linux-kernel - Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK

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Date:   Fri, 29 Jun 2018 13:39:20 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE

On Fri, Jun 29, 2018 at 12:48 PM, Mathieu Desnoyers
<mathieu.desnoyers@...icios.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?