Prevent out-of-bounds write when processing external headers in sicslowpan #1409
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When processing external headers in sicslowpan, a long chain of headers could possibly overflow the uip_buf variable.
In the code below, which follows the now inserted check, the writes to the exthdr struct could result in an out-of-bounds write.
exthdr = (struct uip_ext_hdr *)ip_payload; exthdr->len = (2 + len) / 8 - 1; exthdr->next = next;
A packet test that should trigger the bug is included in the PR.