[PATCH] block: annotate lockless task ioprio accesses

[Nguyen Ngoc Thang]

__get_task_ioprio() reads task->io_context and ioc->ioprio without
task_lock() when called on current, e.g. from init_sync_kiocb() on every
write.  set_task_ioprio() can update both for another task at any time,
including the first publication of a new io_context.  KCSAN reports it:

  BUG: KCSAN: data-race in __kernel_write_iter / set_task_ioprio
  value changed: 0x0000000000000000 -> 0xffff888109813730

A stale or torn value is harmless, a task simply gets the old or new
priority, but the compiler may tear or refetch the accesses and the
io_context fields must be visible before the pointer is.

Publish the pointer with smp_store_release(), read it with READ_ONCE()
(address dependency orders the ioprio load), and mark the ioprio
accesses.

Reported-by: syzbot+feeb9b5352b9ff89618a@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://syzkaller.appspot.com/bug?extid=feeb9b5352b9ff89618a
Signed-off-by: Nguyen Ngoc Thang <ngocthang2710.1999@xxxxxxxxx>
---
Tested with a KCSAN kernel in QEMU. syzbot's coredump repro is too slow to hit
the window, so a thread loops pwrite() on a tmpfs file while the main thread
does the first ioprio_set() on its tid (~35k iterations per run). Same pair,
same "0 -> pointer" value change: 26 reports per run before, 0 in two runs
after. No Fixes: tag, the history I have is grafted.
 block/blk-ioc.c        | 5 +++--
 include/linux/ioprio.h | 5 +++--
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/block/blk-ioc.c b/block/blk-ioc.c
index d15918d7fabb..12ddc672f41d 100644
--- a/block/blk-ioc.c
+++ b/block/blk-ioc.c
@@ -277,9 +277,10 @@ int set_task_ioprio(struct task_struct *task, int ioprio)
 		if (task->io_context)
 			kmem_cache_free(iocontext_cachep, ioc);
 		else
-			task->io_context = ioc;
+			/* Pairs with READ_ONCE() in __get_task_ioprio() */
+			smp_store_release(&task->io_context, ioc);
 	}
-	task->io_context->ioprio = ioprio;
+	WRITE_ONCE(task->io_context->ioprio, ioprio);
 out:
 	task_unlock(task);
 	return 0;
diff --git a/include/linux/ioprio.h b/include/linux/ioprio.h
index 5210e8371238..56371e9ea306 100644
--- a/include/linux/ioprio.h
+++ b/include/linux/ioprio.h
@@ -56,7 +56,8 @@ static inline int task_nice_ioclass(struct task_struct *task)
  */
 static inline int __get_task_ioprio(struct task_struct *p)
 {
-	struct io_context *ioc = p->io_context;
+	/* Paired with smp_store_release() in set_task_ioprio() */
+	struct io_context *ioc = READ_ONCE(p->io_context);
 	int prio;
 
 	if (!ioc)
@@ -66,7 +67,7 @@ static inline int __get_task_ioprio(struct task_struct *p)
 	if (p != current)
 		lockdep_assert_held(&p->alloc_lock);
 
-	prio = ioc->ioprio;
+	prio = READ_ONCE(ioc->ioprio);
 	if (IOPRIO_PRIO_CLASS(prio) == IOPRIO_CLASS_NONE)
 		prio = IOPRIO_PRIO_VALUE(task_nice_ioclass(p),
 					 task_nice_ioprio(p));
-- 
2.43.0