[PATCH v3 0/4] arm64: kprobes: Fix some bugs in arm64 kprobes

[PATCH v3 0/4] arm64: kprobes: Fix some bugs in arm64 kprobes

[Masami Hiramatsu]

Hi,

Here are the v3 patches which fixes kprobe bugs on arm64.
In this version I fixed some issues pointed by James and add Reviewed-by
and Acked-bys.

Background:
Naresh reported that recently ftracetest crashes kernel, and I found
there are 3 different bugs around the crash. In v1 thread, we found
one another bug of RCU and debug exception.

- Kprobes on arm64 doesn't recover pstate.D mask after single stepping.
  So after hitting kprobes, the debug flag status is changed.
- Some symbols which are called from blacklisted function, are not
  blacklisted.
- Debug exception is not visible to RCU, thus rcu_read_lock() cause
  a warning inside it.
- Debug exception handlers on arm64 is using rcu_read_lock(), but
  that is not needed because interrupts are disabled.

This series includes fixes for above bugs.

Thank you,

---

Masami Hiramatsu (4):
      arm64: kprobes: Recover pstate.D in single-step exception handler
      arm64: unwind: Prohibit probing on return_address()
      arm64: Make debug exception handlers visible from RCU
      arm64: Remove unneeded rcu_read_lock from debug handlers


 arch/arm64/include/asm/daifflags.h |    2 ++
 arch/arm64/kernel/debug-monitors.c |   14 +++++++------
 arch/arm64/kernel/probes/kprobes.c |   39 +++++------------------------------
 arch/arm64/kernel/return_address.c |    3 +++
 arch/arm64/kernel/stacktrace.c     |    3 +++
 arch/arm64/mm/fault.c              |   40 ++++++++++++++++++++++++++++++++++++
 6 files changed, 61 insertions(+), 40 deletions(-)

--
Masami Hiramatsu (Linaro) <mhiramat@kernel.org>

[PATCH v3 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler

[Masami Hiramatsu]

kprobes manipulates the interrupted PSTATE for single step, and
doesn't restore it. Thus, if we put a kprobe where the pstate.D
(debug) masked, the mask will be cleared after the kprobe hits.

Moreover, in the most complicated case, this can lead a kernel
crash with below message when a nested kprobe hits.

[  152.118921] Unexpected kernel single-step exception at EL1

When the 1st kprobe hits, do_debug_exception() will be called.
At this point, debug exception (= pstate.D) must be masked (=1).
But if another kprobes hits before single-step of the first kprobe
(e.g. inside user pre_handler), it unmask the debug exception
(pstate.D = 0) and return.
Then, when the 1st kprobe setting up single-step, it saves current
DAIF, mask DAIF, enable single-step, and restore DAIF.
However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
single-step exception happens soon after restoring DAIF.

This has been introduced by commit 7419333fa15e ("arm64: kprobe:
Always clear pstate.D in breakpoint exception handler")

To solve this issue, this stores all DAIF bits and restore it
after single stepping.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
  Changes in v3:
   - Update patch description
   - move PSR_DAIF_MASK in daifflags.h
  Changes in v2:
   - Save and restore all DAIF flags.
   - Operate pstate directly and remove spsr_set_debug_flag().
---
 arch/arm64/include/asm/daifflags.h |    2 ++
 arch/arm64/kernel/probes/kprobes.c |   39 +++++-------------------------------
 2 files changed, 7 insertions(+), 34 deletions(-)

diff --git a/arch/arm64/include/asm/daifflags.h b/arch/arm64/include/asm/daifflags.h
index 987926ed535e..063c964af705 100644
--- a/arch/arm64/include/asm/daifflags.h
+++ b/arch/arm64/include/asm/daifflags.h
@@ -13,6 +13,8 @@
 #define DAIF_PROCCTX		0
 #define DAIF_PROCCTX_NOIRQ	PSR_I_BIT
 #define DAIF_ERRCTX		(PSR_I_BIT | PSR_A_BIT)
+#define DAIF_MASK		(PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
+
 
 /* mask/save/unmask/restore all exceptions, including interrupts. */
 static inline void local_daif_mask(void)
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index bd5dfffca272..bf2259651b67 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -167,33 +167,6 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
 	__this_cpu_write(current_kprobe, p);
 }
 
-/*
- * When PSTATE.D is set (masked), then software step exceptions can not be
- * generated.
- * SPSR's D bit shows the value of PSTATE.D immediately before the
- * exception was taken. PSTATE.D is set while entering into any exception
- * mode, however software clears it for any normal (none-debug-exception)
- * mode in the exception entry. Therefore, when we are entering into kprobe
- * breakpoint handler from any normal mode then SPSR.D bit is already
- * cleared, however it is set when we are entering from any debug exception
- * mode.
- * Since we always need to generate single step exception after a kprobe
- * breakpoint exception therefore we need to clear it unconditionally, when
- * we become sure that the current breakpoint exception is for kprobe.
- */
-static void __kprobes
-spsr_set_debug_flag(struct pt_regs *regs, int mask)
-{
-	unsigned long spsr = regs->pstate;
-
-	if (mask)
-		spsr |= PSR_D_BIT;
-	else
-		spsr &= ~PSR_D_BIT;
-
-	regs->pstate = spsr;
-}
-
 /*
  * Interrupts need to be disabled before single-step mode is set, and not
  * reenabled until after single-step mode ends.
@@ -205,17 +178,17 @@ spsr_set_debug_flag(struct pt_regs *regs, int mask)
 static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
-	kcb->saved_irqflag = regs->pstate;
+	kcb->saved_irqflag = regs->pstate & DAIF_MASK;
 	regs->pstate |= PSR_I_BIT;
+	/* Unmask PSTATE.D for enabling software step exceptions. */
+	regs->pstate &= ~PSR_D_BIT;
 }
 
