Re: [PATCH] pidfd: Stop taking cred_guard_mutex

[Jann Horn <jannh@google.com>]

On Tue, Mar 10, 2020 at 7:54 PM Eric W. Biederman <ebiederm@xmission.com> wrote:
> During exec some file descriptors are closed and the files struct is
> unshared.  But all of that can happen at other times and it has the
> same protections during exec as at ordinary times.  So stop taking the
> cred_guard_mutex as it is useless.
>
> Furthermore he cred_guard_mutex is a bad idea because it is deadlock
> prone, as it is held in serveral while waiting possibly indefinitely
> for userspace to do something.

Please don't. Just use the new exec_update_mutex like everywhere else.

> Cc: Sargun Dhillon <sargun@sargun.me>
> Cc: Christian Brauner <christian.brauner@ubuntu.com>
> Cc: Arnd Bergmann <arnd@arndb.de>
> Fixes: 8649c322f75c ("pid: Implement pidfd_getfd syscall")
> Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
> ---
>  kernel/pid.c | 6 ------
>  1 file changed, 6 deletions(-)
>
> Christian if you don't have any objections I will take this one through
> my tree.
>
> I tried to figure out why this code path takes the cred_guard_mutex and
> the archive on lore.kernel.org was not helpful in finding that part of
> the conversation.

That was my suggestion.

> diff --git a/kernel/pid.c b/kernel/pid.c
> index 60820e72634c..53646d5616d2 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -577,17 +577,11 @@ static struct file *__pidfd_fget(struct task_struct *task, int fd)
>         struct file *file;
>         int ret;
>
> -       ret = mutex_lock_killable(&task->signal->cred_guard_mutex);
> -       if (ret)
> -               return ERR_PTR(ret);
> -
>         if (ptrace_may_access(task, PTRACE_MODE_ATTACH_REALCREDS))
>                 file = fget_task(task, fd);
>         else
>                 file = ERR_PTR(-EPERM);
>
> -       mutex_unlock(&task->signal->cred_guard_mutex);
> -
>         return file ?: ERR_PTR(-EBADF);
>  }

If you make this change, then if this races with execution of a setuid
program that afterwards e.g. opens a unix domain socket, an attacker
will be able to steal that socket and inject messages into
communication with things like DBus. procfs currently has the same
race, and that still needs to be fixed, but at least procfs doesn't
let you open things like sockets because they don't have a working
->open handler, and it enforces the normal permission check for opening files.