CVE-2026-68368: usb: gadget: f_ncm: validate datagram bounds in ncm_unwrap_ntb()

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

usb: gadget: fncm: validate datagram bounds in ncmunwrapntb()

When unpacking host-supplied NTBs, ncmunwrapntb() checks datagram length against framemax but does not verify that the datagram fits within the declared block length. Additionally, when decoding multiple NTBs from a single socket buffer, subsequent block lengths are not checked against the actual remaining buffer data.

With these checks missing, a malicious USB host can specify datagram offsets and lengths that point beyond the block, or supply secondary NTB headers declaring lengths larger than the buffer. skbputdata() then copies adjacent kernel memory from skbsharedinfo into the network skb.

Fix this by verifying that sufficient buffer space remains for the NTB header before parsing, handling zero-length block declarations, ensuring that block lengths never exceed the remaining buffer space, and verifying that each datagram payload stays strictly within the block boundary.