linux-kernel - Re: [PATCH 0/3] Fix some seq_file users that were recently broken

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Message-ID: <87ft27ebuy.fsf@notabene.neil.brown.name>
Date:   Mon, 08 Feb 2021 09:45:09 +1100
From:   NeilBrown <neilb@...e.de>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>,
        Xin Long <lucien.xin@...il.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Andy Lutomirski <luto@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        Marcelo Ricardo Leitner <marcelo.leitner@...il.com>,
        "David S. Miller" <davem@...emloft.net>,
        linux-sctp@...r.kernel.org, netdev@...r.kernel.org
Subject: Re: [PATCH 0/3] Fix some seq_file users that were recently broken

On Fri, Feb 05 2021, Andrew Morton wrote:

> On Fri, 05 Feb 2021 11:36:30 +1100 NeilBrown <neilb@...e.de> wrote:
>
>> A recent change to seq_file broke some users which were using seq_file
>> in a non-"standard" way ...  though the "standard" isn't documented, so
>> they can be excused.  The result is a possible leak - of memory in one
>> case, of references to a 'transport' in the other.
>> 
>> These three patches:
>>  1/ document and explain the problem
>>  2/ fix the problem user in x86
>>  3/ fix the problem user in net/sctp
>> 
>
> 1f4aace60b0e ("fs/seq_file.c: simplify seq_file iteration code and
> interface") was August 2018, so I don't think "recent" applies here?

I must be getting old :-(

>
> I didn't look closely, but it appears that the sctp procfs file is
> world-readable.  So we gave unprivileged userspace the ability to leak
> kernel memory?

Not quite that bad.  The x86 problem allows arbitrary memory to be
leaked, but that is in debugfs (as I'm sure you saw) so is root-only.
The sctp one only allows an sctp_transport to be pinned.  That is not
good and would, e.g., prevent the module from being unloaded.  But
unlike a simple memory leak it won't affect anything outside of sctp.

>
> So I'm thinking that we aim for 5.12-rc1 on all three patches with a cc:stable?

Thanks for looking after these!

NeilBrown

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