Re: [PATCH bpf v2] bpf: Fix UAF in task local storage

[Martin KaFai Lau <martin.lau@linux.dev>]

On 6/1/23 5:26 PM, KP Singh wrote:
> When task local storage was generalized for tracing programs, the
> bpf_task_local_storage callback was moved from a BPF LSM hook
> callback for security_task_free LSM hook to it's own callback. But a
> failure case in bad_fork_cleanup_security was missed which, when
> triggered, led to a dangling task owner pointer and a subsequent
> use-after-free. Move the bpf_task_storage_free to the very end of
> free_task to handle all failure cases.
> 
> This issue was noticed when a BPF LSM program was attached to the
> task_alloc hook on a kernel with KASAN enabled. The program used
> bpf_task_storage_get to copy the task local storage from the current
> task to the new task being created.
> 
> Fixes: a10787e6d58c ("bpf: Enable task local storage for tracing programs")
> Reported-by: Kuba Piecuch <jpiecuch@google.com>
> Signed-off-by: KP Singh <kpsingh@kernel.org>
> ---
> 
> * v1 -> v2
> 
> Move the bpf_task_storage_free to free_task as suggested by Martin
> 
>   kernel/fork.c | 2 +-
>   1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index ed4e01daccaa..cb20f9f596d3 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -627,6 +627,7 @@ void free_task(struct task_struct *tsk)
>   	arch_release_task_struct(tsk);
>   	if (tsk->flags & PF_KTHREAD)
>   		free_kthread_struct(tsk);
> +	bpf_task_storage_free(tsk);

Applied. Thanks for the fix.

A followup question, does it need a "notrace" to be added to 
bpf_task_storage_free to ensure no task storage can be recreated?

>   	free_task_struct(tsk);
>   }
>   EXPORT_SYMBOL(free_task);
> @@ -979,7 +980,6 @@ void __put_task_struct(struct task_struct *tsk)
>   	cgroup_free(tsk);
>   	task_numa_free(tsk, true);
>   	security_task_free(tsk);
> -	bpf_task_storage_free(tsk);
>   	exit_creds(tsk);
>   	delayacct_tsk_free(tsk);
>   	put_signal_struct(tsk->signal);