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1 # Fuzzing Dropbear |
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2 |
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3 Dropbear is process-per-session so it assumes calling `dropbear_exit()` |
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4 is fine at any point to clean up. This makes fuzzing a bit trickier. |
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5 A few pieces of wrapping infrastructure are used to work around this. |
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6 |
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7 The [libfuzzer](http://llvm.org/docs/LibFuzzer.html#fuzz-target) harness |
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8 expects a long running process to continually run a test function with |
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9 a string of crafted input. That process should not leak resources or exit. |
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10 |
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11 ## longjmp |
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12 |
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13 When dropbear runs in fuzz mode it sets up a |
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14 [`setjmp()`](http://man7.org/linux/man-pages/man3/setjmp.3.html) target prior |
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15 to launching the code to be fuzzed, and then [`dropbear_exit()`](dbutil.c#L125) |
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16 calls `longjmp()` back there. This avoids exiting though it doesn't free |
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17 memory or other resources. |
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18 |
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19 ## malloc Wrapper |
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20 |
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21 Dropbear normally uses a [`m_malloc()`](dbmalloc.c) function that is the same as `malloc()` but |
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22 exits if allocation fails. In fuzzing mode this is replaced with a tracking allocator |
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23 that stores all allocations in a linked list. After the `longjmp()` occurs the fuzzer target |
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24 calls [`m_malloc_free_epoch(1, 1)`](dbmalloc.c) to clean up any unreleased memory. |
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25 |
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26 If the fuzz target runs to completion it calls `m_malloc_free_epoch(1, 0)` which will reset |
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27 the tracked allocations but will not free memory - that allows libfuzzer's leak checking |
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28 to detect leaks in normal operation. |
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29 |
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30 ## File Descriptor Input |
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31 |
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32 As a network process Dropbear reads and writes from a socket. The wrappers for |
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33 `read()`/`write()`/`select()` in [fuzz-wrapfd.c](fuzz-wrapfd.c) will read from the |
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34 fuzzer input that has been set up with `wrapfd_add()`. `write()` output is |
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35 currently discarded. |
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36 These also test error paths such as EINTR and short reads with certain probabilities. |
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37 |
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38 This allows running the entire dropbear server process with network input provided by the |
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39 fuzzer, without many modifications to the main code. At the time of writing this |
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40 only runs the pre-authentication stages, though post-authentication could be run similarly. |
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41 |
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42 ## Encryption and Randomness |
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43 |
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44 When running in fuzzing mode Dropbear uses a [fixed seed](dbrandom.c#L185) |
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45 every time so that failures can be reproduced. |
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46 |
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47 Since the fuzzer cannot generate valid encrypted input the packet decryption and |
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48 message authentication calls are disabled, see [packet.c](packet.c). |
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49 MAC failures are set to occur with a low probability to test that error path. |
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50 |
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51 ## Fuzzers |
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52 |
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53 Current fuzzers are |
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54 |
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55 - [fuzzer-preauth](fuzzer-preauth.c) - the fuzzer input is treated as a stream of session input. This will |
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56 test key exchange, packet ordering, authentication attempts etc. |
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57 |
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58 - [fuzzer-preauth_nomaths](fuzzer-preauth_nomaths.c) - the same as fuzzer-preauth but with asymmetric crypto |
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59 routines replaced with dummies for faster runtime. corpora are shared |
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60 between fuzzers by [oss-fuzz](https://github.com/google/oss-fuzz) so this |
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61 will help fuzzer-preauth too. |
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62 |
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63 - [fuzzer-verify](fuzzer-verify.c) - read a key and signature from fuzzer input and verify that signature. |
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64 It would not be expected to pass, though some keys with bad parameters are |
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65 able to validate with a trivial signature - extra checks are added for that. |
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66 |
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67 - [fuzzer-pubkey](fuzzer-pubkey.c) - test parsing of an `authorized_keys` line. |
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68 |
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69 - [fuzzer-kexdh](fuzzer-kexdh.c) - test Diffie-Hellman key exchange where the fuzz input is the |
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70 ephemeral public key that would be received over the network. This is testing `mp_expt_mod()` |
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71 and and other libtommath routines. |
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72 |
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73 - [fuzzer-kexecdh](fuzzer-kexecdh.c) - test Elliptic Curve Diffie-Hellman key exchange like fuzzer-kexdh. |
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74 This is testing libtommath ECC routines. |
