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Subject: Re: [PATCH v2 4/4] x86/static_call: Add inline static call implementation for x86-64
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <20181129174555.dapti2w4acrbm2jp@treble>
Date:   Thu, 29 Nov 2018 09:52:56 -0800
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Ard Biesheuvel <ard.biesheuvel@linaro.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Masami Hiramatsu <mhiramat@kernel.org>,
        Jason Baron <jbaron@akamai.com>, Jiri Kosina <jkosina@suse.cz>,
        David Laight <David.Laight@aculab.com>,
        Borislav Petkov <bp@alien8.de>, julia@ni.com, jeyu@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>
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To:     Josh Poimboeuf <jpoimboe@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Nov 29, 2018, at 9:45 AM, Josh Poimboeuf <jpoimboe@redhat.com> wrote:
>=20
>> On Thu, Nov 29, 2018 at 09:41:33AM -0800, Andy Lutomirski wrote:
>>=20
>>> On Nov 29, 2018, at 9:21 AM, Steven Rostedt <rostedt@goodmis.org> wrote:=

>>>=20
>>> On Thu, 29 Nov 2018 12:20:00 -0500
>>> Steven Rostedt <rostedt@goodmis.org> wrote:
>>>=20
>>>=20
>>>> r8 =3D return address
>>>> r9 =3D function to call
>>>>=20
>>>=20
>>> Bad example, r8 and r9 are args, but r10 and r11 are available.
>>>=20
>>> -- Steve
>>>=20
>>>>   push r8
>>>>   jmp *r9
>>>>=20
>>>> Then have the regs->ip point to that trampoline.
>>=20
>> Cute. That=E2=80=99ll need ORC annotations and some kind of retpoline to r=
eplace the indirect jump, though.
>=20
> I'm going with this idea, but the BP is so rare that I really don't see
> why a retpoline would be needed.
>=20

Without the retpoline in place, you are vulnerable to security researchers c=
ausing you a personal denial of service by finding a way to cause the BP to g=
et hit, mistraining the branch predictor, and writing a paper about it :)=