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Subject[PATCH V2] block: rename 'q->debugfs_dir' and 'q->blk_trace->dir' in blk_unregister_queue()
Date
syzbot is reporting use after free bug in debugfs_remove[1].

This is because in request_queue, 'q->debugfs_dir' and
'q->blk_trace->dir' could be the same dir. And in __blk_release_queue(),
blk_mq_debugfs_unregister() will remove everything inside the dir.

With futher investigation of the reporduce repro, the problem can be
reporduced by following procedure:

1. LOOP_CTL_ADD, create a request_queue q1, blk_mq_debugfs_register() will
create the dir.
2. LOOP_CTL_REMOVE, blk_release_queue() will add q1 to release queue.
3. LOOP_CTL_ADD, create another request_queue q2,blk_mq_debugfs_register()
will fail because the dir aready exist.
4. BLKTRACESETUP, create two files(msg and dropped) inside the dir.
5. call __blk_release_queue() for q1, debugfs_remove_recursive() will
delete the files created in step 4.
6. LOOP_CTL_REMOVE, blk_release_queue() will add q2 to release queue.
And when __blk_release_queue() is called for q2, blk_trace_shutdown() will
try to release the two files created in step 4, wich are aready released
in step 5.

thread1 |kworker |thread2
----------------------- | ------------------------ | --------------------
loop_control_ioctl | |
loop_add | |
blk_mq_debugfs_register| |
debugfs_create_dir | |
loop_control_ioctl | |
loop_remove | |
blk_release_queue | |
schedule_work | |
| |loop_control_ioctl
| | loop_add
| | ...
| |blk_trace_ioctl
| | __blk_trace_setup
| | debugfs_create_file
|__blk_release_queue |
| blk_mq_debugfs_unregister|
| debugfs_remove_recursive|
| |loop_control_ioctl
| | loop_remove
| | ...
|__blk_release_queue |
| blk_trace_shutdown |
| debugfs_remove |

commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression") pushed the
final release of request_queue to a workqueue, so, when loop_add() is
called again(step 3), __blk_release_queue() might not been called yet,
which causes the problem.

Fix the problem by renaming 'q->debugfs_dir' or 'q->blk_trace->dir'
in blk_unregister_queue() if they exist.

[1] https://syzkaller.appspot.com/bug?extid=903b72a010ad6b7a40f2
References: CVE-2019-19770
Fixes: commit dc9edc44de6c ("block: Fix a blk_exit_rl() regression")
Reported-by: syzbot <syz...@syzkaller.appspotmail.com>
Signed-off-by: yu kuai <yukuai3@huawei.com>
---
Changes in V2:
add device name to the new dir name
fix compile err when 'CONFIG_BLK_DEBUG_FS' is not enabled
add treatment of 'q->blk_trace->dir'

block/blk-sysfs.c | 42 ++++++++++++++++++++++++++++++++++++++++++
1 file changed, 42 insertions(+)

diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index fca9b158f4a0..1da8d4a4915a 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -11,6 +11,7 @@
#include <linux/blktrace_api.h>
#include <linux/blk-mq.h>
#include <linux/blk-cgroup.h>
+#include <linux/debugfs.h>

#include "blk.h"
#include "blk-mq.h"
@@ -1011,6 +1012,46 @@ int blk_register_queue(struct gendisk *disk)
}
EXPORT_SYMBOL_GPL(blk_register_queue);

+#ifdef CONFIG_DEBUG_FS
+/*
+ * blk_prepare_release_queue - rename q->debugfs_dir and q->blk_trace->dir
+ * @q: request_queue of which the dir to be renamed belong to.
+ *
+ * Because the final release of request_queue is in a workqueue, the
+ * cleanup might not been finished yet while the same device start to
+ * create. It's not correct if q->debugfs_dir still exist while trying
+ * to create a new one.
+ */
+static void blk_prepare_release_queue(struct request_queue *q)
+{
+ struct dentry *new = NULL;
+ struct dentry **old = NULL;
+ char name[DNAME_INLINE_LEN];
+ int i = 0;
+
+#ifdef CONFIG_BLK_DEBUG_FS
+ if (!IS_ERR_OR_NULL(q->debugfs_dir))
+ old = &q->debugfs_dir;
+#endif
+#ifdef CONFIG_BLK_DEV_IO_TRACE
+ /* q->debugfs_dir and q->blk_trace->dir can't both exist */
+ if (q->blk_trace && !IS_ERR_OR_NULL(q->blk_trace->dir))
+ old = &q->blk_trace->dir;
+#endif
+ if (old == NULL)
+ return;
+ while (new == NULL) {
+ sprintf(name, "ready_to_remove_%s_%d",
+ kobject_name(q->kobj.parent), i++);
+ new = debugfs_rename(blk_debugfs_root, *old,
+ blk_debugfs_root, name);
+ }
+ *old = new;
+}
+#else
+#define blk_prepare_release_queue(q) do { } while (0)
+#endif
+
/**
* blk_unregister_queue - counterpart of blk_register_queue()
* @disk: Disk of which the request queue should be unregistered from sysfs.
@@ -1039,6 +1080,7 @@ void blk_unregister_queue(struct gendisk *disk)
mutex_unlock(&q->sysfs_lock);

mutex_lock(&q->sysfs_dir_lock);
+ blk_prepare_release_queue(q);
/*
* Remove the sysfs attributes before unregistering the queue data
* structures that can be modified through sysfs.
--
2.17.2
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