Re: [PATCH v2 4/5] signal: PIDFD_SIGNAL_TID threads via pidfds

[Christian Brauner]

On Sat, Mar 30, 2019 at 02:34:16AM +0100, Christian Brauner wrote:
> On Sat, Mar 30, 2019 at 02:22:29AM +0100, Christian Brauner wrote:
> > On Sat, Mar 30, 2019 at 02:06:34AM +0100, Jann Horn wrote:
> > > On Fri, Mar 29, 2019 at 4:54 PM Christian Brauner <christian@xxxxxxxxxx> wrote:
> > > > With the addition of pidfd_open() it is possible for users to reference a
> > > > specific thread by doing:
> > > >
> > > > int pidfd = pidfd_open(<tid>, 0);
> > > >
> > > > This means we can extend pidfd_send_signal() to signal a specific thread.
> > > > As promised in the commit for pidfd_send_signal() [1] the extension is
> > > > based on a flag argument, i.e. the scope of the signal delivery is based on
> > > > the flag argument, not on the type of file descriptor.
> > > > To this end the flag PIDFD_SIGNAL_TID is added. With this change we now
> > > > cover most of the functionality of all the other signal sending functions
> > > > combined:
> > > [...]
> > > > diff --git a/include/uapi/linux/wait.h b/include/uapi/linux/wait.h
> > > > index d6c7c0701997..b72f0ef84fe5 100644
> > > > --- a/include/uapi/linux/wait.h
> > > > +++ b/include/uapi/linux/wait.h
> > > [...]
> > > > +/* Flags to pass to pidfd_send_signal */
> > > > +#define PIDFD_SIGNAL_TID 1 /* Send signal to specific thread */
> > > 
> > > nit: s/1/1U/; the flags argument is an `unsigned int`
> > 
> > Will change.
> > 
> > > 
> > > >  #endif /* _UAPI_LINUX_WAIT_H */
> > > > diff --git a/kernel/signal.c b/kernel/signal.c
> > > > index eb97d0cc6ef7..9f93da85b2b9 100644
> > > > --- a/kernel/signal.c
> > > > +++ b/kernel/signal.c
> > > [...]
> > > > +static int pidfd_send_signal_specific(struct pid *pid, int sig,
> > > > +                                     struct kernel_siginfo *info)
> > > > +{
> > > > +       struct task_struct *p;
> > > > +       int error = -ESRCH;
> > > > +
> > > > +       rcu_read_lock();
> > > > +       p = pid_task(pid, PIDTYPE_PID);
> > > > +       if (p)
> > > > +               error = __do_send_specific(p, sig, info);
> > > > +       rcu_read_unlock();
> > > > +
> > > > +       return error;
> > > > +}
> > > > +
> > > >  /**
> > > > - * sys_pidfd_send_signal - send a signal to a process through a task file
> > > > - *                          descriptor
> > > > + * sys_pidfd_send_signal - send a signal to a process through a pidfd
> > > > +
> > > >   * @pidfd:  the file descriptor of the process
> > > >   * @sig:    signal to be sent
> > > >   * @info:   the signal info
> > > >   * @flags:  future flags to be passed
> > > 
> > > nit: comment is outdated, it isn't "future flags" anymore
> > 
> > Will remove.
> > 
> > > 
> > > [...]
> > > > + *   rt_tgsigqueueinfo(<tgid>, <tid>, <sig>, <uinfo>)
> > > > + * - pidfd_send_signal(<pidfd>, <sig>, <info>, PIDFD_SIGNAL_TID);
> > > > + *   which is equivalent to
> > > > + *   rt_tgsigqueueinfo(<tgid>, <tid>, <sig>, <uinfo>)
> > > > + *
> > > >   * In order to extend the syscall to threads and process groups the @flags
> > > >   * argument should be used. In essence, the @flags argument will determine
> > > >   * what is signaled and not the file descriptor itself. Put in other words,
> > > 
> > > nit: again, outdated comment about @flags
> > 
> > Will update.
> > 
> > > 
> > > [...]
> > > > @@ -3626,43 +3695,16 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> > > >                 prepare_kill_siginfo(sig, &kinfo);
> > > >         }
> > > >
> > > > -       ret = kill_pid_info(sig, &kinfo, pid);
> > > > +       if (flags & PIDFD_SIGNAL_TID)
> > > > +               ret = pidfd_send_signal_specific(pid, sig, &kinfo);
> > > > +       else
