linux-kernel - Re: [PATCH] proc: prevent accessing /proc/<PID>/environ until it's ready

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Date:	Thu, 28 Apr 2016 14:30:58 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mathias Krause <minipli@...glemail.com>
Cc:	linux-kernel@...r.kernel.org, Emese Revfy <re.emese@...il.com>,
	Pax Team <pageexec@...email.hu>,
	Al Viro <viro@...iv.linux.org.uk>,
	Mateusz Guzik <mguzik@...hat.com>,
	Alexey Dobriyan <adobriyan@...il.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Jarod Wilson <jarod@...hat.com>
Subject: Re: [PATCH] proc: prevent accessing /proc/<PID>/environ until it's
 ready

On Thu, 28 Apr 2016 21:04:18 +0200 Mathias Krause <minipli@...glemail.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.

> 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.