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FUZZER-NOTES.md

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Fuzzing Dropbear

Dropbear is process-per-session so it assumes calling dropbear_exit() is fine at any point to clean up. This makes fuzzing a bit trickier. A few pieces of wrapping infrastructure are used to work around this.

The libfuzzer harness expects a long running process to continually run a test function with a string of crafted input. That process should not leak resources or exit.

longjmp

When dropbear runs in fuzz mode it sets up a setjmp() target prior to launching the code to be fuzzed, and then dropbear_exit() calls longjmp() back there. This avoids exiting though it doesn't free memory or other resources.

malloc Wrapper

Dropbear normally uses a m_malloc() function that is the same as malloc() but exits if allocation fails. In fuzzing mode this is replaced with a tracking allocator that stores all allocations in a linked list. After the longjmp() occurs the fuzzer target calls m_malloc_free_epoch(1, 1) to clean up any unreleased memory.

If the fuzz target runs to completion it calls m_malloc_free_epoch(1, 0) which will reset the tracked allocations but will not free memory - that allows libfuzzer's leak checking to detect leaks in normal operation.

File Descriptor Input

As a network process Dropbear reads and writes from a socket. The wrappers for read()/write()/select() in fuzz-wrapfd.c will read from the fuzzer input that has been set up with wrapfd_add(). write() output is currently discarded. These also test error paths such as EINTR and short reads with certain probabilities.

This allows running the entire dropbear server process with network input provided by the fuzzer, without many modifications to the main code. At the time of writing this only runs the pre-authentication stages, though post-authentication could be run similarly.

Encryption and Randomness

When running in fuzzing mode Dropbear uses a fixed seed every time so that failures can be reproduced.

Since the fuzzer cannot generate valid encrypted input the packet decryption and message authentication calls are disabled, see packet.c. MAC failures are set to occur with a low probability to test that error path.

Fuzzers

Current fuzzers are

  • fuzzer-preauth - the fuzzer input is treated as a stream of session input. This will test key exchange, packet ordering, authentication attempts etc.

  • fuzzer-preauth_nomaths - the same as fuzzer-preauth but with asymmetric crypto routines replaced with dummies for faster runtime. corpora are shared between fuzzers by oss-fuzz so this will help fuzzer-preauth too.

  • fuzzer-verify - read a key and signature from fuzzer input and verify that signature. It would not be expected to pass, though some keys with bad parameters are able to validate with a trivial signature - extra checks are added for that.

  • fuzzer-pubkey - test parsing of an authorized_keys line.

  • fuzzer-kexdh - test Diffie-Hellman key exchange where the fuzz input is the ephemeral public key that would be received over the network. This is testing mp_expt_mod() and and other libtommath routines.

  • fuzzer-kexecdh - test Elliptic Curve Diffie-Hellman key exchange like fuzzer-kexdh. This is testing libtommath ECC routines.

  • fuzzer-kexcurve25519 - test Curve25519 Elliptic Curve Diffie-Hellman key exchange like fuzzer-kexecdh. This is testing dropbear_curve25519_scalarmult() and other libtommath routines.