Subject: Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention
To: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
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Cc: Andy Lutomirski <luto@amacapital.net>, Ingo Molnar <mingo@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@kernel.org>, Borislav Petkov <bp@suse.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Brian Gerst <brgerst@gmail.com>, Chris Metcalf <cmetcalf@mellanox.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Liang Z Li <liang.z.li@intel.com>,
        Masami Hiramatsu <mhiramat@kernel.org>, Huang Rui <ray.huang@amd.com>,
        Jiri Slaby <jslaby@suse.cz>, Jonathan Corbet <corbet@lwn.net>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        Paul Gortmaker <paul.gortmaker@windriver.com>,
        Vlastimil Babka <vbabka@suse.cz>, Chen Yucong <slaoub@gmail.com>,
        Alexandre Julliard <julliard@winehq.org>,
        Fenghua Yu <fenghua.yu@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Shuah Khan <shuah@kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, linux-msdos@vger.kernel.org,
        wine-devel@winehq.org
From: Stas Sergeev <stsp@list.ru>
Message-ID: <62427c4e-7ac3-69a7-a677-4c7f459d541e@list.ru>
Date: Fri, 10 Mar 2017 01:05:57 +0300
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09.03.2017 04:11, Ricardo Neri пишет:
> On Wed, 2017-03-08 at 19:53 +0300, Stas Sergeev wrote:
>> 08.03.2017 19:46, Andy Lutomirski пишет:
>>>> No no, since I meant prot mode, this is not what I need.
>>>> I would never need to disable UMIP as to allow the
>>>> prot mode apps to do SLDT. Instead it would be good
>>>> to have an ability to provide a replacement for the dummy
>>>> emulation that is currently being proposed for kernel.
>>>> All is needed for this, is just to deliver a SIGSEGV.
>>> That's what I meant.  Turning off FIXUP_UMIP would leave UMIP on but
>>> turn off the fixup, so you'd get a SIGSEGV indicating #GP (or a vm86
>>> GP exit).
>> But then I am confused with the word "compat" in
>> your "COMPAT_MASK0_X86_UMIP_FIXUP" and
>> "sys_adjust_compat_mask(int op, int word, u32 mask);"
>>
>> Leaving UMIP on and only disabling a fixup doesn't
>> sound like a compat option to me. I would expect
>> compat to disable it completely.
> I guess that the _UMIP_FIXUP part makes it clear that emulation, not
> UMIP is disabled, allowing the SIGSEGV be delivered to the user space
> program.
>
> Would having a COMPAT_MASK0_X86_UMIP_FIXUP to disable emulation and a
> COMPAT_MASK0_X86_UMIP to disable UMIP make sense?
>
> Also, wouldn't having a COMPAT_MASK0_X86_UMIP to disable UMIP defeat its
> purpose? Applications could simply use this compat mask to bypass UMIP
> and gain access to the instructions it protects.
I don't think someone will want to completely disable
UMIP, so why do you need such functionality?
My question was only what does "compat" mean
in "COMPAT_MASK0_X86_UMIP_FIXUP", compat with what.