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Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CA+55aFzxCjkSbfRUeX3W_oXSJ6LMUdRVYB=DR2b3rUNkiixM1A@mail.gmail.com>
Date:   Thu, 28 Jun 2018 18:08:20 -0700
Cc:     Andrew Lutomirski <luto@kernel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        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>,
        Ingo Molnar <mingo@redhat.com>, Peter Anvin <hpa@zytor.com>,
        Andi Kleen <andi@firstfloor.org>,
        Christoph Lameter <cl@linux.com>, Ben Maurer <bmaurer@fb.com>,
        Steven 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>, joelaf@google.com
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To:     Linus Torvalds <torvalds@linux-foundation.org>
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> On Jun 28, 2018, at 5:18 PM, Linus Torvalds <torvalds@linux-foundation.org=
> wrote:
>=20
>> On Thu, Jun 28, 2018 at 4:30 PM Andy Lutomirski <luto@kernel.org> wrote:
>>=20
>> The idea is that, if someone screws up and sticks a number like
>> 0xbaadf00d00045678 into their rseq abort_ip in a 32-bit x86 program
>> (when they actually mean 0x00045678), we want to something consistent.
>=20
> I think the "something consistent" is perfectly fine with just "it won't w=
ork".
>=20
> Make it do
>=20
>        if (rseq_cs->abort_ip !=3D (unsigned long)rseq_cs->abort_ip)
>                return -EINVAL;
>=20
> at abort time.

You sure?  Because, unless I remember wrong, a 32-bit user program on a 64-b=
it kernel will actually work at least most of the time even if high bits are=
 set. I=E2=80=99m okay with straight-up promising =E2=80=9Cwill always work=E2=
=80=9D or =E2=80=9Cwill never work=E2=80=9D, but =E2=80=9Csometimes=E2=80=9D=
 is bad.

>=20
> Done.
>=20
> If it's a 32-bit kernel, the above will reject the thing, and if it's
> a 64-bit kernel, it will be a no-op, but the abort won't work in a
> 32-bit caller.
>=20
> Problem solved.
>=20
>             Linus