Re: [RFC 2/2] signal: extend pidfd_send_signal() to allow expedited process killing

[Suren Baghdasaryan <surenb@google.com>]

On Thu, Apr 11, 2019 at 8:18 AM Suren Baghdasaryan <surenb@google.com> wrote:
>
> Thanks for the feedback!
> Just to be clear, this implementation is used in this RFC as a
> reference to explain the intent. To be honest I don't think it will be
> adopted as is even if the idea survives scrutiny.
>
> On Thu, Apr 11, 2019 at 3:30 AM Christian Brauner <christian@brauner.io> wrote:
> >
> > On Wed, Apr 10, 2019 at 06:43:53PM -0700, Suren Baghdasaryan wrote:
> > > Add new SS_EXPEDITE flag to be used when sending SIGKILL via
> > > pidfd_send_signal() syscall to allow expedited memory reclaim of the
> > > victim process. The usage of this flag is currently limited to SIGKILL
> > > signal and only to privileged users.
> > >
> > > Signed-off-by: Suren Baghdasaryan <surenb@google.com>
> > > ---
> > >  include/linux/sched/signal.h |  3 ++-
> > >  include/linux/signal.h       | 11 ++++++++++-
> > >  ipc/mqueue.c                 |  2 +-
> > >  kernel/signal.c              | 37 ++++++++++++++++++++++++++++--------
> > >  kernel/time/itimer.c         |  2 +-
> > >  5 files changed, 43 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> > > index e412c092c1e8..8a227633a058 100644
> > > --- a/include/linux/sched/signal.h
> > > +++ b/include/linux/sched/signal.h
> > > @@ -327,7 +327,8 @@ extern int send_sig_info(int, struct kernel_siginfo *, struct task_struct *);
> > >  extern void force_sigsegv(int sig, struct task_struct *p);
> > >  extern int force_sig_info(int, struct kernel_siginfo *, struct task_struct *);
> > >  extern int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp);
> > > -extern int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid);
> > > +extern int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid,
> > > +                             bool expedite);
> > >  extern int kill_pid_info_as_cred(int, struct kernel_siginfo *, struct pid *,
> > >                               const struct cred *);
> > >  extern int kill_pgrp(struct pid *pid, int sig, int priv);
> > > diff --git a/include/linux/signal.h b/include/linux/signal.h
> > > index 9702016734b1..34b7852aa4a0 100644
> > > --- a/include/linux/signal.h
> > > +++ b/include/linux/signal.h
> > > @@ -446,8 +446,17 @@ int __save_altstack(stack_t __user *, unsigned long);
> > >  } while (0);
> > >
> > >  #ifdef CONFIG_PROC_FS
> > > +
> > > +/*
> > > + * SS_FLAGS values used in pidfd_send_signal:
> > > + *
> > > + * SS_EXPEDITE indicates desire to expedite the operation.
> > > + */
> > > +#define SS_EXPEDITE  0x00000001
> >
> > Does this make sense as an SS_* flag?
> > How does this relate to the signal stack?
>
> It doesn't, so I agree that the name should be changed.
> PIDFD_SIGNAL_EXPEDITE_MM_RECLAIM would seem appropriate.
>
> > Is there any intention to ever use this flag with stack_t?
> >
> > New flags should be PIDFD_SIGNAL_*. (E.g. the thread flag will be
> > PIDFD_SIGNAL_THREAD.)
> > And since this is exposed to userspace in contrast to the mm internal
> > naming it should be something more easily understandable like
> > PIDFD_SIGNAL_MM_RECLAIM{_FASTER} or something.
> >
> > > +
> > >  struct seq_file;
> > >  extern void render_sigset_t(struct seq_file *, const char *, sigset_t *);
> > > -#endif
> > > +
> > > +#endif /* CONFIG_PROC_FS */
> > >
> > >  #endif /* _LINUX_SIGNAL_H */
> > > diff --git a/ipc/mqueue.c b/ipc/mqueue.c
> > > index aea30530c472..27c66296e08e 100644
> > > --- a/ipc/mqueue.c
> > > +++ b/ipc/mqueue.c
> > > @@ -720,7 +720,7 @@ static void __do_notify(struct mqueue_inode_info *info)
> > >                       rcu_read_unlock();
> > >
> > >                       kill_pid_info(info->notify.sigev_signo,
> > > -                                   &sig_i, info->notify_owner);
> > > +                                   &sig_i, info->notify_owner, false);
> > >                       break;
> > >               case SIGEV_THREAD:
> > >                       set_cookie(info->notify_cookie, NOTIFY_WOKENUP);
> > > diff --git a/kernel/signal.c b/kernel/signal.c
> > > index f98448cf2def..02ed4332d17c 100644
> > > --- a/kernel/signal.c
> > > +++ b/kernel/signal.c
> > > @@ -43,6 +43,7 @@
> > >  #include <linux/compiler.h>
> > >  #include <linux/posix-timers.h>
> > >  #include <linux/livepatch.h>
> > > +#include <linux/oom.h>
> > >
> > >  #define CREATE_TRACE_POINTS
> > >  #include <trace/events/signal.h>
> > > @@ -1394,7 +1395,8 @@ int __kill_pgrp_info(int sig, struct kernel_siginfo *info, struct pid *pgrp)
> > >       return success ? 0 : retval;
> > >  }
> > >
> > > -int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid)
> > > +int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid,
> > > +                               bool expedite)
> > >  {
> > >       int error = -ESRCH;
> > >       struct task_struct *p;
> > > @@ -1402,8 +1404,17 @@ int kill_pid_info(int sig, struct kernel_siginfo *info, struct pid *pid)
> > >       for (;;) {
> > >               rcu_read_lock();
> > >               p = pid_task(pid, PIDTYPE_PID);
> > > -             if (p)
> > > +             if (p) {
> > >                       error = group_send_sig_info(sig, info, p, PIDTYPE_TGID);
> > > +
> > > +                     /*
> > > +                      * Ignore expedite_reclaim return value, it is best
> > > +                      * effort only.
> > > +                      */
> > > +                     if (!error && expedite)
> > > +                             expedite_reclaim(p);
> >
> > SIGKILL will take the whole thread group down so the reclaim should make
> > sense here.
> >
>
> This sounds like confirmation. I hope I'm not missing some flaw that
> you are trying to point out.
>
> > > +             }
> > > +
> > >               rcu_read_unlock();
> > >               if (likely(!p || error != -ESRCH))
> > >                       return error;
> > > @@ -1420,7 +1431,7 @@ static int kill_proc_info(int sig, struct kernel_siginfo *info, pid_t pid)
> > >  {
> > >       int error;
> > >       rcu_read_lock();
> > > -     error = kill_pid_info(sig, info, find_vpid(pid));
> > > +     error = kill_pid_info(sig, info, find_vpid(pid), false);
> > >       rcu_read_unlock();
> > >       return error;
> > >  }
> > > @@ -1487,7 +1498,7 @@ static int kill_something_info(int sig, struct kernel_siginfo *info, pid_t pid)
> > >
> > >       if (pid > 0) {
> > >               rcu_read_lock();
> > > -             ret = kill_pid_info(sig, info, find_vpid(pid));
> > > +             ret = kill_pid_info(sig, info, find_vpid(pid), false);
> > >               rcu_read_unlock();
> > >               return ret;
> > >       }
> > > @@ -1704,7 +1715,7 @@ EXPORT_SYMBOL(kill_pgrp);
> > >
> > >  int kill_pid(struct pid *pid, int sig, int priv)
> > >  {
> > > -     return kill_pid_info(sig, __si_special(priv), pid);
> > > +     return kill_pid_info(sig, __si_special(priv), pid, false);
> > >  }
> > >  EXPORT_SYMBOL(kill_pid);
> > >
> > > @@ -3577,10 +3588,20 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> > >       struct pid *pid;
> > >       kernel_siginfo_t kinfo;
> > >
> > > -     /* Enforce flags be set to 0 until we add an extension. */
> > > -     if (flags)
> > > +     /* Enforce no unknown flags. */
> > > +     if (flags & ~SS_EXPEDITE)
> > >               return -EINVAL;
> > >
> > > +     if (flags & SS_EXPEDITE) {
> > > +             /* Enforce SS_EXPEDITE to be used with SIGKILL only. */
> > > +             if (sig != SIGKILL)
> > > +                     return -EINVAL;
> >
> > Not super fond of this being a SIGKILL specific flag but I get why.
>
> Understood. I was thinking that EXPEDITE flag might make sense for
> other signals in the future but from internal feedback sounds like if
> we go this way the flag name should be more specific.
>
> > > +
> > > +             /* Limit expedited killing to privileged users only. */
> > > +             if (!capable(CAP_SYS_NICE))
> > > +                     return -EPERM;
> >
> > Do you have a specific (DOS or other) attack vector in mind that renders
> > ns_capable unsuitable?
> >

