Re: Binder regression caused by commit a43cfc87caaf

[Ondrej Mosnacek]

On Mon, Aug 8, 2022 at 9:52 PM Liam Howlett <liam.howlett@xxxxxxxxxx> wrote:
> * Liam R. Howlett <Liam.Howlett@xxxxxxxxxx> [220808 11:07]:
> > * Ondrej Mosnacek <omosnace@xxxxxxxxxx> [220808 06:13]:
> > > Hello,
> > >
> > > FYI, since commit a43cfc87caaf ("android: binder: stop saving a
> > > pointer to the VMA") (found by git bisect) the binder test in
> > > selinux-testsuite [1] started to trigger a lockdep assert BUG() in
> > > find_vma() - see the end of [2] for an example.
> > >
> > > A minimal reproducer is:
> > > ```
> > > git clone https://github.com/SELinuxProject/selinux-testsuite.git
> > > cd selinux-testsuite/tests/binder
> > > make
> > > setenforce 0  # if SELinux is enabled
> > > ./init_binder.sh || true
> > > ./manager -n -v & sleep 2
> > > ./service_provider -n -v
> > > ```
> > > Requires the equivalent of libselinux-devel, make, gcc, and git-core
> > > Fedora packages.
> > > The last command will trigger the BUG; on good kernels it will
> > > successfully enter the ioctl loop.
> > >
> > > [1] https://github.com/SELinuxProject/selinux-testsuite/
> > > [2] https://s3.us-east-1.amazonaws.com/arr-cki-prod-datawarehouse-public/datawarehouse-public/2022/08/07/redhat:606549366/build_x86_64_redhat:606549366_x86_64/tests/5/results_0001/console.log/console.log
> > >
> >
> > Thanks.  It looks like binder has some paths that are not taking the
> > necessary mmap lock for using VMAs.  I'm looking into it now.
>
> This does not fail for me, are you sure this is the reproducer?  I see
> the manager and service_provider communicate.
>
> Looking at your trace and the code, the bug makes sense and I have
> something that will probably fix the issue, but I'd like to verify.

Hm... it seems it is necessary to have CONFIG_DEBUG_VM=y for the
particular debug check to be active. I guess you don't have that
turned on? It happens to be turned on in Fedora's release kernel, so I
didn't realize there was a specific config dependency.

Thank you for looking into it!

-- 
Ondrej Mosnacek
Senior Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.