On Thu, Mar 12, 2015 at 6:07 AM, Jann Horn <jann@xxxxxxxxx> wrote: > 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? Looks good to me. FWIW, if we wanted to fix this in the kernel, I think it could be easier to add SIG_KILL which would be just like SIG_DFL except always fatal even if masked rather than coming up with complicated changes to sigreturn. --Andy > > --- > 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 > -- Andy Lutomirski AMA Capital Management, LLC -- To unsubscribe from this list: send the line "unsubscribe linux-api" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
