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

[Alexander Bulekov]

On 210118 1334, Christian Schoenebeck wrote:
> On Montag, 18. Januar 2021 00:09:24 CET 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.
> 
> Yes, dealing with signal handlers for that is probably a bit intransparent 
> and 
> would leave a questionable feeling about its reliability.
> 
> What about __attribute__((destructor)) to auto delete the fuzzer directory, 
> like virtio-9p-test.c does for the same task?
> 

My worry is that we will end up deleting it while it is still in use.
The scenario I am worried about:
[parent process ] set up temp directory for virtio-9p
[parent process ] initialize QEMU 
[parent process ] fork() and wait()
[child process 1] Run the fuzzing input.
[child process 1] Once the input has been executed, call _Exit(). This
should skip the atexit()/__attribute((destructor)) handlers. For reasons
related to libfuzzer, we need to do this so that libfuzzer doesn't treat
each child exit()-ing as a crash.
[parent process ] wait() returns.
[parent process ] generate a new input.. fork() and wait()
[child process 2] Run the fuzzing input.
[child process 2] Somewhere we hit an abort(). libfuzzer hooks the abort
and dumps the crashing input and stack trace. Since abort() doesn't call
exit handlers, it will skip over atexit()/__attribute((destructor)) handlers
[parent process ] wait() returns.
[parent process ] generate a new input.. fork() and wait()
[child process 3] Run the fuzzing input.
[child process 3] Somewhere we hit an exit(). This will dump the
input/stacktrace and it will run the exit handlers (removing the shared
9p directory)
[parent process ] wait() returns. generate a new input.. fork() and wait()
[child process 4] ...

Now all the subsequent forked children will refer to a shared directory
that we already removed. Ideally, we would run the cleanup handler only
after the parent process exit()s. I think there are some ways to do
this, by placing the atexit() call in a place only reachable by the
parent. However, on OSS-Fuzz this shouldn't be a problem as everything
is cleaned up automatically anyway..
I am more worried about the fact that files/directories/links that are
created by 9p in the child processes, persist across inputs. I think
Thomas suggested a way to work-around this for PATCH 1/3. We could have
a function that runs in the parent after each wait() returns, that would
remove all the files in the temp directory and scrub the extended
attributes applied by 9p to the shared dir.
-Alex


> >  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 "
> 
> 
>