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

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Message-ID: <CA+rthh98mYF8heKzEN6Y28ATgjXL54_i6DTKpRV4B52r3hmofA@mail.gmail.com>
Date:	Fri, 29 Apr 2016 08:02:24 +0200
From:	Mathias Krause <minipli@...glemail.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"linux-kernel@...r.kernel.org" <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 28 April 2016 at 23:30, Andrew Morton <akpm@...ux-foundation.org> wrote:
> 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.

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