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75 lines
3.5 KiB
Markdown
75 lines
3.5 KiB
Markdown
# Fuzzing Dropbear
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Dropbear is process-per-session so it assumes calling `dropbear_exit()`
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is fine at any point to clean up. This makes fuzzing a bit trickier.
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A few pieces of wrapping infrastructure are used to work around this.
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The [libfuzzer](http://llvm.org/docs/LibFuzzer.html#fuzz-target) harness
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expects a long running process to continually run a test function with
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a string of crafted input. That process should not leak resources or exit.
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## longjmp
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When dropbear runs in fuzz mode it sets up a
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[`setjmp()`](http://man7.org/linux/man-pages/man3/setjmp.3.html) target prior
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to launching the code to be fuzzed, and then [`dropbear_exit()`](dbutil.c#L125)
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calls `longjmp()` back there. This avoids exiting though it doesn't free
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memory or other resources.
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## malloc Wrapper
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Dropbear normally uses a [`m_malloc()`](dbmalloc.c) function that is the same as `malloc()` but
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exits if allocation fails. In fuzzing mode this is replaced with a tracking allocator
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that stores all allocations in a linked list. After the `longjmp()` occurs the fuzzer target
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calls [`m_malloc_free_epoch(1, 1)`](dbmalloc.c) to clean up any unreleased memory.
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If the fuzz target runs to completion it calls `m_malloc_free_epoch(1, 0)` which will reset
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the tracked allocations but will not free memory - that allows libfuzzer's leak checking
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to detect leaks in normal operation.
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## File Descriptor Input
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As a network process Dropbear reads and writes from a socket. The wrappers for
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`read()`/`write()`/`select()` in [fuzz-wrapfd.c](fuzz-wrapfd.c) will read from the
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fuzzer input that has been set up with `wrapfd_add()`. `write()` output is
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currently discarded.
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These also test error paths such as EINTR and short reads with certain probabilities.
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This allows running the entire dropbear server process with network input provided by the
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fuzzer, without many modifications to the main code. At the time of writing this
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only runs the pre-authentication stages, though post-authentication could be run similarly.
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## Encryption and Randomness
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When running in fuzzing mode Dropbear uses a [fixed seed](dbrandom.c#L185)
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every time so that failures can be reproduced.
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Since the fuzzer cannot generate valid encrypted input the packet decryption and
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message authentication calls are disabled, see [packet.c](packet.c).
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MAC failures are set to occur with a low probability to test that error path.
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## Fuzzers
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Current fuzzers are
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- [fuzzer-preauth](fuzzer-preauth.c) - the fuzzer input is treated as a stream of session input. This will
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test key exchange, packet ordering, authentication attempts etc.
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- [fuzzer-preauth_nomaths](fuzzer-preauth_nomaths.c) - the same as fuzzer-preauth but with asymmetric crypto
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routines replaced with dummies for faster runtime. corpora are shared
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between fuzzers by [oss-fuzz](https://github.com/google/oss-fuzz) so this
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will help fuzzer-preauth too.
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- [fuzzer-verify](fuzzer-verify.c) - read a key and signature from fuzzer input and verify that signature.
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It would not be expected to pass, though some keys with bad parameters are
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able to validate with a trivial signature - extra checks are added for that.
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- [fuzzer-pubkey](fuzzer-pubkey.c) - test parsing of an `authorized_keys` line.
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- [fuzzer-kexdh](fuzzer-kexdh.c) - test Diffie-Hellman key exchange where the fuzz input is the
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ephemeral public key that would be received over the network. This is testing `mp_expt_mod()`
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and and other libtommath routines.
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- [fuzzer-kexecdh](fuzzer-kexecdh.c) - test Elliptic Curve Diffie-Hellman key exchange like fuzzer-kexdh.
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This is testing libtommath ECC routines.
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