Re: [PATCH] s390: fix double free of GS and RI CBs on fork() failure

[Vasily Gorbik]

On Wed, Aug 24, 2022 at 10:01:00AM +0200, Heiko Carstens wrote:
> On Tue, Aug 16, 2022 at 11:54:07AM -0400, Brian Foster wrote:
> > The pointers for guarded storage and runtime instrumentation control
> > blocks are stored in the thread_struct of the associated task. These
> > pointers are initially copied on fork() via arch_dup_task_struct()
> > and then cleared via copy_thread() before fork() returns. If fork()
> > happens to fail after the initial task dup and before copy_thread(),
> > the newly allocated task and associated thread_struct memory are
> > freed via free_task() -> arch_release_task_struct(). This results in
> > a double free of the guarded storage and runtime info structs
> > because the fields in the failed task still refer to memory
> > associated with the source task.
> > 
> > This problem can manifest as a BUG_ON() in set_freepointer() (with
> > CONFIG_SLAB_FREELIST_HARDENED enabled) or KASAN splat (if enabled)
> > when running trinity syscall fuzz tests on s390x. To avoid this
> > problem, clear the associated pointer fields in
> > arch_dup_task_struct() immediately after the new task is copied.
> > Note that the RI flag is still cleared in copy_thread() because it
> > resides in thread stack memory and that is where stack info is
> > copied.
> > 
> > Signed-off-by: Brian Foster <bfoster@xxxxxxxxxx>
> > ---
> >  arch/s390/kernel/process.c | 22 ++++++++++++++++------
> >  1 file changed, 16 insertions(+), 6 deletions(-)
> 
> Reviewed-by: Heiko Carstens <hca@xxxxxxxxxxxxx>
> 
> Thanks for debugging and fixing this!
> 
> Vasily, could you pick this one up, and also add the Fixes: and Cc:
> stable tags as proposed by Gerald?
> Thanks!

Applied, Thank you!