For review: seccomp_user_notif(2) manual page

[Jann Horn]

On Sat, Oct 24, 2020 at 2:53 PM Michael Kerrisk (man-pages)
<mtk.manpages at gmail.com> wrote:
> On 10/17/20 2:25 AM, Jann Horn wrote:
> > On Fri, Oct 16, 2020 at 8:29 PM Michael Kerrisk (man-pages)
> > <mtk.manpages at gmail.com> wrote:
[...]
> >> I'm not sure if I should write anything about this small UAPI
> >> breakage in BUGS, or not. Your thoughts?
> >
> > Thinking about it a bit more: Any code that relies on pause() or
> > epoll_wait() not restarting is buggy anyway, right? Because a signal
> > could also arrive directly before entering the syscall, while
> > userspace code is still executing? So one could argue that we're just
> > enlarging a preexisting race. (Unless the signal handler checks the
> > interrupted register state to figure out whether we already entered
> > syscall handling?)
>
> Yes, that all makes sense.
>
> > If userspace relies on non-restarting behavior, it should be using
> > something like epoll_pwait(). And that stuff only unblocks signals
> > after we've already past the seccomp checks on entry.
>
> Thanks for elaborating that detail, since as soon as you talked
> about "enlarging a preexisting race" above, I immediately wondered
> sigsuspend(), pselect(), etc.
>
> (Mind you, I still wonder about the effect on system calls that
> are normally nonrestartable because they have timeouts. My
> understanding is that the kernel doesn't restart those system
> calls because it's impossible for the kernel to restart the call
> with the right timeout value. I wonder what happens when those
> system calls are restarted in the scenario we're discussing.)

Ah, that's an interesting edge case...

> Anyway, returning to your point... So, to be clear (and to
> quickly remind myself in case I one day reread this thread),
> there is not a problem with sigsuspend(), pselect(), ppoll(),
> and epoll_pwait() since:
>
> * Before the syscall, signals are blocked in the target.
> * Inside the syscall, signals are still blocked at the time
>   the check is made for seccomp filters.
> * If a seccomp user-space notification  event kicks, the target
>   is put to sleep with the signals still blocked.
> * The signal will only get delivered after the supervisor either
>   triggers a spoofed success/failure return in the target or the
>   supervisor sends a CONTINUE response to the kernel telling it
>   to execute the target's system call. Either way, there won't be
>   any restarting of the target's system call (and the supervisor
>   thus won't see multiple notifications).
>
> (Right?)

Yeah.

[...]
> > So we should probably document the restarting behavior as something
> > the supervisor has to deal with in the manpage; but for the
> > "non-restarting syscalls can restart from the target's perspective"
> > aspect, it might be enough to document this as quirky behavior that
> > can't actually break correct code? (Or not document it at all. Dunno.)
>
> So, I've added the following to the page:
>
>    Interaction with SA_RESTART signal handlers
>        Consider the following scenario:
>
>        · The target process has used sigaction(2)  to  install  a  signal
>          handler with the SA_RESTART flag.
>
>        · The target has made a system call that triggered a seccomp user-
>          space notification and the target is currently blocked until the
>          supervisor sends a notification response.
>
>        · A  signal  is  delivered to the target and the signal handler is
>          executed.
>
>        · When  (if)  the  supervisor  attempts  to  send  a  notification
>          response,  the SECCOMP_IOCTL_NOTIF_SEND ioctl(2)) operation will
>          fail with the ENOENT error.
>
>        In this scenario, the kernel  will  restart  the  target's  system
>        call.   Consequently,  the  supervisor  will receive another user-
>        space notification.  Thus, depending on how many times the blocked
>        system call is interrupted by a signal handler, the supervisor may
>        receive multiple notifications for the same  system  call  in  the
>        target.
>
>        One  oddity  is  that  system call restarting as described in this
>        scenario will occur even for the blocking system calls  listed  in
>        signal(7) that would never normally be restarted by the SA_RESTART
>        flag.
>
> Does that seem okay?

Sounds good to me.

> In addition, I've queued a cross-reference in signal(7):
>
>        In certain circumstances, the seccomp(2) user-space notifi‐
>        cation  feature can lead to restarting of system calls that
>        would otherwise  never  be  restarted  by  SA_RESTART;  for
>        details, see seccomp_user_notif(2).