exec: Add exec_update_mutex to replace cred_guard_mutex

The cred_guard_mutex is problematic as it is held over possibly
indefinite waits for userspace. The possible indefinite waits for
userspace that I have identified are: The cred_guard_mutex is held in
PTRACE_EVENT_EXIT waiting for the tracer. The cred_guard_mutex is
held over "put_user(0, tsk->clear_child_tid)" in exit_mm(). The
cred_guard_mutex is held over "get_user(futex_offset, ...") in
exit_robust_list. The cred_guard_mutex held over copy_strings.

The functions get_user and put_user can trigger a page fault which can
potentially wait indefinitely in the case of userfaultfd or if
userspace implements part of the page fault path.

In any of those cases the userspace process that the kernel is waiting
for might make a different system call that winds up taking the
cred_guard_mutex and result in deadlock.

Holding a mutex over any of those possibly indefinite waits for
userspace does not appear necessary. Add exec_update_mutex that will
just cover updating the process during exec where the permissions and
the objects pointed to by the task struct may be out of sync.

The plan is to switch the users of cred_guard_mutex to
exec_update_mutex one by one. This lets us move forward while still
being careful and not introducing any regressions.

Link: https://lore.kernel.org/lkml/[e-mail]/
Link: https://lore.kernel.org/lkml/[e-mail]/
Link: https://lore.kernel.org/linux-fsdevel/[e-mail]/
Link: https://lore.kernel.org/lkml/[e-mail]/
Link: https://lore.kernel.org/lkml/[e-mail]/
Ref: 45c1a159b85b ("Add PTRACE_O_TRACEVFORKDONE and PTRACE_O_TRACEEXIT facilities.")
Ref: 456f17cd1a28 ("[PATCH] user-vm-unlock-2.5.31-A2")
Reviewed-by: Kirill Tkhai <[e-mail]>
Signed-off-by: "Eric W. Biederman" <[e-mail]>
Signed-off-by: Bernd Edlinger <[e-mail]>
Signed-off-by: Eric W. Biederman <[e-mail]>

fs/exec.c

···
 }
 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);
···
 		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);
···
 		goto out;
 
 	/*
-	 * After clearing bprm->mm (to mark that current is using the
-	 * prepared mm now), we have nothing left of the original
+	 * After setting bprm->called_exec_mmap (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;
 
 #ifdef CONFIG_POSIX_TIMERS
···
 {
 	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);
 	}
···
 	 * 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);
···
 
 		read_lock(&binfmt_lock);
 		put_binfmt(fmt);
-		if (retval < 0 && !bprm->mm) {
+		if (retval < 0 && bprm->called_exec_mmap) {
 			/* we got to flush_old_exec() and failed after it */
 			read_unlock(&binfmt_lock);
 			force_sigsegv(SIGSEGV);

include/linux/binfmts.h

···
 		 * exec has happened. Used to sanitize execution environment
 		 * and to set AT_SECURE auxv for glibc.
 		 */
-		secureexec:1;
+		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;
 #ifdef __alpha__
 	unsigned int taso:1;
 #endif

include/linux/sched/signal.h

···
 
 	struct mutex cred_guard_mutex;	/* guard against foreign influences on
 					 * credential calculations
-					 * (notably. ptrace) */
+					 * (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.
+					 */
 } __randomize_layout;
 
 /*

init/init_task.c

···
 	.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

···
 	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;
 }