Waiman, Luis, Ada reported that HW breakpoints on ARM64 trigger
"sleeping while atomic" warnings on PREEMPT_RT. The hardware event is
delivered with disabled interrupts and perf intrastrucure expects
disabled interrupts while the overflow callback is invoked.

The callback then sends a SIGTRAP signal for which it acquires
sighand_struct::siglock, a spinlock_t which becomes a sleeping lock and
must not be acquired in atomic context.

Delay the event callback until the return to userland.
Add perf_arch_hwbp_notify(), a generic perf callback which delayes the
actual callback invocation to task_work_add() callback. This callback
invokes the architecture defines callback arch_hwbp_send_sig().
This requires struct callback_head and the functions require
ARCH_NEED_PERF_HW_NOTIF to be defined.

This was reported against ARM64. ARM and LongARCH follow the same
pattern are also converted.

Reported-by: Luis Claudio R. Goncalves <lgoncalv@redhat.com>
Reported-by: Waiman Long <longman@redhat.com>
Closes: https://lore.kernel.org/all/aho0eqjMESuHxECr@redhat.com/
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
---

This is approximately the fifth iteration at which point I think I can
share what I have.

Could this be tested, please? I tested it on arm64 for arm64 based on
Luis' test. ARM compiles my HW lacks support for this I think (gdb
aborts early). I hope the best for LoongARCH.

Are the perf bits okay or is it too gross?

 arch/arm/include/asm/hw_breakpoint.h       |  1 +
 arch/arm/kernel/ptrace.c                   |  6 ++----
 arch/arm64/include/asm/hw_breakpoint.h     |  1 +
 arch/arm64/kernel/ptrace.c                 |  6 ++----
 arch/loongarch/include/asm/hw_breakpoint.h |  1 +
 arch/loongarch/kernel/ptrace.c             |  6 ++----
 include/linux/hw_breakpoint.h              |  3 +++
 include/linux/perf_event.h                 |  3 +++
 kernel/events/core.c                       | 24 ++++++++++++++++++++++
 9 files changed, 39 insertions(+), 12 deletions(-)

diff --git a/arch/arm/include/asm/hw_breakpoint.h b/arch/arm/include/asm/hw_breakpoint.h
index e7f9961c53b2d..5b3a373348ff5 100644
--- a/arch/arm/include/asm/hw_breakpoint.h
+++ b/arch/arm/include/asm/hw_breakpoint.h
@@ -7,6 +7,7 @@
 struct task_struct;
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
+#define ARCH_NEED_PERF_HW_NOTIF
 
 struct arch_hw_breakpoint_ctrl {
 		u32 __reserved	: 9,
diff --git a/arch/arm/kernel/ptrace.c b/arch/arm/kernel/ptrace.c
index 7951b2c06fec6..1650f12a04702 100644
--- a/arch/arm/kernel/ptrace.c
+++ b/arch/arm/kernel/ptrace.c
@@ -347,9 +347,7 @@ static long ptrace_hbp_idx_to_num(int idx)
 /*
  * Handle hitting a HW-breakpoint.
  */
-static void ptrace_hbptriggered(struct perf_event *bp,
-				     struct perf_sample_data *data,
-				     struct pt_regs *regs)
+void arch_hwbp_send_sig(struct perf_event *bp)
 {
 	struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
 	long num;
@@ -424,7 +422,7 @@ static struct perf_event *ptrace_hbp_create(struct task_struct *tsk, int type)
 	attr.bp_type	= type;
 	attr.disabled	= 1;
 
-	return register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL,
+	return register_user_hw_breakpoint(&attr, perf_arch_hwbp_notify, NULL,
 					   tsk);
 }
 
diff --git a/arch/arm64/include/asm/hw_breakpoint.h b/arch/arm64/include/asm/hw_breakpoint.h
index bd81cf17744af..58befb79c885c 100644
--- a/arch/arm64/include/asm/hw_breakpoint.h
+++ b/arch/arm64/include/asm/hw_breakpoint.h
@@ -124,6 +124,7 @@ extern void hw_breakpoint_pmu_read(struct perf_event *bp);
 extern int hw_breakpoint_slots(int type);
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
+#define ARCH_NEED_PERF_HW_NOTIF
 extern void hw_breakpoint_thread_switch(struct task_struct *next);
 extern void ptrace_hw_copy_thread(struct task_struct *task);
 #else
diff --git a/arch/arm64/kernel/ptrace.c b/arch/arm64/kernel/ptrace.c
index 4d08598e2891d..df6122779ada6 100644
--- a/arch/arm64/kernel/ptrace.c
+++ b/arch/arm64/kernel/ptrace.c
@@ -168,9 +168,7 @@ void ptrace_disable(struct task_struct *child)
 /*
  * Handle hitting a HW-breakpoint.
  */
-static void ptrace_hbptriggered(struct perf_event *bp,
-				struct perf_sample_data *data,
-				struct pt_regs *regs)
+void arch_hwbp_send_sig(struct perf_event *bp)
 {
 	struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
 	const char *desc = "Hardware breakpoint trap (ptrace)";
@@ -312,7 +310,7 @@ static struct perf_event *ptrace_hbp_create(unsigned int note_type,
 	attr.bp_type	= type;
 	attr.disabled	= 1;
 
