Revert "exec: Add exec_update_mutex to replace cred_guard_mutex"

This reverts commit b796d94921 which is
commit eea9673250db4e854e9998ef9da6d4584857f0ea.

It breaks the API and we don't need it right now in the 5.4 tree.

Bug: 161946584
Change-Id: I28f98480e9f4a8599b7cfe887401d7515bf5360e
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>

fs/exec.c

@@ -1007,26 +1007,16 @@ ssize_t read_code(struct file *file, unsigned long addr, loff_t pos, size_t len)
 }
 EXPORT_SYMBOL(read_code);
 
-/*
- * Maps the mm_struct mm into the current task struct.
- * On success, this function returns with the mutex
- * exec_update_mutex locked.
- */
 static int exec_mmap(struct mm_struct *mm)
 {
 	struct task_struct *tsk;
 	struct mm_struct *old_mm, *active_mm;
-	int ret;
 
 	/* Notify parent that we're no longer interested in the old VM */
 	tsk = current;
 	old_mm = current->mm;
 	exec_mm_release(tsk, old_mm);
 
-	ret = mutex_lock_killable(&tsk->signal->exec_update_mutex);
-	if (ret)
-		return ret;
-
 	if (old_mm) {
 		sync_mm_rss(old_mm);
 		/*
@@ -1038,11 +1028,9 @@ static int exec_mmap(struct mm_struct *mm)
 		down_read(&old_mm->mmap_sem);
 		if (unlikely(old_mm->core_state)) {
 			up_read(&old_mm->mmap_sem);
-			mutex_unlock(&tsk->signal->exec_update_mutex);
 			return -EINTR;
 		}
 	}
-
 	task_lock(tsk);
 	active_mm = tsk->active_mm;
 	membarrier_exec_mmap(mm);
@@ -1297,12 +1285,11 @@ int flush_old_exec(struct linux_binprm * bprm)
 		goto out;
 
 	/*
-	 * After setting bprm->called_exec_mmap (to mark that current is
-	 * using the prepared mm now), we have nothing left of the original
+	 * After clearing bprm->mm (to mark that current is using the
+	 * prepared mm now), we have nothing left of the original
 	 * process. If anything from here on returns an error, the check
 	 * in search_binary_handler() will SEGV current.
 	 */
-	bprm->called_exec_mmap = 1;
 	bprm->mm = NULL;
 
 	set_fs(USER_DS);
@@ -1436,8 +1423,6 @@ static void free_bprm(struct linux_binprm *bprm)
 {
 	free_arg_pages(bprm);
 	if (bprm->cred) {
-		if (bprm->called_exec_mmap)
-			mutex_unlock(&current->signal->exec_update_mutex);
 		mutex_unlock(&current->signal->cred_guard_mutex);
 		abort_creds(bprm->cred);
 	}
@@ -1487,7 +1472,6 @@ void install_exec_creds(struct linux_binprm *bprm)
 	 * credentials; any time after this it may be unlocked.
 	 */
 	security_bprm_committed_creds(bprm);
-	mutex_unlock(&current->signal->exec_update_mutex);
 	mutex_unlock(&current->signal->cred_guard_mutex);
 }
 EXPORT_SYMBOL(install_exec_creds);
@@ -1679,7 +1663,7 @@ int search_binary_handler(struct linux_binprm *bprm)
 
 		read_lock(&binfmt_lock);
 		put_binfmt(fmt);
-		if (retval < 0 && bprm->called_exec_mmap) {
+		if (retval < 0 && !bprm->mm) {
 			/* we got to flush_old_exec() and failed after it */
 			read_unlock(&binfmt_lock);
 			force_sigsegv(SIGSEGV);

include/linux/binfmts.h

@@ -44,13 +44,7 @@ struct linux_binprm {
 		 * exec has happened. Used to sanitize execution environment
 		 * and to set AT_SECURE auxv for glibc.
 		 */
-		secureexec:1,
-		/*
-		 * Set by flush_old_exec, when exec_mmap has been called.
-		 * This is past the point of no return, when the
-		 * exec_update_mutex has been taken.
-		 */
-		called_exec_mmap:1;
+		secureexec:1;
 #ifdef __alpha__
 	unsigned int taso:1;
 #endif

include/linux/sched/signal.h

@@ -225,14 +225,7 @@ struct signal_struct {
 
 	struct mutex cred_guard_mutex;	/* guard against foreign influences on
 					 * credential calculations
-					 * (notably. ptrace)
-					 * Deprecated do not use in new code.
-					 * Use exec_update_mutex instead.
-					 */
-	struct mutex exec_update_mutex;	/* Held while task_struct is being
-					 * updated during exec, and may have
-					 * inconsistent permissions.
-					 */
+					 * (notably. ptrace) */
 	ANDROID_KABI_RESERVE(1);
 	ANDROID_KABI_RESERVE(2);
 	ANDROID_KABI_RESERVE(3);

init/init_task.c

@@ -27,7 +27,6 @@ static struct signal_struct init_signals = {
 	.multiprocess	= HLIST_HEAD_INIT,
 	.rlim		= INIT_RLIMITS,
 	.cred_guard_mutex = __MUTEX_INITIALIZER(init_signals.cred_guard_mutex),
-	.exec_update_mutex = __MUTEX_INITIALIZER(init_signals.exec_update_mutex),
 #ifdef CONFIG_POSIX_TIMERS
 	.posix_timers = LIST_HEAD_INIT(init_signals.posix_timers),
 	.cputimer	= {

kernel/fork.c

@@ -1597,7 +1597,6 @@ static int copy_signal(unsigned long clone_flags, struct task_struct *tsk)
 	sig->oom_score_adj_min = current->signal->oom_score_adj_min;
 
 	mutex_init(&sig->cred_guard_mutex);
-	mutex_init(&sig->exec_update_mutex);
 
 	return 0;
 }