Re: [v6 PATCH 00/21] x86: Enable User-Mode Instruction Prevention

On Wed, Mar 8, 2017 at 5:11 PM, Ricardo Neri
<[email protected]> wrote:
> 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 was obviously extremely unclear.  The point of the proposed syscall
is to let programs opt out of legacy features.  So there would be a
bit to disable emulation of UMIP-blocked instructions (this giving the
unadulterated #GP).  There would not be a bit to disable UMIP itself.

There's also a flaw in my proposal.  Disable-vsyscall would be per-mm
and disable-umip-emulation would be per-task, so they'd need to be in
separate words to make any sense.  I'll ponder this a bit more.
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