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Subject: Re: [PATCH 4/4] x86,module: Detect CRn and DRn manipulation
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <e390a895-cc09-0e9b-a05d-0c9b7bc6bfbd@redhat.com>
Date:   Tue, 7 Apr 2020 16:51:54 -0700
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        LKML <linux-kernel@vger.kernel.org>, hch@infradead.org,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        mingo <mingo@redhat.com>, bp <bp@alien8.de>, hpa@zytor.com,
        x86 <x86@kernel.org>, "Kenneth R. Crudup" <kenny@panix.com>,
        Jessica Yu <jeyu@kernel.org>,
        Rasmus Villemoes <rasmus.villemoes@prevas.dk>,
        Fenghua Yu <fenghua.yu@intel.com>,
        Xiaoyao Li <xiaoyao.li@intel.com>,
        Thomas Hellstrom <thellstrom@vmware.com>,
        Tony Luck <tony.luck@intel.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        jannh@google.com, keescook@chromium.org, David.Laight@aculab.com,
        Doug Covelli <dcovelli@vmware.com>, mhiramat@kernel.org
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To:     Paolo Bonzini <pbonzini@redhat.com>
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> On Apr 7, 2020, at 3:12 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
>=20
> On 07/04/20 23:27, Peter Zijlstra wrote:
>> On Tue, Apr 07, 2020 at 02:22:11PM -0700, Nadav Amit wrote:
>>> Anyhow, I do not think it is the only use-case which is not covered =
by your
>>> patches (even considering CRs/DRs alone). For example, there is no =
kernel
>>> function to turn on CR4.VMXE, which is required to run hypervisors =
on x86.
>> That needs an exported function; there is no way we'll allow random
>> writes to CR4, there's too much dodgy stuff in there.
>=20
> native_write_cr4 and pv_ops (through which you can do write_cr4) are
> both exported, and so is cpu_tlbstate which is used by __cr4_set_bits
> and friends.  Am I missing something glaringly obvious?

No, I was the one who missed the obvious thing.

Having said that, I still think there are additional cases that need to =
be
handled. For instance, I see that is_erratum_383() in KVM (AMD) flushes =
the
local TLB by writing to CR3 the previous value. I am not familiar with =
the
erratum. Maybe I am missing something again, but I do not see an =
appropriate
exported alternative in the kernel.