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

[Jann Horn]

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().

> +       return membuf_write(&to, cetregs, sizeof(struct cet_user_state));
> +}
[...]
> diff --git a/arch/x86/kernel/ptrace.c b/arch/x86/kernel/ptrace.c
[...]
> @@ -52,7 +52,9 @@ enum x86_regset {
>         REGSET_IOPERM64 = REGSET_XFP,
>         REGSET_XSTATE,
>         REGSET_TLS,
> +       REGSET_CET64 = REGSET_TLS,
>         REGSET_IOPERM32,
> +       REGSET_CET32,
>  };
[...]
> @@ -1229,6 +1231,13 @@ static struct user_regset x86_64_regsets[] __ro_after_init = {
[...]
> +       [REGSET_CET64] = {
> +               .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
> +       },
>  };
[...]
> @@ -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.