 static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
-	if (kcb->saved_irqflag & PSR_I_BIT)
-		regs->pstate |= PSR_I_BIT;
-	else
-		regs->pstate &= ~PSR_I_BIT;
+	regs->pstate &= ~DAIF_MASK;
+	regs->pstate |= kcb->saved_irqflag;
 }
 
 static void __kprobes
@@ -252,8 +225,6 @@ static void __kprobes setup_singlestep(struct kprobe *p,
 
 		set_ss_context(kcb, slot);	/* mark pending ss */
 
-		spsr_set_debug_flag(regs, 0);
-
 		/* IRQs and single stepping do not mix well. */
 		kprobes_save_local_irqflag(kcb, regs);
 		kernel_enable_single_step(regs);

Re: [PATCH v3 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler

[Will Deacon]

On Thu, Jul 25, 2019 at 05:15:54PM +0900, Masami Hiramatsu wrote:
> kprobes manipulates the interrupted PSTATE for single step, and
> doesn't restore it. Thus, if we put a kprobe where the pstate.D
> (debug) masked, the mask will be cleared after the kprobe hits.
> 
> Moreover, in the most complicated case, this can lead a kernel
> crash with below message when a nested kprobe hits.
> 
> [  152.118921] Unexpected kernel single-step exception at EL1
> 
> When the 1st kprobe hits, do_debug_exception() will be called.
> At this point, debug exception (= pstate.D) must be masked (=1).
> But if another kprobes hits before single-step of the first kprobe
> (e.g. inside user pre_handler), it unmask the debug exception
> (pstate.D = 0) and return.
> Then, when the 1st kprobe setting up single-step, it saves current
> DAIF, mask DAIF, enable single-step, and restore DAIF.
> However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
> single-step exception happens soon after restoring DAIF.
> 
> This has been introduced by commit 7419333fa15e ("arm64: kprobe:
> Always clear pstate.D in breakpoint exception handler")
> 
> To solve this issue, this stores all DAIF bits and restore it
> after single stepping.
> 
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
> Reviewed-by: James Morse <james.morse@arm.com>
> Tested-by: James Morse <james.morse@arm.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>   Changes in v3:
>    - Update patch description
>    - move PSR_DAIF_MASK in daifflags.h
>   Changes in v2:
>    - Save and restore all DAIF flags.
>    - Operate pstate directly and remove spsr_set_debug_flag().
> ---
>  arch/arm64/include/asm/daifflags.h |    2 ++
>  arch/arm64/kernel/probes/kprobes.c |   39 +++++-------------------------------
>  2 files changed, 7 insertions(+), 34 deletions(-)

I'm seeing an allmodconfig build failure with this:

arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_save_local_irqflag’:
arch/arm64/kernel/probes/kprobes.c:181:38: error: ‘DAIF_MASK’ undeclared (first use in this function); did you mean ‘BIT_MASK’?
  kcb->saved_irqflag = regs->pstate & DAIF_MASK;
                                      ^~~~~~~~~
                                      BIT_MASK
arch/arm64/kernel/probes/kprobes.c:181:38: note: each undeclared identifier is reported only once for each function it appears in
arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_restore_local_irqflag’:
arch/arm64/kernel/probes/kprobes.c:190:19: error: ‘DAIF_MASK’ undeclared (first use in this function); did you mean ‘BIT_MASK’?
  regs->pstate &= ~DAIF_MASK;
                   ^~~~~~~~~
                   BIT_MASK
make[2]: *** [scripts/Makefile.build:274: arch/arm64/kernel/probes/kprobes.o] Error 1

Will

Re: [PATCH v3 1/4] arm64: kprobes: Recover pstate.D in single-step exception handler

[Masami Hiramatsu]

On Thu, 1 Aug 2019 13:16:22 +0100
Will Deacon <will@kernel.org> wrote:

> On Thu, Jul 25, 2019 at 05:15:54PM +0900, Masami Hiramatsu wrote:
> > kprobes manipulates the interrupted PSTATE for single step, and
> > doesn't restore it. Thus, if we put a kprobe where the pstate.D
> > (debug) masked, the mask will be cleared after the kprobe hits.
> > 
> > Moreover, in the most complicated case, this can lead a kernel
> > crash with below message when a nested kprobe hits.
> > 
> > [  152.118921] Unexpected kernel single-step exception at EL1
> > 
> > When the 1st kprobe hits, do_debug_exception() will be called.
> > At this point, debug exception (= pstate.D) must be masked (=1).
> > But if another kprobes hits before single-step of the first kprobe
> > (e.g. inside user pre_handler), it unmask the debug exception
> > (pstate.D = 0) and return.
> > Then, when the 1st kprobe setting up single-step, it saves current
> > DAIF, mask DAIF, enable single-step, and restore DAIF.
> > However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
> > single-step exception happens soon after restoring DAIF.
> > 
> > This has been introduced by commit 7419333fa15e ("arm64: kprobe:
> > Always clear pstate.D in breakpoint exception handler")
> > 
> > To solve this issue, this stores all DAIF bits and restore it
> > after single stepping.
> > 
> > Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> > Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
> > Reviewed-by: James Morse <james.morse@arm.com>
> > Tested-by: James Morse <james.morse@arm.com>
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > ---
> >   Changes in v3:
> >    - Update patch description
> >    - move PSR_DAIF_MASK in daifflags.h
> >   Changes in v2:
> >    - Save and restore all DAIF flags.
> >    - Operate pstate directly and remove spsr_set_debug_flag().
> > ---
> >  arch/arm64/include/asm/daifflags.h |    2 ++
> >  arch/arm64/kernel/probes/kprobes.c |   39 +++++-------------------------------
> >  2 files changed, 7 insertions(+), 34 deletions(-)
> 
> I'm seeing an allmodconfig build failure with this:
> 
> arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_save_local_irqflag’:
> arch/arm64/kernel/probes/kprobes.c:181:38: error: ‘DAIF_MASK’ undeclared (first use in this function); did you mean ‘BIT_MASK’?
>   kcb->saved_irqflag = regs->pstate & DAIF_MASK;
>                                       ^~~~~~~~~
>                                       BIT_MASK
> arch/arm64/kernel/probes/kprobes.c:181:38: note: each undeclared identifier is reported only once for each function it appears in
> arch/arm64/kernel/probes/kprobes.c: In function ‘kprobes_restore_local_irqflag’:
> arch/arm64/kernel/probes/kprobes.c:190:19: error: ‘DAIF_MASK’ undeclared (first use in this function); did you mean ‘BIT_MASK’?
>   regs->pstate &= ~DAIF_MASK;
>                    ^~~~~~~~~
>                    BIT_MASK
> make[2]: *** [scripts/Makefile.build:274: arch/arm64/kernel/probes/kprobes.o] Error 1