Missed this one. I was thinking of oom-reaper thread as a limited
system resource (one thread which maintains a kill list and reaps
process mms one at a time) and therefore should be protected from
abuse.

> > > +     }
> > > +
> > >       f = fdget_raw(pidfd);
> > >       if (!f.file)
> > >               return -EBADF;
> > > @@ -3614,7 +3635,7 @@ SYSCALL_DEFINE4(pidfd_send_signal, int, pidfd, int, sig,
> > >               prepare_kill_siginfo(sig, &kinfo);
> > >       }
> > >
> > > -     ret = kill_pid_info(sig, &kinfo, pid);
> > > +     ret = kill_pid_info(sig, &kinfo, pid, (flags & SS_EXPEDITE) != 0);
> > >
> > >  err:
> > >       fdput(f);
> > > diff --git a/kernel/time/itimer.c b/kernel/time/itimer.c
> > > index 02068b2d5862..c926483cdb53 100644
> > > --- a/kernel/time/itimer.c
> > > +++ b/kernel/time/itimer.c
> > > @@ -140,7 +140,7 @@ enum hrtimer_restart it_real_fn(struct hrtimer *timer)
> > >       struct pid *leader_pid = sig->pids[PIDTYPE_TGID];
> > >
> > >       trace_itimer_expire(ITIMER_REAL, leader_pid, 0);
> > > -     kill_pid_info(SIGALRM, SEND_SIG_PRIV, leader_pid);
> > > +     kill_pid_info(SIGALRM, SEND_SIG_PRIV, leader_pid, false);
> > >
> > >       return HRTIMER_NORESTART;
> > >  }
> > > --
> > > 2.21.0.392.gf8f6787159e-goog
> > >