Re: linux 5.14.3: free_user_ns causes NULL pointer dereference

[Yu Zhao]

On Mon, Oct 4, 2021 at 11:19 AM Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote:
>
> ebiederm@xxxxxxxxxxxx (Eric W. Biederman) writes:
>
> > Adding Rune Kleveland to the discussion as he also seems to have
> > reproduced the issue.
> >
> > Alex and I have been starring at the code and the reports and this
> > bug is hiding well.  Here is what we have figured out so far.
> >
> > Both the warning from free_user_ns calling dec_ucount that Jordan Glover
> > reported and the KASAN error that Yu Zhao has reported appear to have
> > the same cause.  Using a ucounts structure after it has been freed and
> > reallocated as something else.
> >
> > I have just skimmed through the recent report from Rune Kleveland
> > and it appears also to be a use after free.  Especially since the
> > second failure in the log is slub complaining about trying to free
> > the ucounts data structure.
> >
> > We looked through the users of put_ucounts and we don't see any obvious
> > buggy users that would be freeing the data structure early.
> >
> > Alex has tried to reproduce this so far is not having any luck.
> > Folks can you tell what compiler versions you are using and share your
> > kernel config with us?  That might help.

Thanks. Per your request:

$ aarch64-cros-linux-gnu-clang -v
Chromium OS 12.0_pre422132_p20210405-r9 clang version 13.0.0
(/var/tmp/portage/sys-devel/llvm-12.0_pre422132_p20210405-r9/work/llvm-12.0_pre422132_p20210405/clang
cd442157cff4aad209ae532cbf031abbe10bc1df)
Target: aarch64-cros-linux-gnu
Thread model: posix
InstalledDir: /usr/bin
Found candidate GCC installation:
/usr/bin/../lib/gcc/aarch64-cros-linux-gnu/10.2.0
Found candidate GCC installation:
/usr/bin/../lib64/gcc/aarch64-cros-linux-gnu/10.2.0
Selected GCC installation: /usr/bin/../lib64/gcc/aarch64-cros-linux-gnu/10.2.0
Candidate multilib: .;@m64
Selected multilib: .;@m64

Kernel config attached.

Attachment: kernel.config
Description: Binary data