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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: <f0fc93ca-ba1d-2570-5d79-0c02ee388f9d@citrix.com>
Date:   Thu, 15 Feb 2018 16:51:38 -0800
Cc:     Dave Hansen <dave.hansen@linux.intel.com>,
        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 ML <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>
 <f0fc93ca-ba1d-2570-5d79-0c02ee388f9d@citrix.com>
To:     Andrew Cooper <andrew.cooper3@citrix.com>
Sender: linux-kernel-owner@vger.kernel.org
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Andrew Cooper <andrew.cooper3@citrix.com> wrote:

> On 16/02/2018 00:25, 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.
>>> Do you mean you don't fully understand how PTI gives SMEP-like =
behavior
>>> on non-SMEP hardware?
>> 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
> Being 32bit is itself sufficient protection against Meltdown (as long =
as
> there nothing interesting of the kernels mapped below the 4G =
boundary).
>=20
> However, a 32bit compatibility process try to attack with Spectre/SP2 =
to
> redirect speculation back into userspace, at which point (if =
successful)
> the pipeline will be speculating in 64bit mode, and Meltdown is back =
on
> the table.  SMEP will block this attack vector, irrespective of other
> SP2 defences the kernel may employ, but a fully SP2-defended kernel
> doesn't require SMEP to be safe in this case.

Based on Jann Horn=E2=80=99s description of the branch predictor, it =
basically only
holds the lowest 31-bits of the target address. There might be a subtle
problem if the prediction wrapsaround, but excluding this case, I do not =
see
how Spectre v2 can be used to jump into running user code.