> > > > +               ret = kill_pid_info(sig, &kinfo, pid);
> > > 
> > > nit: maybe give pidfd_send_signal_specific() and kill_pid_info() the
> > > same signatures, since they perform similar operations with the same
> > > argument types?
> > 
> > Yes, let's do
> > pidfd_send_signal_specific.(pid, sig, &kinfo);
> > kill_pid_info..............(pid, sig, &kinfo);
> > 
> > so it matches the argument order of the syscalls itself too.
> 
> Strike that. We should do:
> pidfd_send_signal_specific.(sig, &kinfo, pid);
> kill_pid_info..............(sig, &kinfo, pid);
> 
> because kill_pid_info() is called in multiple places so we would
> needlessly shovle code around.
> 
> > 
> > > 
> > > Something that was already kinda weird in the existing code, but is
> > > getting worse with TIDs is the handling of SI_USER with siginfo.
> > 
> > Right, that's what we discussed earlier.
> > 
> > > Copying context lines from above here:
> > > 
> > >         if (info) {
> > >                 ret = copy_siginfo_from_user_any(&kinfo, info);
> > >                 if (unlikely(ret))
> > >                         goto err;
> > >                 ret = -EINVAL;
> > >                 if (unlikely(sig != kinfo.si_signo))
> > >                         goto err;
> > >                 if ((task_pid(current) != pid) &&
> > >                     (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
> > >                         /* Only allow sending arbitrary signals to yourself. */
> > >                         ret = -EPERM;
> > >                         if (kinfo.si_code != SI_USER)
> > >                                 goto err;
> > >                         /* Turn this into a regular kill signal. */
> > >                         prepare_kill_siginfo(sig, &kinfo);
> > >                 }
> > >         } else {
> > >                 prepare_kill_siginfo(sig, &kinfo);
> > >         }
> > > 
> > > So for signals to PIDs, the rule is that if you send siginfo with
> > > SI_USER to yourself, the siginfo is preserved; otherwise the kernel
> > > silently clobbers it. That's already kind of weird - silent behavior
> > 
> > Clobbers as in "silently replaces it whatever it seems fit?
> > 
> > > difference depending on a security check. But now, for signals to
> > > threads, I think the result is going to be that signalling the thread
> > > group leader preserves information, and signalling any other thread
> > > clobbers it? If so, that seems bad.
> > > 
> > > do_rt_sigqueueinfo() seems to have the same issue, from a glance - but
> > > there, at least the error case is just a -EPERM, not a silent behavior
> > > difference.
> > > 
> > > Would it make sense to refuse sending siginfo with SI_USER to
> > > non-current? If you actually want to send a normal SI_USER signal, you
> > 
> > Yeah.
> > 
> > > can use info==NULL, right? That should create wrongness parity with
> > > do_rt_sigqueueinfo().
> > 
> > So you'd just do (just doing it non-elegantly rn):
> > if ((task_pid(current) != pid) &&
> >     (kinfo.si_code >= 0 || kinfo.si_code == SI_TKILL)) {
> >         ret = -EPERM;
> >         goto err;
> > }
> > 
> > > To improve things further, I guess you'd have to move the comparison
> > > against current into pidfd_send_signal_specific(), or move the task
> > > lookup out of it, or something like that?
> > 
> > Looks like a sane suggestion to me. Would you care to send a patch for
> > that? This is clearly a bugfix suitable for 5.1 so I'd rather not wait
> > until 5.2.

Sorry, that was probably unclear. I was referring to:

> > > Would it make sense to refuse sending siginfo with SI_USER to
> > > non-current? If you actually want to send a normal SI_USER signal, you

This makes perfect sense as a bugfix for 5.1 imo.