Oops, daifflags.h is included via kvm_host.h...
OK, kprobes.c must include daifflags.h directly in this case.

I'll update this patch too.

Thank you,

> 
> Will


-- 
Masami Hiramatsu <mhiramat@kernel.org>

[PATCH v4] arm64: kprobes: Recover pstate.D in single-step exception handler

[Masami Hiramatsu]

kprobes manipulates the interrupted PSTATE for single step, and
doesn't restore it. Thus, if we put a kprobe where the pstate.D
(debug) masked, the mask will be cleared after the kprobe hits.

Moreover, in the most complicated case, this can lead a kernel
crash with below message when a nested kprobe hits.

[  152.118921] Unexpected kernel single-step exception at EL1

When the 1st kprobe hits, do_debug_exception() will be called.
At this point, debug exception (= pstate.D) must be masked (=1).
But if another kprobes hits before single-step of the first kprobe
(e.g. inside user pre_handler), it unmask the debug exception
(pstate.D = 0) and return.
Then, when the 1st kprobe setting up single-step, it saves current
DAIF, mask DAIF, enable single-step, and restore DAIF.
However, since "D" flag in DAIF is cleared by the 2nd kprobe, the
single-step exception happens soon after restoring DAIF.

This has been introduced by commit 7419333fa15e ("arm64: kprobe:
Always clear pstate.D in breakpoint exception handler")

To solve this issue, this stores all DAIF bits and restore it
after single stepping.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Fixes: commit 7419333fa15e ("arm64: kprobe: Always clear pstate.D in breakpoint exception handler")
Reviewed-by: James Morse <james.morse@arm.com>
Tested-by: James Morse <james.morse@arm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
  Changes in v4:
   - Include daifflags.h directly so that kprobe can use DAIF_MASK always.
  Changes in v3:
   - Update patch description
   - move PSR_DAIF_MASK in daifflags.h
  Changes in v2:
   - Save and restore all DAIF flags.
   - Operate pstate directly and remove spsr_set_debug_flag().
---
 arch/arm64/include/asm/daifflags.h |    2 ++
 arch/arm64/kernel/probes/kprobes.c |   40 +++++-------------------------------
 2 files changed, 8 insertions(+), 34 deletions(-)

diff --git a/arch/arm64/include/asm/daifflags.h b/arch/arm64/include/asm/daifflags.h
index 987926ed535e..063c964af705 100644
--- a/arch/arm64/include/asm/daifflags.h
+++ b/arch/arm64/include/asm/daifflags.h
@@ -13,6 +13,8 @@
 #define DAIF_PROCCTX		0
 #define DAIF_PROCCTX_NOIRQ	PSR_I_BIT
 #define DAIF_ERRCTX		(PSR_I_BIT | PSR_A_BIT)
+#define DAIF_MASK		(PSR_D_BIT | PSR_A_BIT | PSR_I_BIT | PSR_F_BIT)
+
 
 /* mask/save/unmask/restore all exceptions, including interrupts. */
 static inline void local_daif_mask(void)
diff --git a/arch/arm64/kernel/probes/kprobes.c b/arch/arm64/kernel/probes/kprobes.c
index bd5dfffca272..c4452827419b 100644
--- a/arch/arm64/kernel/probes/kprobes.c
+++ b/arch/arm64/kernel/probes/kprobes.c
@@ -21,6 +21,7 @@
 #include <asm/ptrace.h>
 #include <asm/cacheflush.h>
 #include <asm/debug-monitors.h>
+#include <asm/daifflags.h>
 #include <asm/system_misc.h>
 #include <asm/insn.h>
 #include <linux/uaccess.h>
@@ -167,33 +168,6 @@ static void __kprobes set_current_kprobe(struct kprobe *p)
 	__this_cpu_write(current_kprobe, p);
 }
 
