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Subject: Re: [PATCH RFC v2 0/6] x86: Disabling PTI in compatibility mode
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <9a825978-fe69-1c10-0da0-0c67dbb9b232@linux.intel.com>
Date:   Thu, 15 Feb 2018 16:48:23 -0800
Cc:     Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andy Lutomirski <luto@kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Willy Tarreau <w@1wt.eu>, x86@kernel.org,
        linux-kernel@vger.kernel.org
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References: <20180215163602.61162-1-namit@vmware.com>
 <27a0082c-fadb-792a-740e-70932d51f1b5@linux.intel.com>
 <91CEEFA7-86C8-4731-BC7E-6AF5CC3A1BA4@gmail.com>
 <9a825978-fe69-1c10-0da0-0c67dbb9b232@linux.intel.com>
To:     Dave Hansen <dave.hansen@linux.intel.com>
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Dave Hansen <dave.hansen@linux.intel.com> wrote:

> On 02/15/2018 04:25 PM, Nadav Amit wrote:
>> Dave Hansen <dave.hansen@linux.intel.com> wrote:
>>=20
>>> On 02/15/2018 08:35 AM, Nadav Amit wrote:
>>>> I removed the PTI disabling while SMEP is unsupported, although I
>>>> must admit I did not fully understand why it is required.
>>>=20
>>> Do you mean you don't fully understand how PTI gives SMEP-like =
behavior
>>> on non-SMEP hardware?
>>=20
>> No. I understand how it provide SMEP-like behavior, and I understand =
the value
>> of SMEP by itself.
>>=20
>> However, I do not understand why SMEP-like protection is required to =
protect
>> processes that run in compatibility-mode from Meltdown/Spectre =
attacks. As
>> far as I understand, the process should not be able to manipulate the =
kernel
>> to execute code in the low 4GB.
>=20
> There are two problems: one is that regardless of Meltdown/Spectre, =
SMEP
> is valuable.  It's valuable to everything, compatibility-mode or not.
>=20
> The second problem is the RSB.  It has a full-width virtual address =
and,
> unlike the other indirect branch prediction, can steer you anywhere
> including to the low 4GB.

Thanks for the explanation. Based on Linus response, I guess this series =
is
nak=E2=80=99d, but still thanks for your patience.

I suspected the RSB might be the reason but it seemed to me that all the =
ROP
opportunities are still there, so I assumed it is not a reason.

Anyhow, thanks again.