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...@xxxxxxxxxxxxxxxxxxxxxxxxx>
Signed-off-by: yu kuai <yukuai3@xxxxxxxxxx>
---
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'
Changes in V3:
handle 'q->debugfs-dir' and 'q->blk_trace->dir' respectivly
remove dev name to avoid memory leek when dev name is too long
warn_on when debugfs_rename failed, don't keep trying in a loop
block/blk-sysfs.c | 51 +++++++++++++++++++++++++++++++++++++++++++++++
1 file changed, 51 insertions(+)
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index fca9b158f4a0..8cd124117e29 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -11,6 +11,8 @@
#include <linux/blktrace_api.h>
#include <linux/blk-mq.h>
#include <linux/blk-cgroup.h>
+#include <linux/debugfs.h>
+#include <linux/atomic.h>
#include "blk.h"
#include "blk-mq.h"
@@ -1011,6 +1013,53 @@ int blk_register_queue(struct gendisk *disk)
}
EXPORT_SYMBOL_GPL(blk_register_queue);
+#ifdef CONFIG_DEBUG_FS
+
+void blk_rename_debugfs_dir(struct dentry **old)
+{
+ static atomic_t i = ATOMIC_INIT(0);
+ struct dentry *new;
+ char name[DNAME_INLINE_LEN];
+ u32 index = atomic_fetch_inc(&i);
+
+ snprintf(name, sizeof(name), "ready_to_remove_%u", index);
+ new = debugfs_lookup(name, blk_debugfs_root);
+ if (WARN_ON(new)) {
+ dput(new);
+ return;
+ }
+ new = debugfs_rename(blk_debugfs_root, *old, blk_debugfs_root, name);
+ if (WARN_ON(!new))
+ return;
+ *old = new;
+}
+/*
+ * 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)
+{
+#ifdef CONFIG_BLK_DEBUG_FS
+ if (!IS_ERR_OR_NULL(q->debugfs_dir))
+ blk_rename_debugfs_dir(&q->debugfs_dir);
+
+#endif
+#ifdef CONFIG_BLK_DEV_IO_TRACE
+ mutex_lock(&q->blk_trace_mutex);
+ if (q->blk_trace && !IS_ERR_OR_NULL(q->blk_trace->dir))
+ blk_rename_debugfs_dir(&q->blk_trace->dir);
+ mutex_unlock(&q->blk_trace_mutex);
+#endif
+}
+#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 +1088,7 @@ 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
