Re: [PATCH] proc: allow killing processes via file descriptors

On Sun, Nov 18, 2018 at 10:23:36PM +0100, Christian Brauner wrote:

On Sun, Nov 18, 2018 at 12:54:10PM -0800, Daniel Colascione wrote:

quoted

On Sun, Nov 18, 2018 at 12:43 PM, Christian Brauner
[off-list ref] wrote:

quoted

On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:

quoted

quoted

On Nov 18, 2018, at 12:44 PM, Daniel Colascione [off-list ref] wrote:

quoted

That is, I'm proposing an API that looks like this:

int process_kill(int procfs_dfd, int signo, const union sigval value)

If, later, process_kill were to *also* accept process-capability FDs,
nothing would break.

Except that this makes it ambiguous to the caller as to whether their current creds are considered.  So it would need to be a different syscall or at least a flag.  Otherwise a lot of those nice theoretical properties go away.

I can add a flag argument
int process_signal(int procfs_dfd, int signo, siginfo_t *info, int flags)
The way I see it process_signal() should be equivalent to kill(pid, signal) for now.
That is siginfo_t is cleared and set to:

info.si_signo = sig;
info.si_errno = 0;
info.si_code = SI_USER;
info.si_pid = task_tgid_vnr(current);
info.si_uid = from_kuid_munged(current_user_ns(), current_uid());

That makes sense. I just don't want to get into a situation where
callers feel that they *have* to use the PID-based APIs to send a
signal because process_kill doesn't offer some bit of functionality.

Yeah.

quoted

Are you imagining something like requiring info t be NULL unless flags
contains some "I have a siginfo_t" value?

Well, I was actually thinking about something like:

/**
 *  sys_process_signal - send a signal to a process trough a process file descriptor
 *  @fd: the file descriptor of the process
 *  @sig: signal to be sent
 *  @info: the signal info
 *  @flags: future flags to be passed
 */
SYSCALL_DEFINE4(process_signal, int, fd, int, sig, siginfo_t __user *, info,
		int, flags)
{
	struct pid *pid;
	struct fd *f;
	kernel_siginfo_t kinfo;

	/* Do not allow users to pass garbage. */
	if (flags)
		return -EINVAL;

	int ret = __copy_siginfo_from_user(sig, &kinfo, info);
	if (unlikely(ret))
		return ret;

	/* For now, enforce that caller's creds are used. */
	kinfo.si_pid = task_tgid_vnr(current);
	kinfo.si_uid = from_kuid_munged(current_user_ns(), current_uid());

	if (signal_impersonates_kernel(kinfo))
		return -EPERM;

	f = fdget(fd);
	if (!f.file)
		return -EBADF;

	pid = f.file->private_data;
	if (!pid)
		return -EBADF;

	return kill_pid_info(sig, kinfo, pid);
}

Just jotted this down here briefly. This will need an fput and so on
obvs.

quoted

BTW: passing SI_USER to rt_sigqueueinfo *should* as long as the
passed-in si_pid and si_uid match what the kernel would set them to in
the kill(2) case. The whole point of SI_USER is that the recipient
knows that it can trust the origin information embedded in the
siginfo_t in the signal handler. If the kernel verifies that a signal
sender isn't actually lying, why not let people send SI_USER with
rt_sigqueueinfo?