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

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Date:	Wed, 11 Mar 2015 15:26:13 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Mikael Pettersson <mikpelinux@...il.com>
Cc:	Jann Horn <jann@...jh.net>, Linux API <linux-api@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Michael Kerrisk <mtk.manpages@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
	Jeff Dike <jdike@...toit.com>,
	Richard Weinberger <richard@....at>,
	Kees Cook <keescook@...omium.org>,
	Will Drewry <wad@...omium.org>
Subject: Re: [PATCH] Don't allow blocking of signals using sigreturn.

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