changeset 1590:68d5d8e84a92

notes on the fuzzer
author Matt Johnston <>
date Mon, 05 Mar 2018 14:07:11 +0800
parents 35af85194268
children b794d277c6da
diffstat 1 files changed, 72 insertions(+), 0 deletions(-) [+]
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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()`](dbutil.c#L125)
+calls `longjmp()` back there. This avoids exiting though it doesn't free 
+memory or other resources.
+## malloc Wrapper
+Dropbear normally uses a [`m_malloc()`](dbmalloc.c) 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)`](dbmalloc.c) 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](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](dbrandom.c#L185)
+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)[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 
+  public key that would be received over the network.
+- fuzzer-kexecdh - test Elliptic Curve Diffie-Hellman key exchange like fuzzer-kexdh