[PATCH] usb: gadget: f_fs: fix use-after-free in ffs_closed() on umount

[Mohammad Mosafer <mohsafer@gmail.com>]

ffs_free_inst() releases the ffs_dev with ffs_release_dev() and only
then re-acquires ffs_dev_lock to free it with _ffs_free_dev().  In
between, the dev is still linked on the ffs_devices list while already
marked unmounted, so a concurrent mount(2) of functionfs finds it by
name in ffs_acquire_dev() and links a fresh ffs_data to the doomed dev
(ffs_data->private_data = dev).  _ffs_free_dev() then kfrees the dev,
and when that mount is torn down, ffs_closed() dereferences the stale
ffs->private_data:

  BUG: KASAN: slab-use-after-free in ffs_data_clear+0x438/0x530
  Write of size 1 at addr ffff88810594664a by task repro/116
    ffs_data_clear+0x438/0x530
    ffs_fs_kill_sb+0x7b/0x510
    deactivate_locked_super+0xa9/0x200
    cleanup_mnt+0x255/0x380
    ... reached via umount(2)
  Freed by task 112:
    kfree+0x127/0x3b0
    ffs_free_inst+0x10c/0x1a0
    usb_put_function_instance+0x8a/0xc0
    configfs_rmdir+0x773/0x9c0
  Allocated by task 113:
    ffs_alloc_inst+0x109/0x360
    function_make+0x138/0x330
    configfs_mkdir+0x48b/0x1090

Hold ffs_dev_lock across the release and the free so that a released
dev is never findable, splitting ffs_release_dev() into a lock-assuming
_ffs_release_dev() (matching the _ffs_* convention in this file) plus a
locking wrapper for the remaining callers.

The race was reproduced with a multi-threaded harness racing configfs
mkdir/rmdir of the ffs instance against mount/umount of functionfs on
a KASAN kernel: the unpatched kernel reports the use-after-free
reliably (2/2 runs), the patched kernel survives an extended soak with
identical churn (2/2 runs clean).

Reported-by: syzbot+6227549bd2c8a1ec8ba0@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=6227549bd2c8a1ec8ba0
Fixes: 5920cda627688c ("usb: gadget: FunctionFS: convert to new function interface with backward compatibility")
Signed-off-by: Mohammad Mosafer <mohsafer@gmail.com>
---
 drivers/usb/gadget/function/f_fs.c | 24 ++++++++++++++++++++----
 1 file changed, 20 insertions(+), 4 deletions(-)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index c64a268e98a4..5e7179b4250e 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -288,6 +288,7 @@ static struct ffs_dev *_ffs_find_dev(const char *name);
 static struct ffs_dev *_ffs_alloc_dev(void);
 static void _ffs_free_dev(struct ffs_dev *dev);
 static int ffs_acquire_dev(const char *dev_name, struct ffs_data *ffs_data);
+static void _ffs_release_dev(struct ffs_dev *ffs_dev);
 static void ffs_release_dev(struct ffs_dev *ffs_dev);
 static int ffs_ready(struct ffs_data *ffs);
 static void ffs_closed(struct ffs_data *ffs);
@@ -4147,8 +4148,17 @@ static void ffs_free_inst(struct usb_function_instance *f)
 	struct f_fs_opts *opts;
 
 	opts = to_f_fs_opts(f);
-	ffs_release_dev(opts->dev);
+
+	/*
+	 * Release and free the dev under a single ffs_dev_lock critical
+	 * section. Between ffs_release_dev() and _ffs_free_dev() the dev
+	 * would still be on the ffs_devices list while already unmounted,
+	 * so a concurrent ffs_acquire_dev() could link a fresh ffs_data to
+	 * the doomed dev, leaving it with a dangling ->private_data that is
+	 * dereferenced in ffs_closed() when that mount is torn down.
+	 */
 	ffs_dev_lock();
+	_ffs_release_dev(opts->dev);
 	_ffs_free_dev(opts->dev);
 	ffs_dev_unlock();
 	kfree(opts);
@@ -4363,10 +4373,11 @@ static int ffs_acquire_dev(const char *dev_name, struct ffs_data *ffs_data)
 	return ret;
 }
 
-static void ffs_release_dev(struct ffs_dev *ffs_dev)
+/*
+ * ffs_dev_lock must be taken by the caller
+ */
+static void _ffs_release_dev(struct ffs_dev *ffs_dev)
 {
-	ffs_dev_lock();
-
 	if (ffs_dev && ffs_dev->mounted) {
 		ffs_dev->mounted = false;
 		if (ffs_dev->ffs_data) {
@@ -4377,7 +4388,12 @@ static void ffs_release_dev(struct ffs_dev *ffs_dev)
 		if (ffs_dev->ffs_release_dev_callback)
 			ffs_dev->ffs_release_dev_callback(ffs_dev);
 	}
+}
 
+static void ffs_release_dev(struct ffs_dev *ffs_dev)
+{
+	ffs_dev_lock();
+	_ffs_release_dev(ffs_dev);
 	ffs_dev_unlock();
 }
 
-- 
2.34.1