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

[Heiko Carstens <hca@linux.ibm.com>]

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@redhat.com>
> ---
>  arch/s390/kernel/process.c | 22 ++++++++++++++++------
>  1 file changed, 16 insertions(+), 6 deletions(-)

Reviewed-by: Heiko Carstens <hca@linux.ibm.com>

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!