Re: [PATCH] seccomp.2: Add note about alarm(2) not being sufficient to limit runtime
From: Andy Lutomirski <hidden>
Date: 2015-03-12 17:31:24
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On Thu, Mar 12, 2015 at 6:07 AM, Jann Horn [off-list ref] wrote:
On Wed, Mar 11, 2015 at 10:43:50PM +0100, Mikael Pettersson wrote:quoted
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
quoted hunk ↗ jump to hunk
--- 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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