fix: parse fragmented sslv2 client hellos #4425
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lrstewart
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camshaft
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Feb 19, 2024
lrstewart
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Feb 19, 2024
tls/s2n_record_read.c
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| /* The first bit of the length must be set to indicate SSLv2, | ||
| * so the raw fragment length must be adjusted. | ||
| */ | ||
| *fragment_length ^= (S2N_TLS_SSLV2_HEADER_FLAG << 8); |
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I wrote a bunch of unit tests for this in s2n_record_read_test, but still please sanity check my math :)
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The shift math is correct 😄. I'm just a bit concerned about the XOR: #4425 (comment)
camshaft
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camshaft
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camshaft
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maddeleine
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maddeleine
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Description of changes:
Fixes an issue where parsing sslv2 messages fails if the first call to conn->recv doesn't return the whole message.
Usually we block in that case and try again later. But we weren't rereading conn->header_in for sslv2, so the second attempt would try to read a whole new header and fail because conn->header_in was already full of the actual header. After I fixed that, I also found that we were removing the sslv2 bit flag while parsing, so the second attempt wouldn't be detected as sslv2.
I also added a conn->header_in reread before attempting to read a new header, just in case there's another code path that leads to us not rereading conn->header_in. conn->header_in is a 5-byte static stuffer, so in order for it to contain a full 5-byte header the read cursor always needs to be 0. I didn't consider that sufficient to solve the problem though, since non-sslv2 headers are available immediately after parsing and some code could theoretically rely on that fact. So I still think the reread at the end of the header parsing methods is necessary.
Testing:
New tests for sslv2 header parsing and fragmented sslv2 messages.
By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.