Re: [RFC PATCH v3 18/24] x86/cet/shstk: User-mode shadow stack support

[Jann Horn]

On Thu, Aug 30, 2018 at 4:44 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> This patch adds basic shadow stack enabling/disabling routines.
> A task's shadow stack is allocated from memory with VM_SHSTK
> flag set and read-only protection.  The shadow stack is
> allocated to a fixed size of RLIMIT_STACK.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
[...]
> +static int set_shstk_ptr(unsigned long addr)
> +{
> +       u64 r;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> +               return -1;
> +
> +       if ((addr >= TASK_SIZE_MAX) || (!IS_ALIGNED(addr, 4)))
> +               return -1;
> +
> +       rdmsrl(MSR_IA32_U_CET, r);
> +       wrmsrl(MSR_IA32_PL3_SSP, addr);
> +       wrmsrl(MSR_IA32_U_CET, r | MSR_IA32_CET_SHSTK_EN);
> +       return 0;
> +}

Here's a really stupid question: Where is the logic for switching
those MSRs on task switch? MSR_IA32_PL3_SSP contains a userspace
pointer, so it has to be switched on task switch, right? I'm sure I'm
missing something obvious, but grepping for places that set
MSR_IA32_PL3_SSP to nonzero values through the entire patchset, I only
see set_shstk_ptr(), which is called from:

 - cet_setup_shstk() (called from arch_setup_features(), which is
called from load_elf_binary())
 - cet_restore_signal() (called on signal handler return)
 - cet_setup_signal() (called from signal handling code)