[PATCH] usb: usbfs: fix use-after-free of usb_device in usbdev_release()

usbdev_release() drops its reference to the struct usb_device before
draining the list of completed async URBs, but that drain path reads back
through the same object: free_async() calls dec_usb_memory_use_count()
for any URB whose buffer came from the usbfs mmap() region, and its first
statement is bus_to_hcd(ps->dev->bus).

After a disconnect the usbfs reference can be the last one, in which case
usb_put_dev() frees the device and the subsequent loop reads offset 80 of
freed memory and uses the result as a struct usb_hcd *, which
hcd_buffer_free_pages() then dereferences.

This is reachable by an unprivileged process that has read/write access to
a /dev/bus/usb node: mmap() the fd, submit one URB with a buffer inside the
mapping, wait for the device to be unplugged, then munmap() and close().
It reproduces on every attempt rather than being a race, because a live
MAP_SHARED vma holds a reference on the struct file, so usbdev_release()
cannot run until the last vma is gone and the freeing branch of
dec_usb_memory_use_count() is always taken.

  BUG: KASAN: slab-use-after-free in dec_usb_memory_use_count+0x3ae/0x410
  Read of size 8 at addr ffff8880122ee050 by task poc/769
  CPU: 1 UID: 1000 PID: 769 Comm: poc Tainted: G    B    6.12.94 #3

  Call Trace:
   dec_usb_memory_use_count+0x3ae/0x410
   free_async+0x2aa/0x4f0
   usbdev_release+0x375/0x460
   __fput+0x3ea/0xb50
   __x64_sys_close+0x86/0x100

  Allocated by task 11:
   usb_alloc_dev+0x55/0xd90
   hub_event+0x2524/0x43d0

  Freed by task 769:
   kfree+0x121/0x360
   device_release+0xd2/0x280
   usb_put_dev+0x23/0x30
   usbdev_release+0x2d8/0x460

Release the device reference after the drain loop instead. Nothing between
the two points requires it to have been dropped.

Fixes: f7d34b445abc ("USB: Add support for usbfs zerocopy.")
Cc: [email protected]
Assisted-by: Claude:claude-opus-5
Signed-off-by: Miguel Peñaranda <[email protected]>
---
 drivers/usb/core/devio.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/usb/core/devio.c b/drivers/usb/core/devio.c
index e191934..486ced7 100644
--- a/drivers/usb/core/devio.c
+++ b/drivers/usb/core/devio.c
@@ -1113,7 +1113,6 @@ static int usbdev_release(struct inode *inode, struct file *file)
 	if (!ps->suspend_allowed)
 		usb_autosuspend_device(dev);
 	usb_unlock_device(dev);
-	usb_put_dev(dev);
 	put_pid(ps->disc_pid);
 	put_cred(ps->cred);
 
@@ -1122,6 +1121,7 @@ static int usbdev_release(struct inode *inode, struct file *file)
 		free_async(as);
 		as = async_getcompleted(ps);
 	}
+	usb_put_dev(dev);
 
 	kfree(ps);
 	return 0;
-- 
2.53.0