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Subject: Re: [PATCH] x86/pkeys: Explicitly treat PK #PF on kernel address as a bad area
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <CAG48ez1iPr0q-PBYnc9Q4GA0uMEyph9THiV2CrN6aRMzqpEfng@mail.gmail.com>
Date:   Thu, 30 Aug 2018 20:08:10 -0700
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        sean.j.christopherson@intel.com,
        kernel list <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        the arch/x86 maintainers <x86@kernel.org>
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Message-Id: <22539689-D7E6-4780-9600-CC05B853EC41@amacapital.net>
References: <20180807172920.8766-1-sean.j.christopherson@intel.com> <b7fe6fad-7ce4-c7c1-31d7-cb609cf45b2f@intel.com> <CAG48ez1iPr0q-PBYnc9Q4GA0uMEyph9THiV2CrN6aRMzqpEfng@mail.gmail.com>
To:     Jann Horn <jannh@google.com>
Sender: linux-kernel-owner@vger.kernel.org
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> On Aug 30, 2018, at 7:38 PM, Jann Horn <jannh@google.com> wrote:
>=20
>> On Tue, 7 Aug 2018 Dave Hansen <dave.hansen@intel.com> wrote:
>>=20
>>> On 08/07/2018 10:29 AM, Sean Christopherson wrote:
>>>      if (unlikely(fault_in_kernel_space(address))) {
>>> +             /*
>>> +              * We should never encounter a protection keys fault on a
>>> +              * kernel address as kernel address are always mapped with=

>>> +              * _PAGE_USER=3D0, i.e. PKRU isn't enforced.
>>> +              */
>>> +             if (WARN_ON_ONCE(error_code & X86_PF_PK))
>>> +                     goto bad_kernel_address;
>>=20
>> I just realized one more thing: the vsyscall page can bite us here.
>> It's at a fault_in_kernel_space() address and we *can* trigger a pkey
>> fault on it if we jump to an instruction that reads from a
>> pkey-protected area.
>>=20
>> We can make a gadget out of unaligned vsyscall instructions that does
>> that.  See:
>>=20
>> 0xffffffffff600002:  shlb   $0x0,0x0(%rax)
>>=20
>> Then, we turn off access to all pkeys, including pkey-0, then jump to
>> the unaligned vsyscall instruction, which reads %rax, which is a kernel
>> address:
>=20
> Andy got rid of the (native) vsyscall page in
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/=
?id=3D076ca272a14cea558b1092ec85cea08510283f2a
> ('x86/vsyscall/64: Drop "native" vsyscalls') a few months ago, right?
> At this point, the vsyscall page should never be executable.

Indeed.

Can one of you cc me on the original patch?=