binder: use cred instead of task for selinux checks

Since binder was integrated with selinux, it has passed
'struct task_struct' associated with the binder_proc
to represent the source and target of transactions.
The conversion of task to SID was then done in the hook
implementations. It turns out that there are race conditions
which can result in an incorrect security context being used.

Fix by using the 'struct cred' saved during binder_open and pass
it to the selinux subsystem.

Cc: stable@vger.kernel.org # 5.14 (need backport for earlier stables)
Fixes: 79af73079d75 ("Add security hooks to binder and implement the hooks for SELinux.")
Suggested-by: Jann Horn <jannh@google.com>
Signed-off-by: Todd Kjos <tkjos@google.com>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>

2 files changed, 20 insertions, 42 deletions

diff --git a/security/security.c b/security/security.c
index 40518c340571..d9d53c1e466a 100644
--- a/security/security.c
+++ b/security/security.c
@@ -747,25 +747,25 @@ static int lsm_superblock_alloc(struct super_block *sb)
 
 /* Security operations */
 
-int security_binder_set_context_mgr(struct task_struct *mgr)
+int security_binder_set_context_mgr(const struct cred *mgr)
 {
 	return call_int_hook(binder_set_context_mgr, 0, mgr);
 }
 
-int security_binder_transaction(struct task_struct *from,
-				struct task_struct *to)
+int security_binder_transaction(const struct cred *from,
+				const struct cred *to)
 {
 	return call_int_hook(binder_transaction, 0, from, to);
 }
 
-int security_binder_transfer_binder(struct task_struct *from,
-				    struct task_struct *to)
+int security_binder_transfer_binder(const struct cred *from,
+				    const struct cred *to)
 {
 	return call_int_hook(binder_transfer_binder, 0, from, to);
 }
 
-int security_binder_transfer_file(struct task_struct *from,
-				  struct task_struct *to, struct file *file)
+int security_binder_transfer_file(const struct cred *from,
+				  const struct cred *to, struct file *file)
 {
 	return call_int_hook(binder_transfer_file, 0, from, to, file);
 }
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
index 49d52d7a7714..b4a1bde20261 100644
--- a/security/selinux/hooks.c
+++ b/security/selinux/hooks.c
@@ -255,29 +255,6 @@ static inline u32 task_sid_obj(const struct task_struct *task)
 	return sid;
 }
 
-/*
- * get the security ID of a task for use with binder
- */
-static inline u32 task_sid_binder(const struct task_struct *task)
-{
-	/*
-	 * In many case where this function is used we should be using the
-	 * task's subjective SID, but we can't reliably access the subjective
-	 * creds of a task other than our own so we must use the objective
-	 * creds/SID, which are safe to access.  The downside is that if a task
-	 * is temporarily overriding it's creds it will not be reflected here;
-	 * however, it isn't clear that binder would handle that case well
-	 * anyway.
-	 *
-	 * If this ever changes and we can safely reference the subjective
-	 * creds/SID of another task, this function will make it easier to
-	 * identify the various places where we make use of the task SIDs in
-	 * the binder code.  It is also likely that we will need to adjust
-	 * the main drivers/android binder code as well.
-	 */
-	return task_sid_obj(task);
-}
-
 static int inode_doinit_with_dentry(struct inode *inode, struct dentry *opt_dentry);
 
 /*
@@ -2067,18 +2044,19 @@ static inline u32 open_file_to_av(struct file *file)
 
 /* Hook functions begin here. */
 
-static int selinux_binder_set_context_mgr(struct task_struct *mgr)
+static int selinux_binder_set_context_mgr(const struct cred *mgr)
 {
 	return avc_has_perm(&selinux_state,
-			    current_sid(), task_sid_binder(mgr), SECCLASS_BINDER,
+			    current_sid(), cred_sid(mgr), SECCLASS_BINDER,
 			    BINDER__SET_CONTEXT_MGR, NULL);
 }
 
-static int selinux_binder_transaction(struct task_struct *from,
-				      struct task_struct *to)
+static int selinux_binder_transaction(const struct cred *from,
+				      const struct cred *to)
 {
 	u32 mysid = current_sid();
-	u32 fromsid = task_sid_binder(from);
+	u32 fromsid = cred_sid(from);
+	u32 tosid = cred_sid(to);
 	int rc;
 
 	if (mysid != fromsid) {
@@ -2089,24 +2067,24 @@ static int selinux_binder_transaction(struct task_struct *from,
 			return rc;
 	}
 
-	return avc_has_perm(&selinux_state, fromsid, task_sid_binder(to),
+	return avc_has_perm(&selinux_state, fromsid, tosid,
 			    SECCLASS_BINDER, BINDER__CALL, NULL);
 }
 
-static int selinux_binder_transfer_binder(struct task_struct *from,
-					  struct task_struct *to)
+static int selinux_binder_transfer_binder(const struct cred *from,
+					  const struct cred *to)
 {
 	return avc_has_perm(&selinux_state,
-			    task_sid_binder(from), task_sid_binder(to),
+			    cred_sid(from), cred_sid(to),
 			    SECCLASS_BINDER, BINDER__TRANSFER,
 			    NULL);
 }
 
-static int selinux_binder_transfer_file(struct task_struct *from,
-					struct task_struct *to,
+static int selinux_binder_transfer_file(const struct cred *from,
+					const struct cred *to,
 					struct file *file)
 {
-	u32 sid = task_sid_binder(to);
+	u32 sid = cred_sid(to);
 	struct file_security_struct *fsec = selinux_file(file);
 	struct dentry *dentry = file->f_path.dentry;
 	struct inode_security_struct *isec;