ipv4: validate checksum_start before completing checksum

Description

In the Linux kernel, the following vulnerability has been resolved: ipv4: validate checksum_start before completing checksum If a packet with bad checksum metadata gets into the ipv4 stack, skb_checksum_help can corrupt the network header and cause a bunch of mischief. This was discovered and reported by Paulos, and has been reporoduced by others independently since. We really shouldn't allow such packets in, but as a defence in depth measure, let's also check before we complete the checksum. A more complete validation at input is forthcoming, but needs more work.

Also Affects

Downstream vendors/products affected by this vulnerability

References

• https://git.kernel.org/stable/c/a464d950f31c89565cbaae038cc60db04fe01956
• https://git.kernel.org/stable/c/0fab003ef4698e0628060a45a48f14b43ba2a40a
• https://git.kernel.org/stable/c/94524b7b32691de2e2ce162c04d6cbc9106e5cbf
• https://git.kernel.org/stable/c/a65f69e0f38ffbed1b128029e09097436416dc8b
• https://git.kernel.org/stable/c/32a8b0adaa18e52aaa0b246b5ffe683075354dbb
• https://git.kernel.org/stable/c/f3e835af6226f1dd12160cbd696081e9b6155de0
• https://git.kernel.org/stable/c/c3e33464746166264334641dc850a36eef7e6326
• https://git.kernel.org/stable/c/6785011f8b16abf40b1e9d8b2e332232b3e68822
• https://pwn.ai/blog/Pwnescape
• https://vercel.com/blog/findings-from-the-1-million-dollar-vercel-sandbox-challenge

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