-	bp = register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL, tsk);
+	bp = register_user_hw_breakpoint(&attr, perf_arch_hwbp_notify, NULL, tsk);
 	if (IS_ERR(bp))
 		return bp;
 
diff --git a/arch/loongarch/include/asm/hw_breakpoint.h b/arch/loongarch/include/asm/hw_breakpoint.h
index 5faa97a87a9e2..2b51922eb5492 100644
--- a/arch/loongarch/include/asm/hw_breakpoint.h
+++ b/arch/loongarch/include/asm/hw_breakpoint.h
@@ -120,6 +120,7 @@ void breakpoint_handler(struct pt_regs *regs);
 void watchpoint_handler(struct pt_regs *regs);
 
 #ifdef CONFIG_HAVE_HW_BREAKPOINT
+#define ARCH_NEED_PERF_HW_NOTIF
 extern void ptrace_hw_copy_thread(struct task_struct *task);
 extern void hw_breakpoint_thread_switch(struct task_struct *next);
 #else
diff --git a/arch/loongarch/kernel/ptrace.c b/arch/loongarch/kernel/ptrace.c
index be38430f7e280..1d5e05a70b8d3 100644
--- a/arch/loongarch/kernel/ptrace.c
+++ b/arch/loongarch/kernel/ptrace.c
@@ -384,9 +384,7 @@ static int lbt_set(struct task_struct *target,
 /*
  * Handle hitting a HW-breakpoint.
  */
-static void ptrace_hbptriggered(struct perf_event *bp,
-				struct perf_sample_data *data,
-				struct pt_regs *regs)
+void arch_hwbp_send_sig(struct perf_event *bp)
 {
 	int i;
 	struct arch_hw_breakpoint *bkpt = counter_arch_bp(bp);
@@ -479,7 +477,7 @@ static struct perf_event *ptrace_hbp_create(unsigned int note_type,
 	attr.bp_type	= type;
 	attr.disabled	= 1;
 
-	bp = register_user_hw_breakpoint(&attr, ptrace_hbptriggered, NULL, tsk);
+	bp = register_user_hw_breakpoint(&attr, perf_arch_hwbp_notify, NULL, tsk);
 	if (IS_ERR(bp))
 		return bp;
 
diff --git a/include/linux/hw_breakpoint.h b/include/linux/hw_breakpoint.h
index db199d653dd1a..bb4c1064a103d 100644
--- a/include/linux/hw_breakpoint.h
+++ b/include/linux/hw_breakpoint.h
@@ -85,6 +85,9 @@ extern int register_perf_hw_breakpoint(struct perf_event *bp);
 extern void unregister_hw_breakpoint(struct perf_event *bp);
 extern void unregister_wide_hw_breakpoint(struct perf_event * __percpu *cpu_events);
 extern bool hw_breakpoint_is_used(void);
+extern void arch_hwbp_send_sig(struct perf_event *bp);
+extern void perf_arch_hwbp_notify(struct perf_event *bp, struct perf_sample_data *data,
+				  struct pt_regs *regs);
 
 extern int dbg_reserve_bp_slot(struct perf_event *bp);
 extern int dbg_release_bp_slot(struct perf_event *bp);
diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 48d851fbd8ea5..bc7fae88a2742 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h
@@ -215,6 +215,9 @@ struct hw_perf_event {
 
 	/* Last sync'ed generation of filters */
 	unsigned long			addr_filters_gen;
+#ifdef ARCH_NEED_PERF_HW_NOTIF
+	struct callback_head		arch_hw_notif;
+#endif
 
 /*
  * hw_perf_event::state flags; used to track the PERF_EF_* state.
diff --git a/kernel/events/core.c b/kernel/events/core.c
index d7f3e2c2ecb1e..1f70cdb52f068 100644
--- a/kernel/events/core.c
+++ b/kernel/events/core.c
@@ -13336,6 +13336,26 @@ static void account_event(struct perf_event *event)
 	account_pmu_sb_event(event);
 }
 
