Subject: Re: [PATCH 3/3] bpf: Make sure that ->comm does not change under us.

From: Daniel Borkmann <[email-masked]>
Date: Mon, 16 Oct 2017 22:50:43 +0200

On 10/16/2017 08:18 PM, Richard Weinberger wrote:
> Sadly we cannot use get_task_comm() since bpf_get_current_comm()
> allows truncation.
>
> Signed-off-by: Richard Weinberger <[email-masked]>
> ---
>   kernel/bpf/helpers.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
> index 511c9d522cfc..4b042b24524d 100644
> --- a/kernel/bpf/helpers.c
> +++ b/kernel/bpf/helpers.c
> @@ -18,6 +18,7 @@
>   #include <linux/sched.h>
>   #include <linux/uidgid.h>
>   #include <linux/filter.h>
> +#include <linux/sched/task.h>
>
>   /* If kernel subsystem is allowing eBPF programs to call this function,
>    * inside its own verifier_ops->get_func_proto() callback it should return
> @@ -149,7 +150,9 @@ BPF_CALL_2(bpf_get_current_comm, char *, buf, u32, size)
>   {
>   	struct task_struct *task = current;
>
> +	task_lock(task);
>   	strncpy(buf, task->comm, size);
> +	task_unlock(task);

Wouldn't this potentially lead to a deadlock? E.g. you attach yourself
to task_lock() / spin_lock() / etc, and then the BPF prog triggers the
bpf_get_current_comm() taking the lock again ...

>   	/* Verifier guarantees that size > 0. For task->comm exceeding
>   	 * size, guarantee that buf is %NUL-terminated. Unconditionally
>