-/*
- * When PSTATE.D is set (masked), then software step exceptions can not be
- * generated.
- * SPSR's D bit shows the value of PSTATE.D immediately before the
- * exception was taken. PSTATE.D is set while entering into any exception
- * mode, however software clears it for any normal (none-debug-exception)
- * mode in the exception entry. Therefore, when we are entering into kprobe
- * breakpoint handler from any normal mode then SPSR.D bit is already
- * cleared, however it is set when we are entering from any debug exception
- * mode.
- * Since we always need to generate single step exception after a kprobe
- * breakpoint exception therefore we need to clear it unconditionally, when
- * we become sure that the current breakpoint exception is for kprobe.
- */
-static void __kprobes
-spsr_set_debug_flag(struct pt_regs *regs, int mask)
-{
-	unsigned long spsr = regs->pstate;
-
-	if (mask)
-		spsr |= PSR_D_BIT;
-	else
-		spsr &= ~PSR_D_BIT;
-
-	regs->pstate = spsr;
-}
-
 /*
  * Interrupts need to be disabled before single-step mode is set, and not
  * reenabled until after single-step mode ends.
@@ -205,17 +179,17 @@ spsr_set_debug_flag(struct pt_regs *regs, int mask)
 static void __kprobes kprobes_save_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
-	kcb->saved_irqflag = regs->pstate;
+	kcb->saved_irqflag = regs->pstate & DAIF_MASK;
 	regs->pstate |= PSR_I_BIT;
+	/* Unmask PSTATE.D for enabling software step exceptions. */
+	regs->pstate &= ~PSR_D_BIT;
 }
 
 static void __kprobes kprobes_restore_local_irqflag(struct kprobe_ctlblk *kcb,
 						struct pt_regs *regs)
 {
-	if (kcb->saved_irqflag & PSR_I_BIT)
-		regs->pstate |= PSR_I_BIT;
-	else
-		regs->pstate &= ~PSR_I_BIT;
+	regs->pstate &= ~DAIF_MASK;
+	regs->pstate |= kcb->saved_irqflag;
 }
 
 static void __kprobes
@@ -252,8 +226,6 @@ static void __kprobes setup_singlestep(struct kprobe *p,
 
 		set_ss_context(kcb, slot);	/* mark pending ss */
 
-		spsr_set_debug_flag(regs, 0);
-
 		/* IRQs and single stepping do not mix well. */
 		kprobes_save_local_irqflag(kcb, regs);
 		kernel_enable_single_step(regs);

[PATCH v3 2/4] arm64: unwind: Prohibit probing on return_address()

[Masami Hiramatsu]

Prohibit probing on return_address() and subroutines which
is called from return_address(), since the it is invoked from
trace_hardirqs_off() which is also kprobe blacklisted.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 Changes in v3:
  - Fix to use NOKPROBE_SYMBOL() for save_return_addr instead
    of nokprobe_inline.
---
 arch/arm64/kernel/return_address.c |    3 +++
 arch/arm64/kernel/stacktrace.c     |    3 +++
 2 files changed, 6 insertions(+)

diff --git a/arch/arm64/kernel/return_address.c b/arch/arm64/kernel/return_address.c
index b21cba90f82d..491184a9f081 100644
--- a/arch/arm64/kernel/return_address.c
+++ b/arch/arm64/kernel/return_address.c
@@ -8,6 +8,7 @@
 
 #include <linux/export.h>
 #include <linux/ftrace.h>
+#include <linux/kprobes.h>
 
 #include <asm/stack_pointer.h>
 #include <asm/stacktrace.h>
@@ -29,6 +30,7 @@ static int save_return_addr(struct stackframe *frame, void *d)
 		return 0;
 	}
 }
+NOKPROBE_SYMBOL(save_return_addr);
 
 void *return_address(unsigned int level)
 {
@@ -52,3 +54,4 @@ void *return_address(unsigned int level)
 		return NULL;
 }
 EXPORT_SYMBOL_GPL(return_address);
+NOKPROBE_SYMBOL(return_address);
diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index 62d395151abe..cd7dab54d17b 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -7,6 +7,7 @@
 #include <linux/kernel.h>
 #include <linux/export.h>
 #include <linux/ftrace.h>
+#include <linux/kprobes.h>
 #include <linux/sched.h>
 #include <linux/sched/debug.h>
 #include <linux/sched/task_stack.h>
@@ -73,6 +74,7 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 
 	return 0;
 }
+NOKPROBE_SYMBOL(unwind_frame);
 
 void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
 		     int (*fn)(struct stackframe *, void *), void *data)
@@ -87,6 +89,7 @@ void notrace walk_stackframe(struct task_struct *tsk, struct stackframe *frame,
 			break;
 	}
 }
+NOKPROBE_SYMBOL(walk_stackframe);
 
 #ifdef CONFIG_STACKTRACE
 struct stack_trace_data {

[PATCH v3 3/4] arm64: Make debug exception handlers visible from RCU

[Masami Hiramatsu]

Make debug exceptions visible from RCU so that synchronize_rcu()
correctly track the debug exception handler.

This also introduces sanity checks for user-mode exceptions as same
as x86's ist_enter()/ist_exit().

The debug exception can interrupt in idle task. For example, it warns
if we put a kprobe on a function called from idle task as below.
The warning message showed that the rcu_read_lock() caused this
problem. But actually, this means the RCU is lost the context which
is already in NMI/IRQ.