+#ifdef ARCH_NEED_PERF_HW_NOTIF
+static void perf_arch_hwbp_send_sig(struct callback_head *head)
+{
+	struct perf_event *bp;
+
+	bp = container_of(head, struct perf_event, hw.arch_hw_notif);
+	arch_hwbp_send_sig(bp);
+	put_event(bp);
+}
+
+void perf_arch_hwbp_notify(struct perf_event *bp, struct perf_sample_data *data,
+			   struct pt_regs *regs)
+{
+	if (WARN_ON_ONCE(!atomic_long_inc_not_zero(&bp->refcount)))
+		return;
+	if (WARN_ON_ONCE(task_work_add(current, &bp->hw.arch_hw_notif, TWA_RESUME)))
+		put_event(bp);
+}
+#endif
+
 /*
  * Allocate and initialize an event structure
  */
@@ -13441,6 +13461,10 @@ perf_event_alloc(struct perf_event_attr *attr, int cpu,
 	}
 
 	if (overflow_handler) {
+#ifdef ARCH_NEED_PERF_HW_NOTIF
+		if (overflow_handler == perf_arch_hwbp_notify)
+			init_task_work(&event->hw.arch_hw_notif, perf_arch_hwbp_send_sig);
+#endif
 		event->overflow_handler	= overflow_handler;
 		event->overflow_handler_context = context;
 	} else if (is_write_backward(event)){
-- 
2.53.0

Thank you for your contribution! Sashiko AI review found 2 potential issue(s) to consider:
- [Critical] Queueing the same hardware breakpoint callback_head multiple times corrupts the task_works list, leading to an infinite loop and Use-After-Free.
- [High] The use of TWA_RESUME fails to interrupt blocking system calls when a watchpoint triggers, potentially hanging the tracee.
--

--- Patch [1]: [RFC PATCH] ARM, ARM64, LOONGARCH: Delay HW BP notification to task_work() ---
commit edeb5ce1fcde3854d06fcd1845bb9a1a8e7ef02e
Author: Sebastian Andrzej Siewior <bigeasy@linutronix.de>

ARM, ARM64, LOONGARCH: Delay HW BP notification to task_work()

This patch delays the delivery of hardware breakpoint events to task_work()
to avoid sleeping while atomic warnings on PREEMPT_RT. It introduces a
generic callback perf_arch_hwbp_notify() that defers the architecture
specific signal generation to a return to user space.

Link: https://lore.kernel.org/all/aho0eqjMESuHxECr@redhat.com/

> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 6d1f8bad7e1c5..9674cd59703fb 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c

[ ... ]

> @@ -13259,6 +13259,26 @@ static void account_event(struct perf_event *event)
>  	account_pmu_sb_event(event);
>  }
>  
> +#ifdef ARCH_NEED_PERF_HW_NOTIF
> +static void perf_arch_hwbp_send_sig(struct callback_head *head)
> +{
> +	struct perf_event *bp;
> +
> +	bp = container_of(head, struct perf_event, hw.arch_hw_notif);
> +	arch_hwbp_send_sig(bp);
> +	put_event(bp);
> +}
> +
> +void perf_arch_hwbp_notify(struct perf_event *bp, struct perf_sample_data *data,
> +			   struct pt_regs *regs)
> +{
> +	if (WARN_ON_ONCE(!atomic_long_inc_not_zero(&bp->refcount)))
> +		return;
> +	if (WARN_ON_ONCE(task_work_add(current, &bp->hw.arch_hw_notif, TWA_RESUME)))

[Severity: Critical]
If a user-space process sets a hardware watchpoint on a buffer and passes it
to a syscall that accesses the buffer multiple times (like copy_to_user),
can this queue the same callback_head multiple times before returning to
user space?

Because arch_hw_notif is a statically embedded callback_head, calling
task_work_add() a second time for the same event without checking if it is
already pending will overwrite its next pointer to point to itself. When
task_work_run() executes, the subsequent execution loop will loop infinitely
because next and work remain the same. 

The callback perf_arch_hwbp_send_sig() also calls put_event(bp), dropping
the refcount to zero and freeing the event. The next iteration reads
work->next and calls work->func(work) from freed memory, resulting in a
use-after-free and CPU lockup.

[Severity: High]
Is TWA_RESUME the correct mode here, or should it use TWA_SIGNAL?

Prior to this patch, hardware breakpoint handlers directly called
force_sig_fault(), which set TIF_SIGPENDING and properly interrupted
blocking system calls (like select or futex) to deliver the SIGTRAP.

TWA_RESUME only sets TIF_NOTIFY_RESUME and leaves signal_pending() false,
meaning the task could potentially block indefinitely without returning to
user space to process the task work, potentially deadlocking the tracee.

> +		put_event(bp);
> +}
> +#endif

[ ... ]

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260713144939.FuCj9yvZ@linutronix.de?part=1