Re: [Xen-devel] [PATCH] fuzz/x86_emulate: fix bounds for input size

[George Dunlap <george.dunlap@xxxxxxxxxx>]

On 02/23/2018 04:30 PM, Wei Liu wrote:
> On Fri, Feb 23, 2018 at 12:57:26AM +0100, Paul Semel wrote:
>> The minimum size for the input size was set to DATA_OFFSET + 1 which was 
>> meaning
>> that we were requesting at least one character of the data array to be 
>> filled.
>> This is not needed for the fuzzer to get working correctly.
> 
> Sorry, I don't follow -- what do you expect the emulator to do if there
> is no instruction to emulate?
> 
>>
>> The maximum size for the input size was set to INPUT_SIZE, which is actually
>> the size of the data array inside the fuzz_corpus structure and so was not
>> abling user (or AFL) to fill in the whole structure. Changing to
>> sizeof(struct fuzz_corpus) correct this problem.
>>
>> Signed-off-by: Paul Semel <semelpaul@xxxxxxxxx>
>> ---
>>  tools/fuzz/x86_instruction_emulator/fuzz-emul.c | 9 +++++----
>>  1 file changed, 5 insertions(+), 4 deletions(-)
>>
>> diff --git a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c 
>> b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
>> index 964682aa1a..f3ce2e7e27 100644
>> --- a/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
>> +++ b/tools/fuzz/x86_instruction_emulator/fuzz-emul.c
>> @@ -33,6 +33,7 @@ struct fuzz_corpus
>>      unsigned char data[INPUT_SIZE];
>>  } input;
>>  #define DATA_OFFSET offsetof(struct fuzz_corpus, data)
>> +#define FUZZ_CORPUS_SIZE (sizeof(struct fuzz_corpus))
>>  
>>  /*
>>   * Internal state of the fuzzing harness.  Calculated initially from the 
>> input
>> @@ -822,13 +823,13 @@ int LLVMFuzzerTestOneInput(const uint8_t *data_p, 
>> size_t size)
>>      /* Reset all global state variables */
>>      memset(&input, 0, sizeof(input));
>>  
>> -    if ( size <= DATA_OFFSET )
>> +    if ( size < DATA_OFFSET )
>>      {
>>          printf("Input too small\n");
>>          return 1;
>>      }
>>  
>> -    if ( size > INPUT_SIZE )
>> +    if ( size > FUZZ_CORPUS_SIZE )
>>      {
>>          printf("Input too large\n");
>>          return 1;
>> @@ -859,9 +860,9 @@ int LLVMFuzzerTestOneInput(const uint8_t *data_p, size_t 
>> size)
>>  
>>  unsigned int fuzz_minimal_input_size(void)
>>  {
>> -    BUILD_BUG_ON(DATA_OFFSET > INPUT_SIZE);
>> +    BUILD_BUG_ON(DATA_OFFSET > FUZZ_CORPUS_SIZE);
> 
> Thinking more about it, this BUILD_BUG_ON is probably irrelevant
> nowadays because we've opted to use struct fuzz_corpus instead of a
> bunch of data structures (when the fuzzer was first implemented). I
> don't think we will go back to the old model in the future so deleting
> this BUILD_BUG_ON should be fine.

We statically allocate an array of data of size INPUT_SIZE in
afl-harness.c; I think that's the purpose of the BUILD_BUG_ON(), to make
sure that that buffer is always big enough for our minimum file size.

 -George

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