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    SubjectRe: [patch V4 part 1 02/36] x86/hw_breakpoint: Prevent data breakpoints on cpu_entry_area
    On Tue, May 5, 2020 at 10:15 PM Thomas Gleixner <tglx@linutronix.de> wrote:
    >
    > From: Andy Lutomirski <luto@kernel.org>
    >
    > 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@infradead.org>
    > Signed-off-by: Andy Lutomirski <luto@kernel.org>
    > Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    > Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    > ---
    > 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@gmail.com>


    > +
    > 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;
    >
    >

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