linux-kernel - Re: [PATCH 1/2] fs/proc: do_task_stat: Fix ESP not readable during coredump

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Message-ID: <20241220145326.Q7Z6NQ7j@linutronix.de>
Date: Fri, 20 Dec 2024 21:53:26 +0700
From: Nam Cao <namcao@...utronix.de>
To: Oleg Nesterov <oleg@...hat.com>
Cc: Shuah Khan <shuah@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dylan Hatch <dylanbhatch@...gle.com>,
	"Eric W . Biederman" <ebiederm@...ssion.com>,
	John Ogness <john.ogness@...utronix.de>,
	Kees Cook <kees@...nel.org>, linux-kernel@...r.kernel.org,
	linux-fsdevel@...r.kernel.org, linux-kselftest@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/2] fs/proc: do_task_stat: Fix ESP not readable during
 coredump

Hi Oleg,

On Tue, Dec 17, 2024 at 04:09:14PM +0100, Oleg Nesterov wrote:
> On 12/17, Oleg Nesterov wrote:
> >
> > On 11/06, Nam Cao wrote:
> > >
> > > @@ -534,6 +517,23 @@ static int do_task_stat(struct seq_file *m, struct pid_namespace *ns,
> > >  		ppid = task_tgid_nr_ns(task->real_parent, ns);
> > >  		pgid = task_pgrp_nr_ns(task, ns);
> > >
> > > +		/*
> > > +		 * esp and eip are intentionally zeroed out.  There is no
> > > +		 * non-racy way to read them without freezing the task.
> > > +		 * Programs that need reliable values can use ptrace(2).
> >
> > OK,
> >
> > but then:
> >
> > > +		 * The only exception is if the task is core dumping because
> > > +		 * a program is not able to use ptrace(2) in that case. It is
> > > +		 * safe because the task has stopped executing permanently.
> > > +		 */
> > > +		if (permitted && task->signal->core_state) {
> > > +			if (try_get_task_stack(task)) {
> > > +				eip = KSTK_EIP(task);
> > > +				esp = KSTK_ESP(task);
> > > +				put_task_stack(task);
> >
> > How can the task->signal->core_state check help ?
> >
> > Suppose we have a task T1 with T1-pid == 100 and you read /proc/100/stat.
> > It is possible that the T1's sub-thread T2 starts the coredumping and sets
> > signal->core_state != NULL.
> >
> > But read(/proc/100/stat) can run before T1 gets SIGKILL from T2 and enters
> > the kernel mode?

Right, I missed that race, thanks for pointing it out.

> Can't the trivial patch below fix the problem?

It can. In fact this is the original fix we had. I thought that checking a
single "core_state" is simpler than checking 3 flags, oh well..

Can you send a proper patch, or should I do it?

Best regards,
Nam

> 
> Oleg.
> 
> 
> --- xfs/proc/array.c
> +++ x/fs/proc/array.c
> @@ -500,7 +500,7 @@
>  		 * a program is not able to use ptrace(2) in that case. It is
>  		 * safe because the task has stopped executing permanently.
>  		 */
> -		if (permitted && (task->flags & (PF_EXITING|PF_DUMPCORE))) {
> +		if (permitted && (task->flags & (PF_EXITING|PF_DUMPCORE|PF_POSTCOREDUMP))) {
>  			if (try_get_task_stack(task)) {
>  				eip = KSTK_EIP(task);
>  				esp = KSTK_ESP(task);
>