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

From: Andy Lutomirski
Date: Wed Mar 11 2015 - 18:26:42 EST

On Wed, Mar 11, 2015 at 2:43 PM, Mikael Pettersson <mikpelinux@xxxxxxxxx> 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 Lutomirski
AMA Capital Management, LLC
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