Re: [PATCH v2 3/3] fuzz: add virtio-9p configurations for fuzzing

[Darren Kenny]

On Sunday, 2021-01-17 at 18:09:24 -05, Alexander Bulekov wrote:
> virtio-9p devices are often used to expose a virtual-filesystem to the
> guest. There have been some bugs reported in this device, such as
> CVE-2018-19364, and CVE-2021-20181. We should fuzz this device
>
> This patch adds two virtio-9p configurations:
>  * One with the widely used -fsdev local driver. This driver leaks some
>    state in the form of files/directories created in the shared dir.
>  * One with the synth driver. While it is not used in the real world, this
>    driver won't leak leak state between fuzz inputs.
>
> Signed-off-by: Alexander Bulekov <alxndr@bu.edu>
> ---
> CC: Christian Schoenebeck <qemu_oss@crudebyte.com>
> CC: Greg Kurz <groug@kaod.org>
>
> I considered adding an atexit handler to remove the temp directory,
> however I am worried that there might be some error that results in a
> call to exit(), rather than abort(), which will cause problems for
> future fork()-ed fuzzers. I don't think there are such calls in the 9p
> code, however there might be something in the APIs used by 9p. As this
> code is primarily for ephemeral OSS-Fuzz conainers, this shouldn't be
> too much of an issue.

As I understand it, this creation of a new directory should happen a lot
less than the amount of actual executions of the target, since it is
only run on the first 'parent' target process executed, prior to the
process cloning operations that the fork execution performs - or any
time that 'parent' target process is renewed, after several thousand
cloned processes.

Is that correct? Or am I misunderstanding when the
generic_fuzzer_virtio_9p_args() function is run?

Thanks,

Darren.

>  tests/qtest/fuzz/generic_fuzz_configs.h | 20 ++++++++++++++++++++
>  1 file changed, 20 insertions(+)
>
> diff --git a/tests/qtest/fuzz/generic_fuzz_configs.h 
> b/tests/qtest/fuzz/generic_fuzz_configs.h
> index 1a133655ee..f99657cdbc 100644
> --- a/tests/qtest/fuzz/generic_fuzz_configs.h
> +++ b/tests/qtest/fuzz/generic_fuzz_configs.h
> @@ -19,6 +19,16 @@ typedef struct generic_fuzz_config {
>      gchar* (*argfunc)(void); /* Result must be freeable by g_free() */
>  } generic_fuzz_config;
>  
> +static inline gchar *generic_fuzzer_virtio_9p_args(void){
> +    char tmpdir[] = "/tmp/qemu-fuzz.XXXXXX";
> +    g_assert_nonnull(mkdtemp(tmpdir));
> +
> +    return g_strdup_printf("-machine q35 -nodefaults "
> +    "-device virtio-9p,fsdev=hshare,mount_tag=hshare "
> +    "-fsdev local,id=hshare,path=%s,security_model=mapped-xattr,"
> +    "writeout=immediate,fmode=0600,dmode=0700", tmpdir);
> +}
> +
>  const generic_fuzz_config predefined_configs[] = {
>      {
>          .name = "virtio-net-pci-slirp",
> @@ -60,6 +70,16 @@ const generic_fuzz_config predefined_configs[] = {
>          .name = "virtio-mouse",
>          .args = "-machine q35 -nodefaults -device virtio-mouse",
>          .objects = "virtio*",
> +    },{
> +        .name = "virtio-9p",
> +        .argfunc = generic_fuzzer_virtio_9p_args,
> +        .objects = "virtio*",
> +    },{
> +        .name = "virtio-9p-synth",
> +        .args = "-machine q35 -nodefaults "
> +        "-device virtio-9p,fsdev=hshare,mount_tag=hshare "
> +        "-fsdev synth,id=hshare",
> +        .objects = "virtio*",
>      },{
>          .name = "e1000",
>          .args = "-M q35 -nodefaults "
> -- 
> 2.28.0