Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
MIME-Version: 1.0
In-Reply-To: <184287091.10022.1530301738384.JavaMail.zimbra@efficios.com>
References: <20180628162359.9054-1-mathieu.desnoyers@efficios.com>
 <1706339668.9644.1530281144560.JavaMail.zimbra@efficios.com>
 <CA+55aFxRJWjbgLeSQ_swi3F5nF1SYgZnC-R7QsOvZdcuSaWeaw@mail.gmail.com>
 <729451355.9702.1530284622326.JavaMail.zimbra@efficios.com>
 <CA+55aFw==YnFJn7iGnKMW=RbPT74YHNa0QDF96mEdMPA2oX9SA@mail.gmail.com>
 <CA+55aFwJWxsE_LL=-8WgB7uH_mYeMSu=boWF4N5KdOW673g4hA@mail.gmail.com>
 <247789350.9741.1530288432573.JavaMail.zimbra@efficios.com>
 <CA+55aFz8KyVqFsVnp-6g+thz7vmm5nK_FSdRTPSks7gsNVNRdQ@mail.gmail.com> <184287091.10022.1530301738384.JavaMail.zimbra@efficios.com>
From:   Andy Lutomirski <luto@amacapital.net>
Date:   Fri, 29 Jun 2018 13:39:20 -0700
Message-ID: <CALCETrXqjrCoHX1KyL0iKGBaAZSGJb1cVdNN1gNigJ8DW9LF8w@mail.gmail.com>
Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE
To:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        linux-api <linux-api@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        Dave Watson <davejwatson@fb.com>, Paul Turner <pjt@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Russell King <linux@arm.linux.org.uk>,
        Ingo Molnar <mingo@redhat.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Andi Kleen <andi@firstfloor.org>,
        Chris Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        rostedt <rostedt@goodmis.org>,
        Josh Triplett <josh@joshtriplett.org>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Will Deacon <will.deacon@arm.com>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Joel Fernandes <joelaf@google.com>
Content-Type: text/plain; charset="UTF-8"
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Fri, Jun 29, 2018 at 12:48 PM, Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
> There are two aspects I'm concerned about here:
>
> 1) security: we don't want 32-bit user-space to feed a 64-bit value over 4GB
>    as abort_ip that may end up causing OOPSes on architectures that would
>    lack proper validation of those values on return to userspace.

I'm not too worried about this.  As long as you're doing it from
signal-delivery context (which you are AFAICT) you're fine.

But I re-read the code and I think I have a really straightforward
solution.  Two choices:

(1) Change instruction_pointer_set() to return an error code if the
address passed in is garbage in a way that could cause unexpected
behavior (like >=2^32 on x86_64 if regs->cs is 32-bit).  It has very
very few callers.

(2) Add instruction_pointer_validate() to go along with
instruction_pointer_set().

That should be enough to solve the problem, right?