  /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events
  /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable
  /sys/kernel/debug/tracing # [  135.122237]
  [  135.125035] =============================
  [  135.125310] WARNING: suspicious RCU usage
  [  135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted
  [  135.125904] -----------------------------
  [  135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle!
  [  135.126839]
  [  135.126839] other info that might help us debug this:
  [  135.126839]
  [  135.127410]
  [  135.127410] RCU used illegally from idle CPU!
  [  135.127410] rcu_scheduler_active = 2, debug_locks = 1
  [  135.128114] RCU used illegally from extended quiescent state!
  [  135.128555] 1 lock held by swapper/0/0:
  [  135.128944]  #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178
  [  135.130499]
  [  135.130499] stack backtrace:
  [  135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20
  [  135.131841] Hardware name: linux,dummy-virt (DT)
  [  135.132224] Call trace:
  [  135.132491]  dump_backtrace+0x0/0x140
  [  135.132806]  show_stack+0x24/0x30
  [  135.133133]  dump_stack+0xc4/0x10c
  [  135.133726]  lockdep_rcu_suspicious+0xf8/0x108
  [  135.134171]  call_break_hook+0x170/0x178
  [  135.134486]  brk_handler+0x28/0x68
  [  135.134792]  do_debug_exception+0x90/0x150
  [  135.135051]  el1_dbg+0x18/0x8c
  [  135.135260]  default_idle_call+0x0/0x44
  [  135.135516]  cpu_startup_entry+0x2c/0x30
  [  135.135815]  rest_init+0x1b0/0x280
  [  135.136044]  arch_call_rest_init+0x14/0x1c
  [  135.136305]  start_kernel+0x4d4/0x500
  [  135.136597]

So make debug exception visible to RCU can fix this warning.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 Changes in v3:
  - Make a comment for debug_exception_enter() clearer.
---
 arch/arm64/mm/fault.c |   40 ++++++++++++++++++++++++++++++++++++++++
 1 file changed, 40 insertions(+)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 9568c116ac7f..ed6c55c87fdc 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr,
 	debug_fault_info[nr].name	= name;
 }
 
+/*
+ * In debug exception context, we explicitly disable preemption.
+ * This serves two purposes: it makes it much less likely that we would
+ * accidentally schedule in exception context and it will force a warning
+ * if we somehow manage to schedule by accident.
+ */
+static void debug_exception_enter(struct pt_regs *regs)
+{
+	if (user_mode(regs)) {
+		RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+	} else {
+		/*
+		 * We might have interrupted pretty much anything.  In
+		 * fact, if we're a debug exception, we can even interrupt
+		 * NMI processing. We don't want this code makes in_nmi()
+		 * to return true, but we need to notify RCU.
+		 */
+		rcu_nmi_enter();
+	}
+
+	preempt_disable();
+
+	/* This code is a bit fragile.  Test it. */
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
+}
+NOKPROBE_SYMBOL(debug_exception_enter);
+
+static void debug_exception_exit(struct pt_regs *regs)
+{
+	preempt_enable_no_resched();
+
+	if (!user_mode(regs))
+		rcu_nmi_exit();
+}
+NOKPROBE_SYMBOL(debug_exception_exit);
+
 #ifdef CONFIG_ARM64_ERRATUM_1463225
 DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
 
@@ -824,6 +860,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
 	if (interrupts_enabled(regs))
 		trace_hardirqs_off();
 
+	debug_exception_enter(regs);
+
 	if (user_mode(regs) && !is_ttbr0_addr(pc))
 		arm64_apply_bp_hardening();
 
@@ -832,6 +870,8 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
 				 inf->sig, inf->code, (void __user *)pc, esr);
 	}
 
+	debug_exception_exit(regs);
+
 	if (interrupts_enabled(regs))
 		trace_hardirqs_on();
 }

Re: [PATCH v3 3/4] arm64: Make debug exception handlers visible from RCU

[Will Deacon]

On Thu, Jul 25, 2019 at 05:16:15PM +0900, Masami Hiramatsu wrote:
> Make debug exceptions visible from RCU so that synchronize_rcu()
> correctly track the debug exception handler.
> 
> This also introduces sanity checks for user-mode exceptions as same
> as x86's ist_enter()/ist_exit().
> 
> The debug exception can interrupt in idle task. For example, it warns
> if we put a kprobe on a function called from idle task as below.
> The warning message showed that the rcu_read_lock() caused this
> problem. But actually, this means the RCU is lost the context which
> is already in NMI/IRQ.
> 
>   /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events
>   /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable
>   /sys/kernel/debug/tracing # [  135.122237]
>   [  135.125035] =============================
>   [  135.125310] WARNING: suspicious RCU usage
>   [  135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted
>   [  135.125904] -----------------------------
>   [  135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle!
>   [  135.126839]
>   [  135.126839] other info that might help us debug this:
>   [  135.126839]
>   [  135.127410]
>   [  135.127410] RCU used illegally from idle CPU!
>   [  135.127410] rcu_scheduler_active = 2, debug_locks = 1
>   [  135.128114] RCU used illegally from extended quiescent state!
>   [  135.128555] 1 lock held by swapper/0/0:
>   [  135.128944]  #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178
>   [  135.130499]
>   [  135.130499] stack backtrace:
>   [  135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20
>   [  135.131841] Hardware name: linux,dummy-virt (DT)
>   [  135.132224] Call trace:
>   [  135.132491]  dump_backtrace+0x0/0x140
>   [  135.132806]  show_stack+0x24/0x30
>   [  135.133133]  dump_stack+0xc4/0x10c
>   [  135.133726]  lockdep_rcu_suspicious+0xf8/0x108
>   [  135.134171]  call_break_hook+0x170/0x178
>   [  135.134486]  brk_handler+0x28/0x68
>   [  135.134792]  do_debug_exception+0x90/0x150
>   [  135.135051]  el1_dbg+0x18/0x8c
>   [  135.135260]  default_idle_call+0x0/0x44
>   [  135.135516]  cpu_startup_entry+0x2c/0x30
>   [  135.135815]  rest_init+0x1b0/0x280
>   [  135.136044]  arch_call_rest_init+0x14/0x1c
>   [  135.136305]  start_kernel+0x4d4/0x500
>   [  135.136597]
> 
> So make debug exception visible to RCU can fix this warning.
> 
> Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
> Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> ---
>  Changes in v3:
>   - Make a comment for debug_exception_enter() clearer.
> ---
>  arch/arm64/mm/fault.c |   40 ++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 40 insertions(+)
> 
> diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> index 9568c116ac7f..ed6c55c87fdc 100644
> --- a/arch/arm64/mm/fault.c
> +++ b/arch/arm64/mm/fault.c
> @@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr,
>  	debug_fault_info[nr].name	= name;
>  }
>  
> +/*
> + * In debug exception context, we explicitly disable preemption.

