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

On 02/23/2018 05:30 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?

Sure, I confused myself on this one, sorry about it !

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 
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 
      /* 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 
unsigned int fuzz_minimal_input_size(void)

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.

Yes, you're right 🙂

I'm going to send my last version of this patch so that you can have it !


- return DATA_OFFSET + 1;
+    return DATA_OFFSET;

Paul Semel

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