Re: [PATCH v7 2/3] pid: Introduce pidfd_getfd syscall

[Christian Brauner]

On Thu, Dec 26, 2019 at 06:03:36PM +0000, Sargun Dhillon wrote:
> This syscall allows for the retrieval of file descriptors from other
> processes, based on their pidfd. This is possible using ptrace, and
> injection of parasitic code to inject code which leverages SCM_RIGHTS
> to move file descriptors between a tracee and a tracer. Unfortunately,
> ptrace comes with a high cost of requiring the process to be stopped,
> and breaks debuggers. This does not require stopping the process under
> manipulation.
> 
> One reason to use this is to allow sandboxers to take actions on file
> descriptors on the behalf of another process. For example, this can be
> combined with seccomp-bpf's user notification to do on-demand fd
> extraction and take privileged actions. One such privileged action
> is binding a socket to a privileged port.
> 
> This also adds the syscall to all architectures at the same time.
> 
> /* prototype */
>   /* flags is currently reserved and should be set to 0 */
>   int sys_pidfd_getfd(int pidfd, int fd, unsigned int flags);
> 
> /* testing */
> Ran self-test suite on x86_64

Fyi, I'm likely going to rewrite/add parts of/to this once I apply.

A few comments below.

> diff --git a/kernel/pid.c b/kernel/pid.c
> index 2278e249141d..4a551f947869 100644
> --- a/kernel/pid.c
> +++ b/kernel/pid.c
> @@ -578,3 +578,106 @@ void __init pid_idr_init(void)
>  	init_pid_ns.pid_cachep = KMEM_CACHE(pid,
>  			SLAB_HWCACHE_ALIGN | SLAB_PANIC | SLAB_ACCOUNT);
>  }
> +
> +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 = ERR_PTR(-EPERM);
> +		goto out;
> +	}
> +
> +	file = fget_task(task, fd);
> +	if (!file)
> +		file = ERR_PTR(-EBADF);
> +
> +out:
> +	mutex_unlock(&task->signal->cred_guard_mutex);
> +	return file;
> +}

Looking at this code now a bit closer, ptrace_may_access() and
fget_task() both take task_lock(task) so this currently does:

task_lock();
/* check access */
task_unlock();

task_lock();
/* get fd */
task_unlock();

which doesn't seem great.

I would prefer if we could do:
task_lock();
/* check access */
/* get fd */
task_unlock();

But ptrace_may_access() doesn't export an unlocked variant so _shrug_.

But we can write this a little cleaner without the goto as:

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 don't like the ?: just do:

if (!file)
	return ERR_PTR(-EBADF);

return file;

though I prefer the shorter ?: syntax which is perfect for shortcutting
returns.

> +
> +static int pidfd_getfd(struct pid *pid, int fd)
> +{
> +	struct task_struct *task;
> +	struct file *file;
> +	int ret, retfd;
> +
> +	task = get_pid_task(pid, PIDTYPE_PID);
> +	if (!task)
> +		return -ESRCH;
> +
> +	file = __pidfd_fget(task, fd);
> +	put_task_struct(task);
> +	if (IS_ERR(file))
> +		return PTR_ERR(file);
> +
> +	retfd = get_unused_fd_flags(O_CLOEXEC);
> +	if (retfd < 0) {
> +		ret = retfd;
> +		goto out;
> +	}
> +
> +	/*
> +	 * security_file_receive must come last since it may have side effects
> +	 * and cannot be reversed.
> +	 */
> +	ret = security_file_receive(file);

So I don't understand the comment here. Can you explain what the side
effects are?
security_file_receive() is called in two places: net/core/scm.c and
net/compat.c. In both places it is called _before_ get_unused_fd_flags()
so I don't know what's special here that would prevent us from doing the
same. If there's no actual reason, please rewrite this functions as:

static int pidfd_getfd(struct pid *pid, int fd)
{
	int ret;
	struct task_struct *task;
	struct file *file;

	task = get_pid_task(pid, PIDTYPE_PID);
	if (!task)
		return -ESRCH;

	file = __pidfd_fget(task, fd);
	put_task_struct(task);
	if (IS_ERR(file))
		return PTR_ERR(file);

	ret = security_file_receive(file);
	if (ret) {
		fput(file);
		return ret;
	}

	ret = get_unused_fd_flags(O_CLOEXEC);
	if (ret < 0)
		fput(file);
	else
		fd_install(ret, file);

	return ret;
}