Maybe add "despite having interrupts disabled"?

> + * This serves two purposes: it makes it much less likely that we would
> + * accidentally schedule in exception context and it will force a warning
> + * if we somehow manage to schedule by accident.
> + */
> +static void debug_exception_enter(struct pt_regs *regs)
> +{
> +	if (user_mode(regs)) {
> +		RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> +	} else {
> +		/*
> +		 * We might have interrupted pretty much anything.  In
> +		 * fact, if we're a debug exception, we can even interrupt
> +		 * NMI processing. We don't want this code makes in_nmi()
> +		 * to return true, but we need to notify RCU.
> +		 */
> +		rcu_nmi_enter();
> +	}
> +
> +	preempt_disable();

If you're addingt new functions for entry/exit, maybe move the
trace_hardirqs_{on,off}() calls in here too?

Will

Re: [PATCH v3 3/4] arm64: Make debug exception handlers visible from RCU

[Masami Hiramatsu]

Hi Will,

On Wed, 31 Jul 2019 18:26:03 +0100
Will Deacon <will@kernel.org> wrote:

> On Thu, Jul 25, 2019 at 05:16:15PM +0900, Masami Hiramatsu wrote:
> > Make debug exceptions visible from RCU so that synchronize_rcu()
> > correctly track the debug exception handler.
> > 
> > This also introduces sanity checks for user-mode exceptions as same
> > as x86's ist_enter()/ist_exit().
> > 
> > The debug exception can interrupt in idle task. For example, it warns
> > if we put a kprobe on a function called from idle task as below.
> > The warning message showed that the rcu_read_lock() caused this
> > problem. But actually, this means the RCU is lost the context which
> > is already in NMI/IRQ.
> > 
> >   /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events
> >   /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable
> >   /sys/kernel/debug/tracing # [  135.122237]
> >   [  135.125035] =============================
> >   [  135.125310] WARNING: suspicious RCU usage
> >   [  135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted
> >   [  135.125904] -----------------------------
> >   [  135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle!
> >   [  135.126839]
> >   [  135.126839] other info that might help us debug this:
> >   [  135.126839]
> >   [  135.127410]
> >   [  135.127410] RCU used illegally from idle CPU!
> >   [  135.127410] rcu_scheduler_active = 2, debug_locks = 1
> >   [  135.128114] RCU used illegally from extended quiescent state!
> >   [  135.128555] 1 lock held by swapper/0/0:
> >   [  135.128944]  #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178
> >   [  135.130499]
> >   [  135.130499] stack backtrace:
> >   [  135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20
> >   [  135.131841] Hardware name: linux,dummy-virt (DT)
> >   [  135.132224] Call trace:
> >   [  135.132491]  dump_backtrace+0x0/0x140
> >   [  135.132806]  show_stack+0x24/0x30
> >   [  135.133133]  dump_stack+0xc4/0x10c
> >   [  135.133726]  lockdep_rcu_suspicious+0xf8/0x108
> >   [  135.134171]  call_break_hook+0x170/0x178
> >   [  135.134486]  brk_handler+0x28/0x68
> >   [  135.134792]  do_debug_exception+0x90/0x150
> >   [  135.135051]  el1_dbg+0x18/0x8c
> >   [  135.135260]  default_idle_call+0x0/0x44
> >   [  135.135516]  cpu_startup_entry+0x2c/0x30
> >   [  135.135815]  rest_init+0x1b0/0x280
> >   [  135.136044]  arch_call_rest_init+0x14/0x1c
> >   [  135.136305]  start_kernel+0x4d4/0x500
> >   [  135.136597]
> > 
> > So make debug exception visible to RCU can fix this warning.
> > 
> > Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
> > Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
> > ---
> >  Changes in v3:
> >   - Make a comment for debug_exception_enter() clearer.
> > ---
> >  arch/arm64/mm/fault.c |   40 ++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 40 insertions(+)
> > 
> > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > index 9568c116ac7f..ed6c55c87fdc 100644
> > --- a/arch/arm64/mm/fault.c
> > +++ b/arch/arm64/mm/fault.c
> > @@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr,
> >  	debug_fault_info[nr].name	= name;
> >  }
> >  
> > +/*
> > + * In debug exception context, we explicitly disable preemption.
> 
> Maybe add "despite having interrupts disabled"?

OK, I'll add it.

> > + * This serves two purposes: it makes it much less likely that we would
> > + * accidentally schedule in exception context and it will force a warning
> > + * if we somehow manage to schedule by accident.
> > + */
> > +static void debug_exception_enter(struct pt_regs *regs)
> > +{
> > +	if (user_mode(regs)) {
> > +		RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> > +	} else {
> > +		/*
> > +		 * We might have interrupted pretty much anything.  In
> > +		 * fact, if we're a debug exception, we can even interrupt
> > +		 * NMI processing. We don't want this code makes in_nmi()
> > +		 * to return true, but we need to notify RCU.
> > +		 */
> > +		rcu_nmi_enter();
> > +	}
> > +
> > +	preempt_disable();
> 
> If you're addingt new functions for entry/exit, maybe move the
> trace_hardirqs_{on,off}() calls in here too?

