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Comment by markasoftware | Hacker News Reader

This is not a new vulnerability, it's just a description of how to exploit the PwnEscape vulnerability in practice on firecracker VMs (disclosed multiple weeks ago and discussed in some less upvoted posts on HN)

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> already discussed at length on HN

Couldn't find it. Care to provide link.

The actual vulnerability got very little discussion. It's clever. Because Linux networking people are obsessed with checksum performance, virtio-net plumbs checksum offload metadata through the host/guest driver interface. Guests can thus deliver the host packets along with instructions on what to checksum. The vulnerability is a missing sanity check that allows checksum instructions to instruct the host to overwrite the header length field in the IPv4 header, rather than the checksum field.

Crucially, this happens in the middle of packet processing, so the kernel code is forced to work with two different ideas of how long the IPv4 header is. All hell predictably breaks loose.

That’s fascinating. I’ve seen this sort of thing on protocol implementations in layer 4+ but it’s rare to find it in layer 3.

Fixed a month ago upstream

https://github.com/torvalds/linux/commit/6785011f8b16abf40b1...

https://www.pwn.ai/blog/pwnescape

Additional color: per the commit, the bug was found by Paulos Yibelo, who runs PwnAI (the author of this piece), and worked on security at AWS for 4 years prior.

OP asked for a link

And honestly, how the &*DK is this ANYTHING related to firecracker?

Ah maybe not on HN, I forget where I learned of it. Either way, multiple weeks ago: https://news.ycombinator.com/item?id=50108845

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