
Date:   Thu, 26 Oct 2023 16:06:24 +0200
From:   Mateusz Guzik <mjguzik@...il.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Abhinav Singh <singhabhinav9051571833@...il.com>,
        akpm@...ux-foundation.org, brauner@...nel.org, surenb@...gle.com,
        michael.christie@...cle.com, mathieu.desnoyers@...icios.com,
        npiggin@...il.com, shakeelb@...gle.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning

On 10/26/23, Michael S. Tsirkin <mst@...hat.com> wrote:
> On Thu, Oct 26, 2023 at 05:46:21PM +0530, Abhinav Singh wrote:
>> This patch fixes the warning about directly dereferencing a pointer
>> tagged with __rcu annotation.
>>
>> Dereferencing the pointers tagged with __rcu directly should
>> always be avoided according to the docs. There is a rcu helper
>> functions rcu_dereference(...) to use when dereferencing a __rcu
>> pointer. This functions returns the non __rcu tagged pointer which
>> can be dereferenced just like a normal pointers.
>>
>> Signed-off-by: Abhinav Singh <singhabhinav9051571833@...il.com>
>
> Well yes but these need to be called under rcu_read_lock.
> Who does it here?
> If no one then maybe you found an actual bug and we need to
> fix it not paper over it.
>

There is no bug here.

p is the newly created thread, ->real_cred was initialized just prior
to this code and there is nobody to whack the creds from under it.

Second bit in the patch changes one real_parent deref, but leaves 2
others just above it. Once more no bug since the entire thing happens
under tasklist_lock, but the patch should either sort all these cases
or none.

I think it would help if the submitter had shown warnings they see.

-- 
Mateusz Guzik <mjguzik gmail.com>