[PATCH] usb: gadget: f_fs: fix use-after-free in ffs_func_unbind

[Shuangpeng Bai <shuangpeng.kernel@gmail.com>]

ffs_func_unbind() continues to access ffs after calling
functionfs_unbind(), including taking ffs->eps_lock for endpoint cleanup.
However, functionfs_unbind() drops the binding reference to ffs.

A concurrent FunctionFS unmount can drop the independent superblock
reference. Either put can then release the final reference and free ffs
before ffs_func_unbind() finishes its endpoint cleanup.

Take a temporary ffs reference for the duration of ffs_func_unbind() and
drop it only after all post-unbind cleanup is complete.

Fixes: a3058a5d82e2 ("usb: gadget: f_fs: remove redundant ffs_data_get()")
Cc: stable@vger.kernel.org
Signed-off-by: Shuangpeng Bai <shuangpeng.kernel@gmail.com>
---
 drivers/usb/gadget/function/f_fs.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/usb/gadget/function/f_fs.c b/drivers/usb/gadget/function/f_fs.c
index 44218be1e676..8acad4564b6c 100644
--- a/drivers/usb/gadget/function/f_fs.c
+++ b/drivers/usb/gadget/function/f_fs.c
@@ -4089,6 +4089,9 @@ static void ffs_func_unbind(struct usb_configuration *c,
 	unsigned count = ffs->eps_count;
 	unsigned long flags;
 
+	/* Keep ffs alive until all post-unbind cleanup is complete. */
+	ffs_data_get(ffs);
+
 	if (ffs->func == func) {
 		ffs_func_eps_disable(func);
 		ffs->func = NULL;
@@ -4122,6 +4125,7 @@ static void ffs_func_unbind(struct usb_configuration *c,
 	func->function.ssp_descriptors = NULL;
 	func->interfaces_nums = NULL;
 
+	ffs_data_put(ffs);
 }
 
 static struct usb_function *ffs_alloc(struct usb_function_instance *fi)
-- 
2.43.0