Re: [PATCH] Don't allow blocking of signals using sigreturn.

[Andy Lutomirski <luto@amacapital.net>]

On Wed, Mar 11, 2015 at 2:43 PM, Mikael Pettersson <mikpelinux@gmail.com> 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.

Which leads to the interesting question: does anyone ever call
sigreturn with a different signal mask than the kernel put there
during signal delivery or, even more strangely, with a totally made up
context?  I suspect that the former does happen, even if the latter
may be rare or completely implausible.

I certainly have code that modifies GPRs in the context prior to sigreturn.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC