net: ppp: Add bound checking for skb data on ppp_sync_txmung

Vulnerability Description

In the Linux kernel, the following vulnerability has been resolved: net: ppp: Add bound checking for skb data on ppp_sync_txmung Ensure we have enough data in linear buffer from skb before accessing initial bytes. This prevents potential out-of-bounds accesses when processing short packets. When ppp_sync_txmung receives an incoming package with an empty payload: (remote) gef➤ p *(struct pppoe_hdr *) (skb->head + skb->network_header) $18 = { type = 0x1, ver = 0x1, code = 0x0, sid = 0x2, length = 0x0, tag = 0xffff8880371cdb96 } from the skb struct (trimmed) tail = 0x16, end = 0x140, head = 0xffff88803346f400 "4", data = 0xffff88803346f416 ":\377", truesize = 0x380, len = 0x0, data_len = 0x0, mac_len = 0xe, hdr_len = 0x0, it is not safe to access data[2]. [pabeni@redhat.com: fixed subj typo]

References

• https://git.kernel.org/stable/c/529401c8f12ecc35f9ea5d946d5a5596cf172b48
• https://git.kernel.org/stable/c/de5a4f0cba58625e88b7bebd88f780c8c0150997
• https://git.kernel.org/stable/c/99aa698dec342a07125d733e39aab4394b3b7e05
• https://git.kernel.org/stable/c/b78f2b458f56a5a4d976c8e01c43dbf58d3ea2ca
• https://git.kernel.org/stable/c/fbaffe8bccf148ece8ad67eb5d7aa852cabf59c8
• https://git.kernel.org/stable/c/b4c836d33ca888695b2f2665f948bc1b34fbd533
• https://git.kernel.org/stable/c/1f6eb9fa87a781d5370c0de7794ae242f1a95ee5
• https://git.kernel.org/stable/c/6e8a6bf43cea4347121ab21bb1ed8d7bef7e732e
• https://git.kernel.org/stable/c/aabc6596ffb377c4c9c8f335124b92ea282c9821