linux-kernel - [PATCH 1/3] X86/db: Change __this_cpu_read() to this_cpu_read() in hw_breakpoint

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Date:   Mon, 13 Dec 2021 12:22:13 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     linux-kernel@...r.kernel.org
Cc:     x86@...nel.org, Lai Jiangshan <laijs@...ux.alibaba.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>
Subject: [PATCH 1/3] X86/db: Change __this_cpu_read() to this_cpu_read() in hw_breakpoint_active()

From: Lai Jiangshan <laijs@...ux.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@...ux.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