security: Fix setting of PF_SUPERPRIV by __capable()

Fix the setting of PF_SUPERPRIV by __capable() as it could corrupt the flags
the target process if that is not the current process and it is trying to
change its own flags in a different way at the same time.

__capable() is using neither atomic ops nor locking to protect t->flags.  This
patch removes __capable() and introduces has_capability() that doesn't set
PF_SUPERPRIV on the process being queried.

This patch further splits security_ptrace() in two:

 (1) security_ptrace_may_access().  This passes judgement on whether one
     process may access another only (PTRACE_MODE_ATTACH for ptrace() and
     PTRACE_MODE_READ for /proc), and takes a pointer to the child process.
     current is the parent.

 (2) security_ptrace_traceme().  This passes judgement on PTRACE_TRACEME only,
     and takes only a pointer to the parent process.  current is the child.

     In Smack and commoncap, this uses has_capability() to determine whether
     the parent will be permitted to use PTRACE_ATTACH if normal checks fail.
     This does not set PF_SUPERPRIV.

Two of the instances of __capable() actually only act on current, and so have
been changed to calls to capable().

Of the places that were using __capable():

 (1) The OOM killer calls __capable() thrice when weighing the killability of a
     process.  All of these now use has_capability().

 (2) cap_ptrace() and smack_ptrace() were using __capable() to check to see
     whether the parent was allowed to trace any process.  As mentioned above,
     these have been split.  For PTRACE_ATTACH and /proc, capable() is now
     used, and for PTRACE_TRACEME, has_capability() is used.

 (3) cap_safe_nice() only ever saw current, so now uses capable().

 (4) smack_setprocattr() rejected accesses to tasks other than current just
     after calling __capable(), so the order of these two tests have been
     switched and capable() is used instead.

 (5) In smack_file_send_sigiotask(), we need to allow privileged processes to
     receive SIGIO on files they're manipulating.

 (6) In smack_task_wait(), we let a process wait for a privileged process,
     whether or not the process doing the waiting is privileged.

I've tested this with the LTP SELinux and syscalls testscripts.

Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Serge Hallyn <serue@us.ibm.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Acked-by: Andrew G. Morgan <morgan@kernel.org>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 34 insertions, 15 deletions

diff --git a/security/smack/smack_lsm.c b/security/smack/smack_lsm.c
index 1b40e558f98..87d75417ea9 100644
--- a/security/smack/smack_lsm.c
+++ b/security/smack/smack_lsm.c
@@ -87,27 +87,46 @@ struct inode_smack *new_inode_smack(char *smack)
  */
 
 /**
- * smack_ptrace - Smack approval on ptrace
- * @ptp: parent task pointer
+ * smack_ptrace_may_access - Smack approval on PTRACE_ATTACH
  * @ctp: child task pointer
  *
  * Returns 0 if access is OK, an error code otherwise
  *
  * Do the capability checks, and require read and write.
  */
-static int smack_ptrace(struct task_struct *ptp, struct task_struct *ctp,
-			unsigned int mode)
+static int smack_ptrace_may_access(struct task_struct *ctp, unsigned int mode)
 {
 	int rc;
 
-	rc = cap_ptrace(ptp, ctp, mode);
+	rc = cap_ptrace_may_access(ctp, mode);
 	if (rc != 0)
 		return rc;
 
-	rc = smk_access(ptp->security, ctp->security, MAY_READWRITE);
-	if (rc != 0 && __capable(ptp, CAP_MAC_OVERRIDE))
+	rc = smk_access(current->security, ctp->security, MAY_READWRITE);
+	if (rc != 0 && capable(CAP_MAC_OVERRIDE))
 		return 0;
+	return rc;
+}
+
+/**
+ * smack_ptrace_traceme - Smack approval on PTRACE_TRACEME
+ * @ptp: parent task pointer
+ *
+ * Returns 0 if access is OK, an error code otherwise
+ *
+ * Do the capability checks, and require read and write.
+ */
+static int smack_ptrace_traceme(struct task_struct *ptp)
+{
+	int rc;
+
+	rc = cap_ptrace_traceme(ptp);
+	if (rc != 0)
+		return rc;
 
+	rc = smk_access(ptp->security, current->security, MAY_READWRITE);
+	if (rc != 0 && has_capability(ptp, CAP_MAC_OVERRIDE))
+		return 0;
 	return rc;
 }
 
@@ -923,7 +942,7 @@ static int smack_file_send_sigiotask(struct task_struct *tsk,
 	 */
 	file = container_of(fown, struct file, f_owner);
 	rc = smk_access(file->f_security, tsk->security, MAY_WRITE);
-	if (rc != 0 && __capable(tsk, CAP_MAC_OVERRIDE))
+	if (rc != 0 && has_capability(tsk, CAP_MAC_OVERRIDE))
 		return 0;
 	return rc;
 }
@@ -1164,12 +1183,12 @@ static int smack_task_wait(struct task_struct *p)
 	 * account for the smack labels having gotten to
 	 * be different in the first place.
 	 *
-	 * This breaks the strict subjet/object access
+	 * This breaks the strict subject/object access
 	 * control ideal, taking the object's privilege
 	 * state into account in the decision as well as
 	 * the smack value.
 	 */
-	if (capable(CAP_MAC_OVERRIDE) || __capable(p, CAP_MAC_OVERRIDE))
+	if (capable(CAP_MAC_OVERRIDE) || has_capability(p, CAP_MAC_OVERRIDE))
 		return 0;
 
 	return rc;
@@ -2016,9 +2035,6 @@ static int smack_setprocattr(struct task_struct *p, char *name,
 {
 	char *newsmack;
 
-	if (!__capable(p, CAP_MAC_ADMIN))
-		return -EPERM;
-
 	/*
 	 * Changing another process' Smack value is too dangerous
 	 * and supports no sane use case.
@@ -2026,6 +2042,9 @@ static int smack_setprocattr(struct task_struct *p, char *name,
 	if (p != current)
 		return -EPERM;
 
+	if (!capable(CAP_MAC_ADMIN))
+		return -EPERM;
+
 	if (value == NULL || size == 0 || size >= SMK_LABELLEN)
 		return -EINVAL;
 
@@ -2552,7 +2571,8 @@ static void smack_release_secctx(char *secdata, u32 seclen)
 struct security_operations smack_ops = {
 	.name =				"smack",
 
-	.ptrace = 			smack_ptrace,
+	.ptrace_may_access =		smack_ptrace_may_access,
+	.ptrace_traceme =		smack_ptrace_traceme,
 	.capget = 			cap_capget,
 	.capset_check = 		cap_capset_check,
 	.capset_set = 			cap_capset_set,
@@ -2729,4 +2749,3 @@ static __init int smack_init(void)
  * all processes and objects when they are created.
  */
 security_initcall(smack_init);
-