Re: [PATCH v11 6/9] x86/cet: Add PTRACE interface for CET

[Yu, Yu-cheng]

On 9/2/2020 1:03 PM, Jann Horn wrote:
> On Tue, Aug 25, 2020 at 2:30 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
> > Add REGSET_CET64/REGSET_CET32 to get/set CET MSRs:
> >
> >      IA32_U_CET (user-mode CET settings) and
> >      IA32_PL3_SSP (user-mode Shadow Stack)
> [...]
> > diff --git a/arch/x86/kernel/fpu/regset.c b/arch/x86/kernel/fpu/regset.c
> [...]
> > +int cetregs_get(struct task_struct *target, const struct user_regset *regset,
> > +               struct membuf to)
> > +{
> > +       struct fpu *fpu = &target->thread.fpu;
> > +       struct cet_user_state *cetregs;
> > +
> > +       if (!boot_cpu_has(X86_FEATURE_SHSTK))
> > +               return -ENODEV;
> > +
> > +       fpu__prepare_read(fpu);
> > +       cetregs = get_xsave_addr(&fpu->state.xsave, XFEATURE_CET_USER);
> > +       if (!cetregs)
> > +               return -EFAULT;
>
> Can this branch ever be hit without a kernel bug? If yes, I think
> -EFAULT is probably a weird error code to choose here. If no, this
> should probably use WARN_ON(). Same thing in cetregs_set().

When a thread is not CET-enabled, its CET state does not exist.  I 
looked at EFAULT, and it means "Bad address".  Maybe this can be ENODEV, 
which means "No such device"?

[...]

> > @@ -1284,6 +1293,13 @@ static struct user_regset x86_32_regsets[] __ro_after_init = {
> [...]
> > +       [REGSET_CET32] = {
> > +               .core_note_type = NT_X86_CET,
> > +               .n = sizeof(struct cet_user_state) / sizeof(u64),
> > +               .size = sizeof(u64), .align = sizeof(u64),
> > +               .active = cetregs_active, .regset_get = cetregs_get,
> > +               .set = cetregs_set
> > +       },
> >   };
>
> Why are there different identifiers for 32-bit CET and 64-bit CET when
> they operate on the same structs and have the same handlers? If
> there's a good reason for that, the commit message should probably
> point that out.

Yes, the reason for two regsets is that fill_note_info() does not expect 
any holes in a regsets.  I will put this in the commit log.

Thanks,
Yu-cheng