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Hallyn" , Linux API , kernel list , Arnd Bergmann , "Eric W. Biederman" , Konstantin Khlebnikov , Kees Cook , Alexey Dobriyan , Thomas Gleixner , Michael Kerrisk-manpages , Jonathan Kowalski , "Dmitry V. Levin" , Andrew Morton , Oleg Nesterov , Nagarathnam Muthusamy , Aleksa Sarai , Al Viro , "Joel Fernandes (Google)" , Daniel Colascione , Florian Weimer Subject: Re: [PATCH v2 4/5] signal: PIDFD_SIGNAL_TID threads via pidfds Message-ID: <20190330012229.yt3hecmgaj2r6vp7@brauner.io> References: <20190329155425.26059-1-christian@brauner.io> <20190329155425.26059-5-christian@brauner.io> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: NeoMutt/20180716 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sat, Mar 30, 2019 at 02:06:34AM +0100, Jann Horn wrote: > On Fri, Mar 29, 2019 at 4:54 PM Christian Brauner wrote: > > With the addition of pidfd_open() it is possible for users to reference a > > specific thread by doing: > > > > int pidfd = pidfd_open(, 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(, , , ) > > + * - pidfd_send_signal(, , , PIDFD_SIGNAL_TID); > > + * which is equivalent to > > + * rt_tgsigqueueinfo(, , , ) > > + * > > * 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. > > 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.