From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: stable@vger.kernel.org
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	patches@lists.linux.dev,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Marco Elver <elver@google.com>,
	Will Deacon <will@kernel.org>,
	Sasha Levin <sashal@kernel.org>
Subject: [PATCH 6.12 1033/1376] arm64: Disable KCSAN instrumentation in delay.o
Date: Sat, 12 Sep 2026 08:57:39 +0200
Message-ID: <20260912065630.583054196@linuxfoundation.org>
In-Reply-To: <20260912065607.535295758@linuxfoundation.org>
References: <20260912065607.535295758@linuxfoundation.org>
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6.12-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Marco Elver <elver@google.com>

[ Upstream commit 5eaec4cf41a8f5ac1a0c69a607cdc5035a797f73 ]

KCSAN relies on udelay() for injecting delays. To avoid recursively
triggering a watchpoint, where KCSAN sets up watchpoint on an address
that is accessed by udelay() in the same thread, disable instrumentation
in arm64's delay implementation.

Paul found a manifestation of this as follows:

| BUG: KCSAN: data-race in __delay / set_need_resched_current
|
| read (marked) to 0xffff000005899b48 of 8 bytes by interrupt on cpu 8:
|  __delay+0xb0/0x378
|  __udelay+0x4c/0x60
|  kcsan_setup_watchpoint+0x3b4/0x820
|  __tsan_unaligned_write4+0x228/0x26c
|  set_need_resched_current+0x138/0x1a8
|  rcu_exp_handler+0x418/0x4a0
|  __flush_smp_call_function_queue+0x36c/0x4a0
|  generic_smp_call_function_single_interrupt+0x20/0x30
|  ipi_handler+0xec/0x558
|  handle_percpu_devid_irq+0x220/0x2a0
|  generic_handle_domain_irq+0x84/0xb4
|  gic_handle_irq+0x64/0x144
|  call_on_irq_stack+0x30/0x48
|  do_interrupt_handler+0x80/0xb8
|  el1_interrupt+0x3c/0x60
|  el1h_64_irq_handler+0x18/0x24
|  el1h_64_irq+0x6c/0x70
|  smp_call_function_single+0x18c/0x25c
|  sync_rcu_exp_select_node_cpus+0x534/0x8bc
|  rcu_exp_sel_wait_wake+0x358/0xef4
|  wait_rcu_exp_gp+0x30/0x44
|  kthread_worker_fn+0x1b4/0x5dc
|  kthread+0x1d8/0x204
|  ret_from_fork+0x10/0x20
|
| write to 0xffff000005899b4c of 4 bytes by interrupt on cpu 8:
|  set_need_resched_current+0x138/0x1a8
|  [...]

This matches what is already done in arch/x86/lib/Makefile.

Reported-by: "Paul E. McKenney" <paulmck@kernel.org>
Fixes: dd03762ab608 ("arm64: Enable KCSAN")
Signed-off-by: Marco Elver <elver@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 arch/arm64/lib/Makefile | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/arm64/lib/Makefile b/arch/arm64/lib/Makefile
index 13e6a2829116c..68d91c2021f19 100644
--- a/arch/arm64/lib/Makefile
+++ b/arch/arm64/lib/Makefile
@@ -1,4 +1,8 @@
 # SPDX-License-Identifier: GPL-2.0
+
+# KCSAN uses udelay for introducing watchpoint delay; avoid recursion.
+KCSAN_SANITIZE_delay.o := n
+
 lib-y		:= clear_user.o delay.o copy_from_user.o		\
 		   copy_to_user.o copy_page.o				\
 		   clear_page.o csum.o insn.o memchr.o memcpy.o		\
-- 
2.53.0