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From:   Linus Torvalds <torvalds@linux-foundation.org>
Date:   Fri, 29 Jun 2018 10:03:10 -0700
Message-ID: <CA+55aFz8KyVqFsVnp-6g+thz7vmm5nK_FSdRTPSks7gsNVNRdQ@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:     Andy Lutomirski <luto@amacapital.net>,
        Andrew Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux API <linux-api@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Paul 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 - ARM Linux <linux@arm.linux.org.uk>,
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On Fri, Jun 29, 2018 at 9:07 AM Mathieu Desnoyers
<mathieu.desnoyers@efficios.com> wrote:
>
> This code is not invoked from syscalls, but rather on return from
> interrupt/trap after a preemption.

But when we register the rseq, we could easily check that the top bits
of the IP is clear, no?

Sure, user space can change it after the fact, but at that point it's
literally "user space is being intentionally stupid".

The real worry is that 32-bit compat code never initializes those bits
at all, no?

                Linus