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Date: Fri, 29 Apr 2016 07:59:00 +0200
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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 23:26, 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().
>>
>> This bug was found by the PaX size_overflow plugin that detected the
>> arithmetic underflow of 'this_len = env_end - (env_start + src)' when
>> env_end is still zero.
>
> So what are the implications of this?  From my reading, a craftily
> constructed application could occasionally read arbitrarily large
> amounts of kernel memory?

I don't think access_remote_vm() is capable of that. So, the only
consequence is, userland trying to access /proc/<PID>/environ of a not
yet fully set up process may get inconsistent data as we're in the
middle of copying in the environment variables.

Regards,
Mathias