file capabilities: remove cap_task_kill()

The original justification for cap_task_kill() was as follows:

	check_kill_permission() does appropriate uid equivalence checks.
	However with file capabilities it becomes possible for an
	unprivileged user to execute a file with file capabilities
	resulting in a more privileged task with the same uid.

However now that cap_task_kill() always returns 0 (permission
granted) when p->uid==current->uid, the whole hook is worthless,
and only likely to create more subtle problems in the corner cases
where it might still be called but return -EPERM.  Those cases
are basically when uids are different but euid/suid is equivalent
as per the check in check_kill_permission().

One example of a still-broken application is 'at' for non-root users.

This patch removes cap_task_kill().

Signed-off-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Earlier-version-tested-by: Luiz Fernando N. Capitulino <lcapitulino@mandriva.com.br>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

4 files changed, 1 insertions, 48 deletions

diff --git a/include/linux/security.h b/include/linux/security.h
index b07357ca2137..c673dfd4dffc 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -57,7 +57,6 @@ extern int cap_inode_need_killpriv(struct dentry *dentry);
 extern int cap_inode_killpriv(struct dentry *dentry);
 extern int cap_task_post_setuid (uid_t old_ruid, uid_t old_euid, uid_t old_suid, int flags);
 extern void cap_task_reparent_to_init (struct task_struct *p);
-extern int cap_task_kill(struct task_struct *p, struct siginfo *info, int sig, u32 secid);
 extern int cap_task_setscheduler (struct task_struct *p, int policy, struct sched_param *lp);
 extern int cap_task_setioprio (struct task_struct *p, int ioprio);
 extern int cap_task_setnice (struct task_struct *p, int nice);
@@ -2187,7 +2186,7 @@ static inline int security_task_kill (struct task_struct *p,
                                       struct siginfo *info, int sig,
                                       u32 secid)
 {
-        return cap_task_kill(p, info, sig, secid);
+        return 0;
 }
 
 static inline int security_task_wait (struct task_struct *p)
diff --git a/security/capability.c b/security/capability.c
index 9e99f36a8b5c..2c6e06d18fab 100644
--- a/security/capability.c
+++ b/security/capability.c
@@ -40,7 +40,6 @@ static struct security_operations capability_ops = {
         .inode_need_killpriv =          cap_inode_need_killpriv,
         .inode_killpriv =               cap_inode_killpriv,
 
-        .task_kill =                    cap_task_kill,
         .task_setscheduler =            cap_task_setscheduler,
         .task_setioprio =               cap_task_setioprio,
         .task_setnice =                 cap_task_setnice,
diff --git a/security/commoncap.c b/security/commoncap.c
index bb0c095f5761..06d5c9469ba3 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -540,41 +540,6 @@ int cap_task_setnice (struct task_struct *p, int nice)
         return cap_safe_nice(p);
 }
 
-int cap_task_kill(struct task_struct *p, struct siginfo *info,
-                                int sig, u32 secid)
-{
-        if (info != SEND_SIG_NOINFO && (is_si_special(info) || SI_FROMKERNEL(info)))
-                return 0;
-
-        /*
-         * Running a setuid root program raises your capabilities.
-         * Killing your own setuid root processes was previously
-         * allowed.
-         * We must preserve legacy signal behavior in this case.
-         */
-        if (p->uid == current->uid)
-                return 0;
-
-        /* sigcont is permitted within same session */
-        if (sig == SIGCONT && (task_session_nr(current) == task_session_nr(p)))
-                return 0;
-
-        if (secid)
-                /*
-                 * Signal sent as a particular user.
-                 * Capabilities are ignored.  May be wrong, but it's the
-                 * only thing we can do at the moment.
-                 * Used only by usb drivers?
-                 */
-                return 0;
-        if (cap_issubset(p->cap_permitted, current->cap_permitted))
-                return 0;
-        if (capable(CAP_KILL))
-                return 0;
-
-        return -EPERM;
-}
-
 /*
  * called from kernel/sys.c for prctl(PR_CABSET_DROP)
  * done without task_capability_lock() because it introduces
@@ -605,11 +570,6 @@ int cap_task_setnice (struct task_struct *p, int nice)
 {
         return 0;
 }
-int cap_task_kill(struct task_struct *p, struct siginfo *info,
-                                int sig, u32 secid)
-{
-        return 0;
-}
 #endif
 
 void cap_task_reparent_to_init (struct task_struct *p)
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 38d707593b31..732ba27923c4 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -1117,11 +1117,6 @@ static int smack_task_movememory(struct task_struct *p)
 static int smack_task_kill(struct task_struct *p, struct siginfo *info,
                            int sig, u32 secid)
 {
-        int rc;
-
-        rc = cap_task_kill(p, info, sig, secid);
-        if (rc != 0)
-                return rc;
         /*
          * Special cases where signals really ought to go through
          * in spite of policy. Stephen Smalley suggests it may