Re: [PATCH] Smack: ignore null signal in smack_task_kill

[Casey Schaufler]

On 4/4/2016 2:14 AM, Rafal Krypa wrote:
> Kill with signal number 0 is commonly used for checking PID existence.
> Smack treated such cases like any other kills, although no signal is
> actually delivered when sig == 0.
>
> Checking permissions when sig == 0 didn't prevent an unprivileged caller
> from learning whether PID exists or not. When it existed, kernel returned
> EPERM, when it didn't - ESRCH. The only effect of policy check in such
> case is noise in audit logs.
>
> This change lets Smack silently ignore kill() invocations with sig == 0.
>
> Signed-off-by: Rafal Krypa <r.krypa@xxxxxxxxxxx>

Acked-by: Casey Schaufler <casey@xxxxxxxxxxxxxxxx>

Applied to git://github.com/cschaufler/smack-next.git#smack-for-4.8

> ---
>  security/smack/smack_lsm.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
> index 11f7901..3d9bbed 100644
> --- a/security/smack/smack_lsm.c
> +++ b/security/smack/smack_lsm.c
> @@ -2227,6 +2227,9 @@ static int smack_task_kill(struct task_struct *p, struct siginfo *info,
>  	struct smack_known *tkp = smk_of_task_struct(p);
>  	int rc;
>  
> +	if (!sig)
> +		return 0; /* null signal; existence test */
> +
>  	smk_ad_init(&ad, __func__, LSM_AUDIT_DATA_TASK);
>  	smk_ad_setfield_u_tsk(&ad, p);
>  	/*