Re: [PATCH] seccomp.2: Add note about alarm(2) not being sufficient to limit runtime

[Mikael Pettersson]

Jann Horn writes:
 > On Wed, Mar 11, 2015 at 10:43:50PM +0100, Mikael Pettersson wrote:
 > > Jann Horn writes:
 > >  > Or should I throw this patch away and write a patch
 > >  > for the prctl() manpage instead that documents that
 > >  > being able to call sigreturn() implies being able to
 > >  > effectively call sigprocmask(), at least on some
 > >  > architectures like X86?
 > > 
 > > Well, that is the semantics of sigreturn().  It is essentially
 > > setcontext() [which includes the actions of sigprocmask()], but
 > > with restrictions on parameter placement (at least on x86).
 > > 
 > > You could introduce some setting to restrict that aspect for
 > > seccomp processes, but you can't change this for normal processes
 > > without breaking things.
 > 
 > Then I think it's probably better and easier to just document the existing
 > behavior? If a new setting would have to be introduced and developers would
 > need to be aware of that, it's probably easier to just tell everyone to use
 > SIGKILL.
 > 
 > Does this manpage patch look good?

LGTM

Acked-by: Mikael Pettersson <mikpelinux@xxxxxxxxx>

 > 
 > ---
 >  man2/seccomp.2 | 25 +++++++++++++++++++++++++
 >  1 file changed, 25 insertions(+)
 > 
 > diff --git a/man2/seccomp.2 b/man2/seccomp.2
 > index 702ceb8..f762d07 100644
 > --- a/man2/seccomp.2
 > +++ b/man2/seccomp.2
 > @@ -64,6 +64,31 @@ Strict secure computing mode is useful for number-crunching
 >  applications that may need to execute untrusted byte code, perhaps
 >  obtained by reading from a pipe or socket.
 >  
 > +Note that although the calling thread can no longer call
 > +.BR sigprocmask (2),
 > +it can use
 > +.BR sigreturn (2)
 > +to block all signals apart from
 > +.BR SIGKILL
 > +and
 > +.BR SIGSTOP .
 > +Therefore, to reliably terminate it,
 > +.BR SIGKILL
 > +has to be used, meaning that e.g.
 > +.BR alarm (2)
 > +is not sufficient for restricting its runtime. Instead, use
 > +.BR timer_create (2)
 > +with
 > +.BR SIGEV_SIGNAL
 > +and
 > +.BR sigev_signo
 > +set to
 > +.BR SIGKILL
 > +or use
 > +.BR setrlimit (2)
 > +to set the hard limit for
 > +.BR RLIMIT_CPU .
 > +
 >  This operation is available only if the kernel is configured with
 >  .BR CONFIG_SECCOMP
 >  enabled.
 > -- 
 > 2.1.4

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