Messages in this thread Patch in this message | | | From | Lai Jiangshan <> | Subject | [PATCH 1/3] X86/db: Change __this_cpu_read() to this_cpu_read() in hw_breakpoint_active() | Date | Mon, 13 Dec 2021 12:22:13 +0800 |
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From: Lai Jiangshan <laijs@linux.alibaba.com>
__this_cpu_read() can not be instrumented except its own debugging code when CONFIG_DEBUG_PREEMPT. The debugging code will call __this_cpu_preempt_check(). __this_cpu_preempt_check() itself is also noinstr, so __this_cpu_read() can be used in noinstr.
But these is one exception when exc_debug_kernel() calls local_db_save() which calls hw_breakpoint_active() which calls __this_cpu_read(). If the data accessed by __this_cpu_preempt_check() is also watched by hw_breakpoints, it would cause recursive #DB.
this_cpu_read() in X86 is also non instrumentable, and it doesn't access to any extra data except the percpu cpu_dr7, and cpu_dr7 is disallowed to be watched in arch_build_bp_info(). So this_cpu_read() is safe to be used when hw_breakpoints is still active, and __this_cpu_read() here should be changed to this_cpu_read().
This problem can only happen when the system owner uses a kernel with CONFIG_DEBUG_PREEMPT enabled and deliberately use hw_breakpoints on the data that __this_cpu_preempt_check() accesses. Sot it is just a problem with no significance.
One might suggest that, all the data accessed by noinstr functions should be marked in denylist for hw_breakpoints. That would complexify the noinstrment framework and add hurdles to anyone that who want to add a new noinstr function. All we need is to suppress #DB in the IST interrupt entry path until safe place where #DB is disabled in hardware or #DB handler can handle well even it hits data accessed by noinstr function. Changing __this_cpu_read() to this_cpu_read() is fit for it.
Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com> --- arch/x86/include/asm/debugreg.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h index cfdf307ddc01..20189ce41578 100644 --- a/arch/x86/include/asm/debugreg.h +++ b/arch/x86/include/asm/debugreg.h @@ -87,7 +87,7 @@ static inline void hw_breakpoint_disable(void) static __always_inline bool hw_breakpoint_active(void) { - return __this_cpu_read(cpu_dr7) & DR_GLOBAL_ENABLE_MASK; + return this_cpu_read(cpu_dr7) & DR_GLOBAL_ENABLE_MASK; } extern void hw_breakpoint_restore(void); -- 2.19.1.6.gb485710b
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