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> the tables are marked as read-only once dynamic linking completes

Alas this won't work. Dynamic linking is lazy, there's no moment when it's "complete". The correct function pointers get loaded and inserted into the table (in place of stubs) when called the first time, which can be arbitrarily far into the future. In fact in most large library ecosystems (gtk apps, etc...) most of the linked functions are never called at all.

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Best practice is to resolve them early now: https://www.redhat.com/en/blog/hardening-elf-binaries-using-...

in this case it takes advantage of the fact that sshd is compiled with early binding not lazy. The ifunc resolver function is called early, right after the dynamic libraries are loaded. It's eager binding (that's what LD_BIND_NOW=1 / -W,-z,now are doing) is a security feature, GOT table will be readonly early. Didn't help with security in this case lol

just to add a detail:

with lazy binding the function resolver might not even be called at all, the link tree is like sshd.elf -> systemd.so -> lzma.so. If systemd uses a ifunc symbol in lzma but sshd is not using that symbol: if lazy binding the resolver will never run, if eager binding the resolver will run. Something the backdoor also took advantage of

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