[PATCH v2] selinux: no recursive read_lock of policy_rwlock in security_genfs_sid()

[Waiman Long]

v1->v2:
 - Add an internal helper to switch on/off lock acquisition instead
   of modifying the external API.

With introduction of fair queued rwlock, recursive read_lock() may hang
the offending process if there is a write_lock() somewhere in between.

With recursive read_lock checking enabled, the following error was
reported:

=============================================
[ INFO: possible recursive locking detected ]
3.16.0-rc1 #2 Tainted: G            E
---------------------------------------------
load_policy/708 is trying to acquire lock:
 (policy_rwlock){.+.+..}, at: [<ffffffff8125b32a>]
security_genfs_sid+0x3a/0x170

but task is already holding lock:
 (policy_rwlock){.+.+..}, at: [<ffffffff8125b48c>]
security_fs_use+0x2c/0x110

other info that might help us debug this:
 Possible unsafe locking scenario:

       CPU0
       ----
  lock(policy_rwlock);
  lock(policy_rwlock);

This patch fixes the occurrence of recursive read_lock() of
policy_rwlock in security_genfs_sid() by adding a helper function
which has a 5th argument to indicate if the rwlock has been taken.

Signed-off-by: Waiman Long <Waiman.Long@xxxxxx>
---
 security/selinux/ss/services.c |   36 ++++++++++++++++++++++++++++--------
 1 files changed, 28 insertions(+), 8 deletions(-)

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 4bca494..5f4c1f3 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -2277,20 +2277,22 @@ out:
 }
 
 /**
- * security_genfs_sid - Obtain a SID for a file in a filesystem
+ * __security_genfs_sid - Helper to obtain a SID for a file in a filesystem
  * @fstype: filesystem type
  * @path: path from root of mount
  * @sclass: file security class
  * @sid: SID for path
+ * @locked: true if policy_rwlock taken
  *
  * Obtain a SID to use for a file in a filesystem that
  * cannot support xattr or use a fixed labeling behavior like
  * transition SIDs or task SIDs.
  */
-int security_genfs_sid(const char *fstype,
-		       char *path,
-		       u16 orig_sclass,
-		       u32 *sid)
+static inline int __security_genfs_sid(const char *fstype,
+				       char *path,
+				       u16 orig_sclass,
+				       u32 *sid,
+				       int locked)
 {
 	int len;
 	u16 sclass;
@@ -2301,7 +2303,8 @@ int security_genfs_sid(const char *fstype,
 	while (path[0] == '/' && path[1] == '/')
 		path++;
 
-	read_lock(&policy_rwlock);
+	if (!locked)
+		read_lock(&policy_rwlock);
 
 	sclass = unmap_class(orig_sclass);
 	*sid = SECINITSID_UNLABELED;
@@ -2336,11 +2339,27 @@ int security_genfs_sid(const char *fstype,
 	*sid = c->sid[0];
 	rc = 0;
 out:
-	read_unlock(&policy_rwlock);
+	if (!locked)
+		read_unlock(&policy_rwlock);
 	return rc;
 }
 
 /**
+ * security_genfs_sid - Obtain a SID for a file in a filesystem
+ * @fstype: filesystem type
+ * @path: path from root of mount
+ * @sclass: file security class
+ * @sid: SID for path
+ */
+int security_genfs_sid(const char *fstype,
+		       char *path,
+		       u16 orig_sclass,
+		       u32 *sid)
+{
+	return __security_genfs_sid(fstype, path, orig_sclass, sid, false);
+}
+
+/**
  * security_fs_use - Determine how to handle labeling for a filesystem.
  * @sb: superblock in question
  */
@@ -2370,7 +2389,8 @@ int security_fs_use(struct super_block *sb)
 		}
 		sbsec->sid = c->sid[0];
 	} else {
-		rc = security_genfs_sid(fstype, "/", SECCLASS_DIR, &sbsec->sid);
+		rc = __security_genfs_sid(fstype, "/", SECCLASS_DIR,
+					  &sbsec->sid, true);
 		if (rc) {
 			sbsec->behavior = SECURITY_FS_USE_NONE;
 			rc = 0;
-- 
1.7.1

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