OK, let's move it in these functions.

Thank you!

> 
> Will


-- 
Masami Hiramatsu <mhiramat@kernel.org>

Re: [PATCH v3 3/4] arm64: Make debug exception handlers visible from RCU

[Will Deacon]

Hi Masami,

On Thu, Aug 01, 2019 at 02:32:25PM +0900, Masami Hiramatsu wrote:
> On Wed, 31 Jul 2019 18:26:03 +0100
> Will Deacon <will@kernel.org> wrote:
> > On Thu, Jul 25, 2019 at 05:16:15PM +0900, Masami Hiramatsu wrote:
> > > diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
> > > index 9568c116ac7f..ed6c55c87fdc 100644
> > > --- a/arch/arm64/mm/fault.c
> > > +++ b/arch/arm64/mm/fault.c
> > > @@ -777,6 +777,42 @@ void __init hook_debug_fault_code(int nr,
> > >  	debug_fault_info[nr].name	= name;
> > >  }
> > >  
> > > +/*
> > > + * In debug exception context, we explicitly disable preemption.
> > 
> > Maybe add "despite having interrupts disabled"?
> 
> OK, I'll add it.
> 
> > > + * This serves two purposes: it makes it much less likely that we would
> > > + * accidentally schedule in exception context and it will force a warning
> > > + * if we somehow manage to schedule by accident.
> > > + */
> > > +static void debug_exception_enter(struct pt_regs *regs)
> > > +{
> > > +	if (user_mode(regs)) {
> > > +		RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
> > > +	} else {
> > > +		/*
> > > +		 * We might have interrupted pretty much anything.  In
> > > +		 * fact, if we're a debug exception, we can even interrupt
> > > +		 * NMI processing. We don't want this code makes in_nmi()
> > > +		 * to return true, but we need to notify RCU.
> > > +		 */
> > > +		rcu_nmi_enter();
> > > +	}
> > > +
> > > +	preempt_disable();
> > 
> > If you're addingt new functions for entry/exit, maybe move the
> > trace_hardirqs_{on,off}() calls in here too?
> 
> OK, let's move it in these functions.

Brill, thanks. Please just resend this patch, as I can pick the other three
up as they are.

Will

[PATCH v4] arm64: Make debug exception handlers visible from RCU

[Masami Hiramatsu]

Make debug exceptions visible from RCU so that synchronize_rcu()
correctly track the debug exception handler.

This also introduces sanity checks for user-mode exceptions as same
as x86's ist_enter()/ist_exit().

The debug exception can interrupt in idle task. For example, it warns
if we put a kprobe on a function called from idle task as below.
The warning message showed that the rcu_read_lock() caused this
problem. But actually, this means the RCU is lost the context which
is already in NMI/IRQ.

  /sys/kernel/debug/tracing # echo p default_idle_call >> kprobe_events
  /sys/kernel/debug/tracing # echo 1 > events/kprobes/enable
  /sys/kernel/debug/tracing # [  135.122237]
  [  135.125035] =============================
  [  135.125310] WARNING: suspicious RCU usage
  [  135.125581] 5.2.0-08445-g9187c508bdc7 #20 Not tainted
  [  135.125904] -----------------------------
  [  135.126205] include/linux/rcupdate.h:594 rcu_read_lock() used illegally while idle!
  [  135.126839]
  [  135.126839] other info that might help us debug this:
  [  135.126839]
  [  135.127410]
  [  135.127410] RCU used illegally from idle CPU!
  [  135.127410] rcu_scheduler_active = 2, debug_locks = 1
  [  135.128114] RCU used illegally from extended quiescent state!
  [  135.128555] 1 lock held by swapper/0/0:
  [  135.128944]  #0: (____ptrval____) (rcu_read_lock){....}, at: call_break_hook+0x0/0x178
  [  135.130499]
  [  135.130499] stack backtrace:
  [  135.131192] CPU: 0 PID: 0 Comm: swapper/0 Not tainted 5.2.0-08445-g9187c508bdc7 #20
  [  135.131841] Hardware name: linux,dummy-virt (DT)
  [  135.132224] Call trace:
  [  135.132491]  dump_backtrace+0x0/0x140
  [  135.132806]  show_stack+0x24/0x30
  [  135.133133]  dump_stack+0xc4/0x10c
  [  135.133726]  lockdep_rcu_suspicious+0xf8/0x108
  [  135.134171]  call_break_hook+0x170/0x178
  [  135.134486]  brk_handler+0x28/0x68
  [  135.134792]  do_debug_exception+0x90/0x150
  [  135.135051]  el1_dbg+0x18/0x8c
  [  135.135260]  default_idle_call+0x0/0x44
  [  135.135516]  cpu_startup_entry+0x2c/0x30
  [  135.135815]  rest_init+0x1b0/0x280
  [  135.136044]  arch_call_rest_init+0x14/0x1c
  [  135.136305]  start_kernel+0x4d4/0x500
  [  135.136597]

So make debug exception visible to RCU can fix this warning.

