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Fix buffer overread in preloaded hostuuid parsing
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Authored by jrtc27 on Dec 22 2021, 4:10 PM.
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Details

Summary

Commit b6be9566d236 stopped prison0_init writing outside of the
preloaded hostuuid's bounds. However, the preloaded data will not
(normally) have a NUL in it, and so validate_uuid will walk off the end
of the buffer in its call to sscanf. Previously if there was any
whitespace in the string we'd at least know there's a NUL one past the
end due to the off-by-one error, but now no such byte is guaranteed.

Fix this by copying to a temporary buffer and explicitly adding a NUL.

Whilst here, change the strlcpy call to use a far less suspicious
argument for dstsize; in practice it's fine, but it's an unusual pattern
and not necessary.

Found by: CHERI
MFC after: 1 week

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