Re: [PATCH 4/9] allow killing tasks in your own or child userns

[ebiederm@xmission.com (Eric W. Biederman)]

"Serge E. Hallyn" <serge@hallyn.com> writes:

> Changelog:
> 	Dec  8: Fixed bug in my check_kill_permission pointed out by
> 	        Eric Biederman.
> 	Dec 13: Apply Eric's suggestion to pass target task into kill_ok_by_cred()
> 	        for clarity
> 	Dec 31: address comment by Eric Biederman:
> 		don't need cred/tcred in check_kill_permission.
> 	Jan  1: use const cred struct.
> 	Jan 11: Per Bastian Blank's advice, clean up kill_ok_by_cred().
> 	Feb 16: kill_ok_by_cred: fix bad parentheses

Acked-by: "Eric W. Biederman" <ebiederm@xmission.com>

>
> Signed-off-by: Serge E. Hallyn <serge.hallyn@canonical.com>
> ---
>  kernel/signal.c |   30 ++++++++++++++++++++++--------
>  1 files changed, 22 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 4e3cff1..ffe4bdf 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -636,13 +636,33 @@ static inline bool si_fromuser(const struct siginfo *info)
>  }
>  
>  /*
> + * called with RCU read lock from check_kill_permission()
> + */
> +static inline int kill_ok_by_cred(struct task_struct *t)
> +{
> +	const struct cred *cred = current_cred();
> +	const struct cred *tcred = __task_cred(t);
> +
> +	if (cred->user->user_ns == tcred->user->user_ns &&
> +	    (cred->euid == tcred->suid ||
> +	     cred->euid == tcred->uid ||
> +	     cred->uid  == tcred->suid ||
> +	     cred->uid  == tcred->uid))
> +		return 1;
> +
> +	if (ns_capable(tcred->user->user_ns, CAP_KILL))
> +		return 1;
> +
> +	return 0;
> +}
> +
> +/*
>   * Bad permissions for sending the signal
>   * - the caller must hold the RCU read lock
>   */
>  static int check_kill_permission(int sig, struct siginfo *info,
>  				 struct task_struct *t)
>  {
> -	const struct cred *cred, *tcred;
>  	struct pid *sid;
>  	int error;
>  
> @@ -656,14 +676,8 @@ static int check_kill_permission(int sig, struct siginfo *info,
>  	if (error)
>  		return error;
>  
> -	cred = current_cred();
> -	tcred = __task_cred(t);
>  	if (!same_thread_group(current, t) &&
> -	    (cred->euid ^ tcred->suid) &&
> -	    (cred->euid ^ tcred->uid) &&
> -	    (cred->uid  ^ tcred->suid) &&
> -	    (cred->uid  ^ tcred->uid) &&
> -	    !capable(CAP_KILL)) {
> +	    !kill_ok_by_cred(t)) {
>  		switch (sig) {
>  		case SIGCONT:
>  			sid = task_session(t);