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From:   Andy Lutomirski <luto@amacapital.net>
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Subject: Re: [patch V4 part 4 02/24] x86/int3: Avoid atomic instrumentation
Date:   Thu, 14 May 2020 07:55:02 -0700
Message-Id: <9DBF3103-4D31-41AA-AC02-9A6F1849AA00@amacapital.net>
References: <20200514131530.GQ2957@hirez.programming.kicks-ass.net>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Alexandre Chartre <alexandre.chartre@oracle.com>,
        Frederic Weisbecker <frederic@kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Petr Mladek <pmladek@suse.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Joel Fernandes <joel@joelfernandes.org>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Juergen Gross <JGross@suse.com>,
        Brian Gerst <brgerst@gmail.com>,
        Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Will Deacon <will@kernel.org>
In-Reply-To: <20200514131530.GQ2957@hirez.programming.kicks-ass.net>
To:     Peter Zijlstra <peterz@infradead.org>
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk


> On May 14, 2020, at 6:15 AM, Peter Zijlstra <peterz@infradead.org> wrote:
>=20
> =EF=BB=BFOn Thu, May 14, 2020 at 02:51:32PM +0200, Thomas Gleixner wrote:
>> Peter Zijlstra <peterz@infradead.org> writes:
>>> On Wed, May 13, 2020 at 09:57:52PM -0700, Andy Lutomirski wrote:
>>>>> On Tue, May 5, 2020 at 7:15 AM Thomas Gleixner <tglx@linutronix.de> wr=
ote:
>>>>>>=20
>>>>>> From: Peter Zijlstra <peterz@infradead.org>
>>>>>>=20
>>>>>> Use arch_atomic_*() and READ_ONCE_NOCHECK() to ensure nothing untowar=
d
>>>>>> creeps in and ruins things.
>>>>>>=20
>>>>>> That is; this is the INT3 text poke handler, strictly limit the code
>>>>>> that runs in it, lest it inadvertenly hits yet another INT3.
>>>>>=20
>>>>>=20
>>>>> Acked-by: Andy Lutomirski <luto@kernel.org>
>>>>>=20
>>>>> Does objtool catch this error?
>>>=20
>>> It does not. I'll put it on the (endless) todo list..
>>=20
>> Well, at least it detects when that code calls out into something which
>> is not in the non-instrumentable section.
>=20
> True, but the more specific problem is that noinstr code can use
> jump_label/static_call just fine.
>=20
> So a more specific test is validating none of that happens in the INT3
> handler before poke_int3_handler(). Which is what I think Andy was
> after.

Exactly.  I admit that sleep-deprived Andy was actually thinking =E2=80=9Ctg=
lx and/or PeterZ found this by inspection, and somewhere it escaped objtool=E2=
=80=99s notice,=E2=80=9D which is sort of the same thing :)=