struct bpf_prog *bpf_patch_insn_single(struct bpf_prog *prog, u32 off,
				       const struct bpf_insn *patch, u32 len)
{
	u32 insn_adj_cnt, insn_rest, insn_delta = len - 1;
	const u32 cnt_max = S16_MAX;
	struct bpf_prog *prog_adj;

	/* Since our patchlet doesn't expand the image, we're done. */
	if (insn_delta == 0) {
		memcpy(prog->insnsi + off, patch, sizeof(*patch));
		return prog;
	}

	insn_adj_cnt = prog->len + insn_delta;

	/* Reject anything that would potentially let the insn->off
	 * target overflow when we have excessive program expansions.
	 * We need to probe here before we do any reallocation where
	 * we afterwards may not fail anymore.
	 */
	if (insn_adj_cnt > cnt_max &&
	    bpf_adj_branches(prog, off, insn_delta, true))
		return NULL;

	/* Several new instructions need to be inserted. Make room
	 * for them. Likely, there's no need for a new allocation as
	 * last page could have large enough tailroom.
	 */
	prog_adj = bpf_prog_realloc(prog, bpf_prog_size(insn_adj_cnt),
				    GFP_USER);
	if (!prog_adj)
		return NULL;

	prog_adj->len = insn_adj_cnt;

	/* Patching happens in 3 steps:
	 *
	 * 1) Move over tail of insnsi from next instruction onwards,
	 *    so we can patch the single target insn with one or more
	 *    new ones (patching is always from 1 to n insns, n > 0).
	 * 2) Inject new instructions at the target location.
	 * 3) Adjust branch offsets if necessary.
	 */
	insn_rest = insn_adj_cnt - off - len;

	memmove(prog_adj->insnsi + off + len, prog_adj->insnsi + off + 1,
		sizeof(*patch) * insn_rest);
	memcpy(prog_adj->insnsi + off, patch, sizeof(*patch) * len);

	/* We are guaranteed to not fail at this point, otherwise
	 * the ship has sailed to reverse to the original state. An
	 * overflow cannot happen at this point.
	 */
	BUG_ON(bpf_adj_branches(prog_adj, off, insn_delta, false));

	return prog_adj;
}
The program copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer, leading to a buffer overflow.