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Date:   Wed, 11 Mar 2020 12:10:18 -0700
From:   Kees Cook <keescook@chromium.org>
To:     Bernd Edlinger <bernd.edlinger@hotmail.de>
Cc:     "Eric W. Biederman" <ebiederm@xmission.com>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Andrew Morton <akpm@linux-foundation.org>,
        Alexey Dobriyan <adobriyan@gmail.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Oleg Nesterov <oleg@redhat.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Andrei Vagin <avagin@gmail.com>,
        Ingo Molnar <mingo@kernel.org>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Yuyang Du <duyuyang@gmail.com>,
        David Hildenbrand <david@redhat.com>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        David Howells <dhowells@redhat.com>,
        James Morris <jamorris@linux.microsoft.com>,
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        Christian Kellner <christian@kellner.me>,
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Subject: Re: [PATCH 2/4] proc: Use new infrastructure to fix deadlocks in
 execve
Message-ID: <202003111208.640025F75@keescook>
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On Tue, Mar 10, 2020 at 06:45:32PM +0100, Bernd Edlinger wrote:
> This changes lock_trace to use the new exec_update_mutex
> instead of cred_guard_mutex.
> 
> This fixes possible deadlocks when the trace is accessing
> /proc/$pid/stack for instance.
> 
> This should be safe, as the credentials are only used for reading,
> and task->mm is updated on execve under the new exec_update_mutex.
> 
> Signed-off-by: Bernd Edlinger <bernd.edlinger@hotmail.de>

I have the same question here as in 3/4. I should probably rescind my
Reviewed-by until I'm convinced about the security-safety of this -- why
is this not a race against cred changes?

-Kees

> ---
>  fs/proc/base.c | 6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/proc/base.c b/fs/proc/base.c
> index ebea950..4fdfe4f 100644
> --- a/fs/proc/base.c
> +++ b/fs/proc/base.c
> @@ -403,11 +403,11 @@ static int proc_pid_wchan(struct seq_file *m, struct pid_namespace *ns,
>  
>  static int lock_trace(struct task_struct *task)
>  {
> -	int err = mutex_lock_killable(&task->signal->cred_guard_mutex);
> +	int err = mutex_lock_killable(&task->signal->exec_update_mutex);
>  	if (err)
>  		return err;
>  	if (!ptrace_may_access(task, PTRACE_MODE_ATTACH_FSCREDS)) {
> -		mutex_unlock(&task->signal->cred_guard_mutex);
> +		mutex_unlock(&task->signal->exec_update_mutex);
>  		return -EPERM;
>  	}
>  	return 0;
> @@ -415,7 +415,7 @@ static int lock_trace(struct task_struct *task)
>  
>  static void unlock_trace(struct task_struct *task)
>  {
> -	mutex_unlock(&task->signal->cred_guard_mutex);
> +	mutex_unlock(&task->signal->exec_update_mutex);
>  }
>  
>  #ifdef CONFIG_STACKTRACE
> -- 
> 1.9.1

-- 
Kees Cook