Re: [patch V4 part 1 02/36] x86/hw_breakpoint: Prevent data breakpoints on cpu_entry_area

[Lai Jiangshan]

On Tue, May 5, 2020 at 10:15 PM Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>
> From: Andy Lutomirski <luto@xxxxxxxxxx>
>
> A data breakpoint near the top of an IST stack will cause unresoverable
> recursion.  A data breakpoint on the GDT, IDT, or TSS is terrifying.
> Prevent either of these from happening.
>
> Co-developed-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> ---
>  arch/x86/kernel/hw_breakpoint.c |   25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>
> --- a/arch/x86/kernel/hw_breakpoint.c
> +++ b/arch/x86/kernel/hw_breakpoint.c
> @@ -227,10 +227,35 @@ int arch_check_bp_in_kernelspace(struct
>         return (va >= TASK_SIZE_MAX) || ((va + len - 1) >= TASK_SIZE_MAX);
>  }
>
> +/*
> + * Checks whether the range from addr to end, inclusive, overlaps the CPU
> + * entry area range.
> + */
> +static inline bool within_cpu_entry_area(unsigned long addr, unsigned long end)
> +{
> +       return end >= CPU_ENTRY_AREA_PER_CPU &&
> +              addr < (CPU_ENTRY_AREA_PER_CPU + CPU_ENTRY_AREA_TOTAL_SIZE);
> +}

Hello

These two lines:
s/CPU_ENTRY_AREA_PER_CPU/CPU_ENTRY_AREA_BASE/g
or
s/CPU_ENTRY_AREA_PER_CPU/CPU_ENTRY_AREA_RO_IDT/g

or otherwise the RO_IDT is not being protected.

sees:
#define CPU_ENTRY_AREA_PER_CPU (CPU_ENTRY_AREA_RO_IDT + PAGE_SIZE)

#define CPU_ENTRY_AREA_TOTAL_SIZE (CPU_ENTRY_AREA_ARRAY_SIZE + PAGE_SIZE)
                                     ^ sizeof PER_CPU           ^ RO_IDT


Reviewed-by: Lai Jiangshan <jiangshanlai@xxxxxxxxx>


> +
>  static int arch_build_bp_info(struct perf_event *bp,
>                               const struct perf_event_attr *attr,
>                               struct arch_hw_breakpoint *hw)
>  {
> +       unsigned long bp_end;
> +
> +       bp_end = attr->bp_addr + attr->bp_len - 1;
> +       if (bp_end < attr->bp_addr)
> +               return -EINVAL;
> +
> +       /*
> +        * Prevent any breakpoint of any type that overlaps the
> +        * cpu_entry_area.  This protects the IST stacks and also
> +        * reduces the chance that we ever find out what happens if
> +        * there's a data breakpoint on the GDT, IDT, or TSS.
> +        */
> +       if (within_cpu_entry_area(attr->bp_addr, bp_end))
> +               return -EINVAL;
> +
>         hw->address = attr->bp_addr;
>         hw->mask = 0;
>
>