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

[Paul Semel <semelpaul@xxxxxxxxx>]

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 
> > 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.
Yes, you're right 🙂

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

> Wei.
>
> >   
> > -    return DATA_OFFSET + 1;
> > +    return DATA_OFFSET;
> >   }
> >   
> >   /*
> > --
> > 2.16.1
--
Paul Semel

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