linux-kernel - Re: [RFC PATCH] rseq: Avoid infinite recursion when delivering SIGSEGV

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Message-ID: <20180621115240.GE22505@arm.com>
Date:   Thu, 21 Jun 2018 12:52:40 +0100
From:   Will Deacon <will.deacon@....com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>
Subject: Re: [RFC PATCH] rseq: Avoid infinite recursion when delivering
 SIGSEGV

On Wed, Jun 20, 2018 at 12:55:35PM -0400, Mathieu Desnoyers wrote:
> ----- On Jun 20, 2018, at 12:36 PM, Will Deacon will.deacon@....com wrote:
> 
> > When delivering a signal to a task that is using rseq, we call into
> > __rseq_handle_notify_resume() so that the registers pushed in the
> > sigframe are updated to reflect the state of the restartable sequence
> > (for example, ensuring that the signal returns to the abort handler if
> > necessary).
> > 
> > However, if the rseq management fails due to an unrecoverable fault when
> > accessing userspace or certain combinations of RSEQ_CS_* flags, then we
> > will attempt to deliver a SIGSEGV. This has the potential for infinite
> > recursion if the rseq code continuously fails on signal delivery.
> > 
> > Avoid this problem by using force_sigsegv() instead of force_sig(), which
> > is explicitly designed to reset the SEGV handler to SIG_DFL in the case
> > of a recursive fault.
> 
> Your approach looks good. One nit below implementation-wise:

Nit accepted :) v2 on its way.

Will