[PATCH 1/1] file capabilities: remove cap_task_kill() (-mmotm)

[Serge Hallyn]

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@xxxxxxxxxx>
Acked-by: Andrew G. Morgan <morgan@xxxxxxxxxx>
Tested-by: Luiz Fernando N. Capitulino <lcapitulino@xxxxxxxxxxxxxxx>
---
 include/linux/security.h   |    3 +--
 security/capability.c      |    1 -
 security/commoncap.c       |   33 ---------------------------------
 security/smack/smack_lsm.c |    5 -----
 4 files changed, 1 insertions(+), 41 deletions(-)

diff --git a/include/linux/security.h b/include/linux/security.h
index 2231526..13fd76a 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -59,7 +59,6 @@ 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_prctl(int option, unsigned long arg2, unsigned long arg3,
 			  unsigned long arg4, unsigned long arg5, long *rc_p);
 extern int cap_task_setscheduler (struct task_struct *p, int policy, struct sched_param *lp);
@@ -2276,7 +2275,7 @@ 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 8340655..38ac54e 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 200361d..e8c3f5e 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -537,34 +537,6 @@ 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)
-{
-	/*
-	 * 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;
-
-	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
@@ -596,11 +568,6 @@ 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
 
 int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 4365fad..2a5eb83 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;
 	/*
 	 * Sending a signal requires that the sender
 	 * can write the receiver.
-- 
1.5.2.5

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