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

[Stas Sergeev]

09.03.2017 04:11, Ricardo Neri ÐÐÑÐÑ:
> On Wed, 2017-03-08 at 19:53 +0300, Stas Sergeev wrote:
> > 08.03.2017 19:46, Andy Lutomirski ÐÐÑÐÑ:
> > > > 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.