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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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