Re: [RFC PATCH v4 3/9] x86/cet/ibt: Add IBT legacy code bitmap allocation function

[Andy Lutomirski]

On Fri, Sep 21, 2018 at 8:10 AM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> Indirect branch tracking provides an optional legacy code bitmap
> that indicates locations of non-IBT compatible code.  When set,
> each bit in the bitmap represents a page in the linear address is
> legacy code.
>
> We allocate the bitmap only when the application requests it.
> Most applications do not need the bitmap.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/kernel/cet.c | 45 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 45 insertions(+)
>
> diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
> index 6adfe795d692..a65d9745af08 100644
> --- a/arch/x86/kernel/cet.c
> +++ b/arch/x86/kernel/cet.c
> @@ -314,3 +314,48 @@ void cet_disable_ibt(void)
>         wrmsrl(MSR_IA32_U_CET, r);
>         current->thread.cet.ibt_enabled = 0;
>  }
> +
> +int cet_setup_ibt_bitmap(void)
> +{
> +       u64 r;
> +       unsigned long bitmap;
> +       unsigned long size;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_IBT))
> +               return -EOPNOTSUPP;
> +
> +       if (!current->thread.cet.ibt_bitmap_addr) {
> +               /*
> +                * Calculate size and put in thread header.
> +                * may_expand_vm() needs this information.
> +                */
> +               size = TASK_SIZE / PAGE_SIZE / BITS_PER_BYTE;
> +               current->thread.cet.ibt_bitmap_size = size;
> +               bitmap = do_mmap_locked(0, size, PROT_READ | PROT_WRITE,
> +                                       MAP_ANONYMOUS | MAP_PRIVATE,
> +                                       VM_DONTDUMP);
> +
> +               if (bitmap >= TASK_SIZE) {
> +                       current->thread.cet.ibt_bitmap_size = 0;
> +                       return -ENOMEM;
> +               }
> +
> +               current->thread.cet.ibt_bitmap_addr = bitmap;
> +       }
> +
> +       /*
> +        * Lower bits of MSR_IA32_CET_LEG_IW_EN are for IBT
> +        * settings.  Clear lower bits even bitmap is already
> +        * page-aligned.
> +        */
> +       bitmap = current->thread.cet.ibt_bitmap_addr;
> +       bitmap &= PAGE_MASK;
> +
> +       /*
> +        * Turn on IBT legacy bitmap.
> +        */
> +       rdmsrl(MSR_IA32_U_CET, r);
> +       r |= (MSR_IA32_CET_LEG_IW_EN | bitmap);
> +       wrmsrl(MSR_IA32_U_CET, r);
> +       return 0;

Why are you writing the MSRs in the case where the bitmap was already allocated?