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

[Eric W. Biederman]

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

> Changelog:
> 	Dec 8: Fixed bug in my check_kill_permission pointed out by
> 	       Eric Biederman.
>
> To test:
> 	1. Test killing tasks as usual.  No change.
> 	2. Clone a new user namespace without a new pidns.
> 	   a. You CAN kill -CONT tasks in your thread group but outside
> 	      your user ns.
> 	   b. You can NOT otherwise kill tasks outside your user_ns.
> 	   c. Inside your new userns, signal semantics are as normal
> 	      with respect to userids, CAP_KILL, and thread groups.
>
> Signed-off-by: Serge E. Hallyn <serge.hallyn at canonical.com>
> ---
>  kernel/signal.c |   27 ++++++++++++++++++++++-----
>  1 files changed, 22 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/signal.c b/kernel/signal.c
> index 4e3cff1..677025c 100644
> --- a/kernel/signal.c
> +++ b/kernel/signal.c
> @@ -635,6 +635,27 @@ static inline bool si_fromuser(const struct siginfo *info)
>  		(!is_si_special(info) && SI_FROMUSER(info));
>  }
>  
> +static inline int kill_ok_by_cred(struct cred *cred, struct cred *tcred)
> +{
Nit: You should just pass in the target task here.
Making it abundantly clear where current and tcred come from.
ns_capable implicitly uses current which is a little surprising
when everything else is being passed in, but makes perfect sense
in this context.

> +	if (cred->user->user_ns != tcred->user->user_ns) {
> +		/* userids are not equivalent - either you have the
> +		   capability to the target user ns or you don't */
> +		if (ns_capable(tcred->user->user_ns, CAP_KILL))
> +			return 1;
> +		return 0;
> +	}
> +
> +	/* same user namespace - usual credentials checks apply */
> +	if ((cred->euid ^ tcred->suid) &&
> +	    (cred->euid ^ tcred->uid) &&
> +	    (cred->uid  ^ tcred->suid) &&
> +	    (cred->uid  ^ tcred->uid) &&
> +	    !ns_capable(tcred->user->user_ns, CAP_KILL))
> +		return 0;
> +
> +	return 1;
> +}
> +
>  /*
>   * Bad permissions for sending the signal
>   * - the caller must hold the RCU read lock
> @@ -659,11 +680,7 @@ static int check_kill_permission(int sig, struct siginfo *info,
>  	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(cred, tcred)) {
>  		switch (sig) {
>  		case SIGCONT:
>  			sid = task_session(t);
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