block: genhd: don't call probe function with major_names_lock held

syzbot is reporting circular locking problem at blk_request_module() [1],
for blk_request_module() is calling probe function with major_names_lock
held while major_names_lock is held during module's __init and __exit
functions.

                                         loop_exit() {
                                           mutex_lock(&loop_ctl_mutex);
  blk_request_module() {
    mutex_lock(&major_names_lock);
    loop_probe() {
      mutex_lock(&loop_ctl_mutex); // Blocked by loop_exit()
      mutex_unlock(&loop_ctl_mutex);
    }
    mutex_unlock(&major_names_lock);
                                           unregister_blkdev() {
                                             mutex_lock(&major_names_lock); // Blocked by blk_request_module()
                                             mutex_unlock(&major_names_lock);
                                           }
                                           mutex_unlock(&loop_ctl_mutex);
  }
                                         }

Based on an assumption that a probe callback passed to __register_blkdev()
belongs to a module which calls __register_blkdev(), drop major_names_lock
before calling probe function by holding a reference to that module which
contains that probe function. If there is a module where this assumption
does not hold, such module can call ____register_blkdev() directly.

  blk_request_module() {
    mutex_lock(&major_names_lock);
    // Block loop_exit()
    mutex_unlock(&major_names_lock);
    loop_probe() {
      mutex_lock(&loop_ctl_mutex);
      mutex_unlock(&loop_ctl_mutex);
    }
    // Unblock loop_exit()
  }
                                         loop_exit() {
                                           mutex_lock(&loop_ctl_mutex);
                                           unregister_blkdev() {
                                             mutex_lock(&major_names_lock);
                                             mutex_unlock(&major_names_lock);
                                           }
                                           mutex_unlock(&loop_ctl_mutex);
                                         }

Note that regardless of this patch, it is up to probe function to
serialize module's __init function and probe function in that module
by using e.g. a mutex. This patch simply makes sure that module's __exit
function won't be called when probe function is about to be called.

While Desmond Cheong Zhi Xi already proposed a patch for breaking ABCD
circular dependency [2], I consider that this patch is still needed for
safely breaking AB-BA dependency upon module unloading. (Note that syzbot
does not test module unloading code because the syzbot kernels are built
with almost all modules built-in. We need manual inspection.)

By doing kmalloc(GFP_KERNEL) in ____register_blkdev() before holding
major_names_lock, we could convert major_names_lock from a mutex to
a spinlock. But that is beyond the scope of this patch.

Link: https://syzkaller.appspot.com/bug?id=7bd106c28e846d1023d4ca915718b1a0905444cb [1]
Link: https://lkml.kernel.org/r/20210617160904.570111-1-desmondcheongzx@gmail.com [2]
Reported-by: syzbot <syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Tested-by: syzbot <syzbot+6a8a0d93c91e8fbf2e80@syzkaller.appspotmail.com>

block/genhd.c

@@ -171,6 +171,7 @@ static struct blk_major_name {
 	int major;
 	char name[16];
 	void (*probe)(dev_t devt);
+	struct module *owner;
 } *major_names[BLKDEV_MAJOR_HASH_SIZE];
 static DEFINE_MUTEX(major_names_lock);
 
@@ -199,7 +200,8 @@ void blkdev_show(struct seq_file *seqf, off_t offset)
  * @major: the requested major device number [1..BLKDEV_MAJOR_MAX-1]. If
  *         @major = 0, try to allocate any unused major number.
  * @name: the name of the new block device as a zero terminated string
- * @probe: allback that is called on access to any minor number of @major
+ * @probe: callback that is called on access to any minor number of @major
+ * @owner: the owner of @probe function (i.e. THIS_MODULE or NULL).
  *
  * The @name must be unique within the system.
  *
@@ -216,8 +218,8 @@ void blkdev_show(struct seq_file *seqf, off_t offset)
  *
  * Use register_blkdev instead for any new code.
  */
-int __register_blkdev(unsigned int major, const char *name,
-		void (*probe)(dev_t devt))
+int ____register_blkdev(unsigned int major, const char *name,
+			void (*probe)(dev_t devt), struct module *owner)
 {
 	struct blk_major_name **n, *p;
 	int index, ret = 0;
@@ -257,6 +259,7 @@ int __register_blkdev(unsigned int major, const char *name,
 
 	p->major = major;
 	p->probe = probe;
+	p->owner = owner;
 	strlcpy(p->name, name, sizeof(p->name));
 	p->next = NULL;
 	index = major_to_index(major);
@@ -279,7 +282,7 @@ int __register_blkdev(unsigned int major, const char *name,
 	mutex_unlock(&major_names_lock);
 	return ret;
 }
-EXPORT_SYMBOL(__register_blkdev);
+EXPORT_SYMBOL(____register_blkdev);
 
 void unregister_blkdev(unsigned int major, const char *name)
 {
@@ -685,14 +688,29 @@ void blk_request_module(dev_t devt)
 {
 	unsigned int major = MAJOR(devt);
 	struct blk_major_name **n;
+	void (*probe_fn)(dev_t devt);
 
 	mutex_lock(&major_names_lock);
 	for (n = &major_names[major_to_index(major)]; *n; n = &(*n)->next) {
-		if ((*n)->major == major && (*n)->probe) {
-			(*n)->probe(devt);
-			mutex_unlock(&major_names_lock);
-			return;
-		}
+		if ((*n)->major != major || !(*n)->probe)
+			continue;
+		if (!try_module_get((*n)->owner))
+			break;
+		/*
+		 * Calling probe function with major_names_lock held causes
+		 * circular locking dependency problem. Thus, call it after
+		 * releasing major_names_lock.
+		 */
+		probe_fn = (*n)->probe;
+		mutex_unlock(&major_names_lock);
+		/*
+		 * Assuming that unregister_blkdev() is called from module's
+		 * __exit function, a module refcount taken above allows us
+		 * to safely call probe function without major_names_lock held.
+		 */
+		probe_fn(devt);
+		module_put((*n)->owner);
+		return;
 	}
 	mutex_unlock(&major_names_lock);
 

include/linux/genhd.h

@@ -277,10 +277,12 @@ extern void put_disk(struct gendisk *disk);
 
 #define alloc_disk(minors) alloc_disk_node(minors, NUMA_NO_NODE)
 
-int __register_blkdev(unsigned int major, const char *name,
-		void (*probe)(dev_t devt));
+int ____register_blkdev(unsigned int major, const char *name,
+			void (*probe)(dev_t devt), struct module *owner);
+#define __register_blkdev(major, name, probe) \
+	____register_blkdev(major, name, probe, THIS_MODULE)
 #define register_blkdev(major, name) \
-	__register_blkdev(major, name, NULL)
+	____register_blkdev(major, name, NULL, NULL)
 void unregister_blkdev(unsigned int major, const char *name);
 
 bool bdev_check_media_change(struct block_device *bdev);