BACKPORT: 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>
(cherry-picked from upstream 18d872f77cecec2677a394170f26aaeb08562cee)

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 6ab31c4d7cc238900c9833be030731772898ca13..1dfc9f1700ab20c37d70eb0e56e6a57570f22ade 100644 (file)
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -2234,6 +2234,9 @@ 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);
        /*