Re: [PATCH v27 06/10] x86/cet/ibt: Update arch_prctl functions for Indirect Branch Tracking

[Edgecombe, Rick P]

On Tue, 2021-07-20 at 10:09 -0700, Yu, Yu-cheng wrote:
> On 7/19/2021 11:21 AM, Edgecombe, Rick P wrote:
> > On Fri, 2021-05-21 at 15:15 -0700, Yu-cheng Yu wrote:
> > > From: "H.J. Lu" <hjl.tools@xxxxxxxxx>
> > > 
> > > Update ARCH_X86_CET_STATUS and ARCH_X86_CET_DISABLE for Indirect
> > > Branch
> > > Tracking.
> > > 
> > > Signed-off-by: H.J. Lu <hjl.tools@xxxxxxxxx>
> > > Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> > > Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> > > ---
> > >   arch/x86/kernel/cet_prctl.c | 5 +++++
> > >   1 file changed, 5 insertions(+)
> > > 
> > > diff --git a/arch/x86/kernel/cet_prctl.c
> > > b/arch/x86/kernel/cet_prctl.c
> > > index b426d200e070..bd3c80d402e7 100644
> > > --- a/arch/x86/kernel/cet_prctl.c
> > > +++ b/arch/x86/kernel/cet_prctl.c
> > > @@ -22,6 +22,9 @@ static int cet_copy_status_to_user(struct
> > > thread_shstk *shstk, u64 __user *ubuf)
> > >                  buf[2] = shstk->size;
> > >          }
> > >   
> > > +       if (shstk->ibt)
> > > +               buf[0] |= GNU_PROPERTY_X86_FEATURE_1_IBT;
> > > +
> > Can you have IBT enabled but not shadow stack via kernel
> > parameters?
> > Outside this diff it has:
> > if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> >         return -ENOTSUPP;
> 
> If shadow stack is disabled by the kernel parameter, IBT is also
> disabled.
Thanks for the clarification.

> 
> > So if "no_user_shstk" is set, this can't be used for IBT. But the
> > kernel would attempt to enable IBT.
> 
> It will not.
Oh yea, I see the cpuid_deps part now. Sorry.

> 
> > Also if so, the CR4 bit enabling logic needs adjusting in this IBT
> > series. If not, we should probably mention this in the docs and
> > enforce
> > it. It would then follow the logic in Kconfig, so maybe the
> > simplest.
> > Like maybe instead of no_user_shstk, just no_user_cet?
> 
> If shadow stack is disabled (from either Kconfig or kernel 
> command-line), then IBT is also disabled.  However, we still need two
> kernel parameters because no_user_ibt can be useful sometimes.  I
> will 
> add a sentence in the document to indicate that IBT depends on shadow
> stack.
> 
> 
Yea, no_user_ibt seems useful. I meant that renaming no_user_shstk to
no_user_cet (or similar) would be more clear and self documenting,
since it intends to disable all user cet features and not just
shadowstack. And leaving no_user_ibt as is. Documentation works as well
though. Not major in any case.