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    Subject[PATCH 5.10 108/120] x86/debug: Prevent data breakpoints on __per_cpu_offset
    Date
    From: Lai Jiangshan <laijs@linux.alibaba.com>

    commit c4bed4b96918ff1d062ee81fdae4d207da4fa9b0 upstream.

    When FSGSBASE is enabled, paranoid_entry() fetches the per-CPU GSBASE value
    via __per_cpu_offset or pcpu_unit_offsets.

    When a data breakpoint is set on __per_cpu_offset[cpu] (read-write
    operation), the specific CPU will be stuck in an infinite #DB loop.

    RCU will try to send an NMI to the specific CPU, but it is not working
    either since NMI also relies on paranoid_entry(). Which means it's
    undebuggable.

    Fixes: eaad981291ee3("x86/entry/64: Introduce the FIND_PERCPU_BASE macro")
    Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
    Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
    Cc: stable@vger.kernel.org
    Link: https://lore.kernel.org/r/20210204152708.21308-1-jiangshanlai@gmail.com
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    arch/x86/kernel/hw_breakpoint.c | 14 ++++++++++++++
    1 file changed, 14 insertions(+)

    --- a/arch/x86/kernel/hw_breakpoint.c
    +++ b/arch/x86/kernel/hw_breakpoint.c
    @@ -269,6 +269,20 @@ static inline bool within_cpu_entry(unsi
    CPU_ENTRY_AREA_TOTAL_SIZE))
    return true;

    + /*
    + * When FSGSBASE is enabled, paranoid_entry() fetches the per-CPU
    + * GSBASE value via __per_cpu_offset or pcpu_unit_offsets.
    + */
    +#ifdef CONFIG_SMP
    + if (within_area(addr, end, (unsigned long)__per_cpu_offset,
    + sizeof(unsigned long) * nr_cpu_ids))
    + return true;
    +#else
    + if (within_area(addr, end, (unsigned long)&pcpu_unit_offsets,
    + sizeof(pcpu_unit_offsets)))
    + return true;
    +#endif
    +
    for_each_possible_cpu(cpu) {
    /* The original rw GDT is being used after load_direct_gdt() */
    if (within_area(addr, end, (unsigned long)get_cpu_gdt_rw(cpu),

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