[PATCH 1/3] X86/db: Change __this_cpu_read() to this_cpu_read() in hw_breakpoint_active()

Lai Jiangshan
2021-12-13T04:22:17Z

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