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

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.

Fixes: ddf8abd25994 ("USB: f_fs: the FunctionFS driver")
Cc: stable <stable@kernel.org>
Assisted-by: Claude:claude-opus-4-7
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Link: https://patch.msgid.link/20260419160359.1577270-1-michael.bommarito@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>