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Subject[PATCH -rcu] kcsan: Make KCSAN compatible with lockdep
From
To avoid unexpected reentry into lockdep's IRQ tracing code via KCSAN
(if TRACE_IRQFLAGS is enabled), (1) use raw versions of
local_irq_{save,restore} in kcsan_setup_watchpoint(), and (2) disable
lockdep in kcsan_report() to avoid IRQ flags tracing upon generating the
report.

Tested with:

CONFIG_LOCKDEP=y
CONFIG_DEBUG_LOCKDEP=y

Where previously, the following warning (and variants with different
stack traces) was consistently generated, with the fix introduced in
this patch, the warning cannot be reproduced.

WARNING: CPU: 0 PID: 2 at kernel/locking/lockdep.c:4406 check_flags.part.0+0x101/0x220
Modules linked in:
CPU: 0 PID: 2 Comm: kthreadd Not tainted 5.5.0-rc1+ #11
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.12.0-1 04/01/2014
RIP: 0010:check_flags.part.0+0x101/0x220
<snip>
Call Trace:
lock_is_held_type+0x69/0x150
freezer_fork+0x20b/0x370
cgroup_post_fork+0x2c9/0x5c0
copy_process+0x2675/0x3b40
_do_fork+0xbe/0xa30
? _raw_spin_unlock_irqrestore+0x40/0x50
? match_held_lock+0x56/0x250
? kthread_park+0xf0/0xf0
kernel_thread+0xa6/0xd0
? kthread_park+0xf0/0xf0
kthreadd+0x321/0x3d0
? kthread_create_on_cpu+0x130/0x130
ret_from_fork+0x3a/0x50
irq event stamp: 64
hardirqs last enabled at (63): [<ffffffff9a7995d0>] _raw_spin_unlock_irqrestore+0x40/0x50
hardirqs last disabled at (64): [<ffffffff992a96d2>] kcsan_setup_watchpoint+0x92/0x460
softirqs last enabled at (32): [<ffffffff990489b8>] fpu__copy+0xe8/0x470
softirqs last disabled at (30): [<ffffffff99048939>] fpu__copy+0x69/0x470

Reported-by: Qian Cai <cai@lca.pw>
Signed-off-by: Marco Elver <elver@google.com>
---
kernel/kcsan/core.c | 4 ++--
kernel/kcsan/report.c | 11 +++++++++++
kernel/locking/Makefile | 3 +++
3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 87bf857c8893..e75f3dbf627e 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -337,7 +337,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
* detection point of view) to simply disable preemptions to ensure
* as many tasks as possible run on other CPUs.
*/
- local_irq_save(irq_flags);
+ raw_local_irq_save(irq_flags);

watchpoint = insert_watchpoint((unsigned long)ptr, size, is_write);
if (watchpoint == NULL) {
@@ -429,7 +429,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)

kcsan_counter_dec(KCSAN_COUNTER_USED_WATCHPOINTS);
out_unlock:
- local_irq_restore(irq_flags);
+ raw_local_irq_restore(irq_flags);
out:
user_access_restore(ua_flags);
}
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index b5b4feea49de..57ab7ef9786c 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -2,6 +2,7 @@

#include <linux/jiffies.h>
#include <linux/kernel.h>
+#include <linux/lockdep.h>
#include <linux/preempt.h>
#include <linux/printk.h>
#include <linux/sched.h>
@@ -410,6 +411,14 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
{
unsigned long flags = 0;

+ /*
+ * With TRACE_IRQFLAGS, lockdep's internal IRQ flags state becomes
+ * corrupted if we do not turn off lockdep. The likely cause is
+ * unexpected reentry into IRQ tracing code via KCSAN if we detect a
+ * data race.
+ */
+ lockdep_off();
+
kcsan_disable_current();
if (prepare_report(&flags, ptr, size, access_type, cpu_id, type)) {
if (print_report(ptr, size, access_type, value_change, cpu_id, type) && panic_on_warn)
@@ -418,4 +427,6 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,
release_report(&flags, type);
}
kcsan_enable_current();
+
+ lockdep_on();
}
diff --git a/kernel/locking/Makefile b/kernel/locking/Makefile
index 45452facff3b..6d11cfb9b41f 100644
--- a/kernel/locking/Makefile
+++ b/kernel/locking/Makefile
@@ -5,6 +5,9 @@ KCOV_INSTRUMENT := n

obj-y += mutex.o semaphore.o rwsem.o percpu-rwsem.o

+# Avoid recursion lockdep -> KCSAN -> ... -> lockdep.
+KCSAN_SANITIZE_lockdep.o := n
+
ifdef CONFIG_FUNCTION_TRACER
CFLAGS_REMOVE_lockdep.o = $(CC_FLAGS_FTRACE)
CFLAGS_REMOVE_lockdep_proc.o = $(CC_FLAGS_FTRACE)
--
2.25.0.rc1.283.g88dfdc4193-goog
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