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Date: Fri, 29 Apr 2016 08:02:24 +0200
Message-ID: <CA+rthh98mYF8heKzEN6Y28ATgjXL54_i6DTKpRV4B52r3hmofA@mail.gmail.com>
Subject: Re: [PATCH] proc: prevent accessing /proc/<PID>/environ until it's ready
From: Mathias Krause <minipli@googlemail.com>
To: Andrew Morton <akpm@linux-foundation.org>
Cc: "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Emese Revfy <re.emese@gmail.com>, Pax Team <pageexec@freemail.hu>,
        Al Viro <viro@zeniv.linux.org.uk>, Mateusz Guzik <mguzik@redhat.com>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Cyrill Gorcunov <gorcunov@openvz.org>, Jarod Wilson <jarod@redhat.com>
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On 28 April 2016 at 23:30, Andrew Morton <akpm@linux-foundation.org> wrote:
> On Thu, 28 Apr 2016 21:04:18 +0200 Mathias Krause <minipli@googlemail.com> wrote:
>
>> If /proc/<PID>/environ gets read before the envp[] array is fully set
>> up in create_{aout,elf,elf_fdpic,flat}_tables(), we might end up trying
>> to read more bytes than are actually written, as env_start will already
>> be set but env_end will still be zero, making the range calculation
>> underflow, allowing to read beyond the end of what has been written.
>>
>> Fix this as it is done for /proc/<PID>/cmdline by testing env_end for
>> zero. It is, apparently, intentionally set last in create_*_tables().
>
> Also, if this is indeed our design then
>
> a) the various create_*_tables() should have comments in there which
>    explain this subtlety to the reader.  Or, better, they use a common
>    helper function for this readiness-signaling operation because..
>
> b) we'll need some barriers there to ensure that the environ_read()
>    caller sees the create_*_tables() writes in the correct order.

I totally agree that this kind of "synchronization" is rather fragile.
Adding comments won't help much, I fear. Rather a dedicated flag,
signaling "process ready for inspection" may be needed. So far, that's
what env_end is (ab-)used for.

Regards,
Mathias