Smack: ignore null signal in smack_task_kill

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@samsung.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>

1 files changed, 3 insertions, 0 deletions

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 6777295f4b2b..e96080eaacbb 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);
         /*