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

[Stas Sergeev]

11.03.2017 00:04, Andy Lutomirski ÐÐÑÐÑ:
> On Fri, Mar 10, 2017 at 2:30 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
> > 10.03.2017 05:41, Andy Lutomirski ÐÐÑÐÑ:
> >
> > > On Wed, Mar 8, 2017 at 5:11 PM, Ricardo Neri
> > > <ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> > > > 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 was obviously extremely unclear.  The point of the proposed syscall
> > > is to let programs opt out of legacy features.
> > I guess both "compat" and "legacy" are misleading
> > here. Maybe these are "x86-specific" or "hypervisor-specific",
> > but a mere enabling of UMIP doesn't immediately make
> > the use of SLDT instruction a legacy IMHO.
> Sure it is. :)  Using SLDT from user mode is a legacy ability that
> just happens to still work on existing CPUs and kernels.  Once UMIP
> goes in, it will officially be obsolete
Yes, but the names you suggest, imply that "UMIP_FIXUP"
is legacy or compat, which I find misleading because it have
just appeared. Maybe something like "COMPAT_X86_UMIP_INSNS_EMU"?