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

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11.03.2017 00:04, Andy Lutomirski пишет:
On Fri, Mar 10, 2017 at 2:30 AM, Stas Sergeev <stsp@xxxxxxx> wrote:
10.03.2017 05:41, Andy Lutomirski пишет:

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 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
"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

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"?
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