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    SubjectRe: [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:
    > > On Sun, Nov 18, 2018 at 12:43 PM, Christian Brauner
    > > <christian@brauner.io> wrote:
    > > > On Sun, Nov 18, 2018 at 01:28:41PM -0700, Andy Lutomirski wrote:
    > > >>
    > > >>
    > > >> > On Nov 18, 2018, at 12:44 PM, Daniel Colascione <dancol@google.com> wrote:
    > > >> >
    > > >>
    > > >> >
    > > >> > 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.
    >
    > >
    > > 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.

    >
    > >
    > > 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?

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