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

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Date:	Thu, 12 Mar 2015 08:22:18 +0100
From:	Mikael Pettersson <mikpelinux@...il.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Mikael Pettersson <mikpelinux@...il.com>,
	Jann Horn <jann@...jh.net>,
	Linux API <linux-api@...r.kernel.org>,
	"linux-kernel\@vger.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.

Andy Lutomirski writes:
 > 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

Yes.  Either a sigfillset();sigdelset(SIGSEGV), or a copy of the
thread's sigmask from a previous sigframe.

 > or, even more strangely, with a totally made up
 > context?

Not "totally made up", but certainly with adjustments(*) made to
both GPRs and PC.  In a different piece of SW: FPU controls.

(*) Rolling back or force-committing a micro-transaction until
PC+GPRs represent the state at an original instruction boundary.
This was in a product using dynamic binary instrumentation.
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