Wind River Support Network

Description

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.