[PATCH] Smack: ignore null signal in smack_task_kill

[Rafal Krypa]

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>
---
 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);
 	/*
-- 
2.8.0.rc3