Re: Interesting race condition...

Jesper Juhl
2004-07-30T00:20:27Z

On Fri, 30 Jul 2004, Roger Luethi wrote:

> On Thu, 22 Jul 2004 22:04:46 -0500, Rob Landley wrote:
>> I just saw a funky thing. Here's the cut and past from the xterm...
>>
>> [root@(none) root]# ps ax | grep hack
>> 9964 pts/1 R 0:00 grep hack HOSTNAME= SHELL=/bin/bash TERM=xterm HISTSIZE=1000 USER=root LS_COLORS=no=00:fi=00:di=00;34:ln=00;36:pi=40;33:so=00;35:bd=40;33;01:cd=40;33;01:or=01;05;37;41:mi=01;05;37;41:ex=00;32:*.cmd=00;32:*.exe=00;32:*.com=00;32:*.btm=00;32:*.bat=00;32:*.sh=00;32:*.csh=00;32:*.tar=00;31:*.tgz=
>> [root@(none) root]# ps ax | grep hack
>> 9966 pts/1 S 0:00 grep hack
>>
>> Seems like some kind of race condition, dunno if it's in Fedore Core 1's ps
>> or the 2.6.7 kernel or what...
>
> If somebody posted a solution for this, I didn't see it. There's a race in
> the kernel, and considering the permissions on /proc/PID/{cmdline,environ}
> a security bug as well: If you win the race with a starting process, you
> can read its environment.
>
> This should plug the hole. Can you give it a spin?
>
> Roger
>
> --- linux-2.6.8-rc2-bk1/fs/proc/base.c.orig 2004-07-30 01:43:23.535967505 +0200
> +++ linux-2.6.8-rc2-bk1/fs/proc/base.c 2004-07-30 01:43:27.428303752 +0200
> @@ -329,6 +329,8 @@ static int proc_pid_cmdline(struct task_
> struct mm_struct *mm = get_task_mm(task);
> if (!mm)
> goto out;
> + if (!mm->arg_end)
> + goto out; /* Shh! No looking before we're done */
>
> len = mm->arg_end - mm->arg_start;
>

I might be reading the code wrong or not be fully aware of the
circumstances here, so please bear with me, but I took a look at
get_task_mm() and it can return NULL. If it does you'd end up
dereferencing a NULL pointer with "if (!mm->arg_end)" - or is that
garanteed not to happen here?

Wouldn't something like

if (!mm || !mm->arg_end)
goto out;

be safer?

Just my two pennies..

- Jesper Juhl <juhl-lkml@dif.dk>