Reported-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 Changes in v4:
  - Update comment of debug_exception_enter().
  - Move trace_hardirqs_off/on() into debug_exception_enter/exit().
 Changes in v3:
  - Make a comment for debug_exception_enter() clearer.
---
 arch/arm64/mm/fault.c |   57 ++++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 49 insertions(+), 8 deletions(-)

diff --git a/arch/arm64/mm/fault.c b/arch/arm64/mm/fault.c
index 9568c116ac7f..cfd65b63f36f 100644
--- a/arch/arm64/mm/fault.c
+++ b/arch/arm64/mm/fault.c
@@ -777,6 +777,53 @@ void __init hook_debug_fault_code(int nr,
 	debug_fault_info[nr].name	= name;
 }
 
+/*
+ * In debug exception context, we explicitly disable preemption despite
+ * having interrupts disabled.
+ * This serves two purposes: it makes it much less likely that we would
+ * accidentally schedule in exception context and it will force a warning
+ * if we somehow manage to schedule by accident.
+ */
+static void debug_exception_enter(struct pt_regs *regs)
+{
+	/*
+	 * Tell lockdep we disabled irqs in entry.S. Do nothing if they were
+	 * already disabled to preserve the last enabled/disabled addresses.
+	 */
+	if (interrupts_enabled(regs))
+		trace_hardirqs_off();
+
+	if (user_mode(regs)) {
+		RCU_LOCKDEP_WARN(!rcu_is_watching(), "entry code didn't wake RCU");
+	} else {
+		/*
+		 * We might have interrupted pretty much anything.  In
+		 * fact, if we're a debug exception, we can even interrupt
+		 * NMI processing. We don't want this code makes in_nmi()
+		 * to return true, but we need to notify RCU.
+		 */
+		rcu_nmi_enter();
+	}
+
+	preempt_disable();
+
+	/* This code is a bit fragile.  Test it. */
+	RCU_LOCKDEP_WARN(!rcu_is_watching(), "exception_enter didn't work");
+}
+NOKPROBE_SYMBOL(debug_exception_enter);
+
+static void debug_exception_exit(struct pt_regs *regs)
+{
+	preempt_enable_no_resched();
+
+	if (!user_mode(regs))
+		rcu_nmi_exit();
+
+	if (interrupts_enabled(regs))
+		trace_hardirqs_on();
+}
+NOKPROBE_SYMBOL(debug_exception_exit);
+
 #ifdef CONFIG_ARM64_ERRATUM_1463225
 DECLARE_PER_CPU(int, __in_cortex_a76_erratum_1463225_wa);
 
@@ -817,12 +864,7 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
 	if (cortex_a76_erratum_1463225_debug_handler(regs))
 		return;
 
-	/*
-	 * Tell lockdep we disabled irqs in entry.S. Do nothing if they were
-	 * already disabled to preserve the last enabled/disabled addresses.
-	 */
-	if (interrupts_enabled(regs))
-		trace_hardirqs_off();
+	debug_exception_enter(regs);
 
 	if (user_mode(regs) && !is_ttbr0_addr(pc))
 		arm64_apply_bp_hardening();
@@ -832,7 +874,6 @@ asmlinkage void __exception do_debug_exception(unsigned long addr_if_watchpoint,
 				 inf->sig, inf->code, (void __user *)pc, esr);
 	}
 
-	if (interrupts_enabled(regs))
-		trace_hardirqs_on();
+	debug_exception_exit(regs);
 }
 NOKPROBE_SYMBOL(do_debug_exception);

[PATCH v3 4/4] arm64: Remove unneeded rcu_read_lock from debug handlers

[Masami Hiramatsu]

Remove rcu_read_lock()/rcu_read_unlock() from debug exception
handlers since we are sure those are not preemptible and
interrupts are off.

Acked-by: Paul E. McKenney <paulmck@linux.ibm.com>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
---
 arch/arm64/kernel/debug-monitors.c |   14 ++++++++------
 1 file changed, 8 insertions(+), 6 deletions(-)

diff --git a/arch/arm64/kernel/debug-monitors.c b/arch/arm64/kernel/debug-monitors.c
index f8719bd30850..48222a4760c2 100644
--- a/arch/arm64/kernel/debug-monitors.c
+++ b/arch/arm64/kernel/debug-monitors.c
@@ -207,16 +207,16 @@ static int call_step_hook(struct pt_regs *regs, unsigned int esr)
 
 	list = user_mode(regs) ? &user_step_hook : &kernel_step_hook;
 
-	rcu_read_lock();
-
+	/*
+	 * Since single-step exception disables interrupt, this function is
+	 * entirely not preemptible, and we can use rcu list safely here.
+	 */
 	list_for_each_entry_rcu(hook, list, node)	{
 		retval = hook->fn(regs, esr);
 		if (retval == DBG_HOOK_HANDLED)
 			break;
 	}
 
-	rcu_read_unlock();
-
 	return retval;
 }
 NOKPROBE_SYMBOL(call_step_hook);
@@ -305,14 +305,16 @@ static int call_break_hook(struct pt_regs *regs, unsigned int esr)
 
 	list = user_mode(regs) ? &user_break_hook : &kernel_break_hook;
 
-	rcu_read_lock();
+	/*
+	 * Since brk exception disables interrupt, this function is
+	 * entirely not preemptible, and we can use rcu list safely here.
+	 */
 	list_for_each_entry_rcu(hook, list, node) {
 		unsigned int comment = esr & ESR_ELx_BRK64_ISS_COMMENT_MASK;
 
 		if ((comment & ~hook->mask) == hook->imm)
 			fn = hook->fn;
 	}
-	rcu_read_unlock();
 
 	return fn ? fn(regs, esr) : DBG_HOOK_ERROR;
 }