usb: gadget: f_fs: copy only received bytes on short ep0 read

In the Linux kernel, the following vulnerability has been resolved:

usb: gadget: f_fs: copy only received bytes on short ep0 read

ffs_ep0_read() allocates its control-OUT data buffer with

kmalloc() (not kzalloc) at the Length value from the Setup

packet, then copies that full len to userspace regardless of

how many bytes were actually received:

data = kmalloc(len, GFP_KERNEL);

...

ret = __ffs_ep0_queue_wait(ffs, data, len);

if ((ret > 0) && (copy_to_user(buf, data, len)))

ret = -EFAULT;

__ffs_ep0_queue_wait() returns req->actual, which on a short

control OUT transfer is strictly less than len. The

copy_to_user() call still copies len bytes, so on a short OUT

the last (len - ret) bytes of the kmalloc() buffer --

uninitialised slab residue -- are delivered to the FunctionFS

daemon.

Short ep0 OUT completions are specified USB control-transfer

behavior and are produced by in-tree UDCs:

* dwc2 continues on req->actual < req->length for ep0 DATA OUT

(short-not-ok is the only ep0-OUT stall path).

* aspeed_udc ends ep0 OUT on rx_len < ep->ep.maxpacket.

* renesas_usbf logs "ep0 short packet" and completes the

request.

* dwc3 stalls on short IN but not on short OUT.

A short ep0 OUT is therefore not evidence of a broken UDC; it is

a normal condition f_fs has to cope with. The sibling gadgetfs

implementation in drivers/usb/gadget/legacy/inode.c already does

this correctly via min(len, dev->req->actual) before

copy_to_user(). This patch brings f_fs.c to the same safe

pattern rather than trimming at a defensive layer.

The bug is reached from the FunctionFS device node, which in

real deployments is owned by the privileged gadget daemon

(adbd, UMS, composite gadget services, etc.); it is not

reachable from unprivileged userspace. Linux host stacks

normally reject short-wLength control OUTs before they reach

the gadget, so reproducing this required a build that

bypasses that host-side check. With the bypass in place, a

1-byte payload on a 64-byte Setup produces 63 bytes of

non-canary slab residue in the daemon's read buffer.

Fix by copying only ret (actually received) bytes to

userspace.