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

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Message-ID: <CACVxJT9J36Rd1_ayTXp+jdo=A90O0P4noNuF85+dsAz0ZWaZPw@mail.gmail.com>
Date:	Fri, 29 Apr 2016 13:11:14 +0300
From:	Alexey Dobriyan <adobriyan@...il.com>
To:	Mateusz Guzik <mguzik@...hat.com>
Cc:	Mathias Krause <minipli@...glemail.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	Emese Revfy <re.emese@...il.com>,
	Pax Team <pageexec@...email.hu>,
	Al Viro <viro@...iv.linux.org.uk>,
	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, Apr 28, 2016 at 10:20 PM, Mateusz Guzik <mguzik@...hat.com> wrote:
> On Thu, Apr 28, 2016 at 09:04:18PM +0200, Mathias Krause 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.
>>
>> Reported-at: https://forums.grsecurity.net/viewtopic.php?f=3&t=4363
>> Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=116461
>> ---
>>  fs/proc/base.c |    3 ++-
>>  1 file changed, 2 insertions(+), 1 deletion(-)
>>
>> diff --git a/fs/proc/base.c b/fs/proc/base.c
>> index 4f764c2ac1a5..45f2162e55b2 100644
>> --- a/fs/proc/base.c
>> +++ b/fs/proc/base.c
>> @@ -955,7 +955,8 @@ static ssize_t environ_read(struct file *file, char __user *buf,
>>       struct mm_struct *mm = file->private_data;
>>       unsigned long env_start, env_end;
>>
>> -     if (!mm)
>> +     /* Ensure the process spawned far enough to have an environment. */
>> +     if (!mm || !mm->env_end)
>>               return 0;
>>
>>       page = (char *)__get_free_page(GFP_TEMPORARY);
>
> In this case get_cmdline in mm/util.c should also be patched for
> completness. It tests for arg_end, but later accesses env_end.

Sort of. get_cmdline() is only really used in audit code applied
to an exiting process which has cmdline setup long ago.

Should have rewrote /proc/*/environ as well... :-(

    Alexey