exit: Move force_uaccess back into do_exit

With kernel threads on architectures that still have set_fs/get_fs
running as KERNEL_DS moving force_uaccess_begin does not appear safe.
Calling force_uaccess_begin is a noop on anything people care about.

Update the comment to explain why this code while looking like an
obvious candidate for moving to make_task_dead probably needs to
remain in do_exit until set_fs/get_fs are entirely removed from the
kernel.

Fixes: 05ea0424f0e2 ("exit: Move oops specific logic from do_exit into make_task_dead")
Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Link: https://lkml.kernel.org/r/YdUxGKRcSiDy8jGg@zeniv-ca.linux.org.uk
Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>

diff --git a/kernel/exit.c b/kernel/exit.c
index db4eeb7fc680b5f58f00e76c016fb34cd742ebf5..fc0726cb22dbcb3e59bfc9dee0c41098c27ca409 100644 (file)
--- a/kernel/exit.c
+++ b/kernel/exit.c
@@ -737,6 +737,20 @@ void __noreturn do_exit(long code)
 
        WARN_ON(blk_needs_flush_plug(tsk));
 
+       /*
+        * If do_dead is called because this processes oopsed, it's possible
+        * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
+        * continuing. Amongst other possible reasons, this is to prevent
+        * mm_release()->clear_child_tid() from writing to a user-controlled
+        * kernel address.
+        *
+        * On uptodate architectures force_uaccess_begin is a noop.  On
+        * architectures that still have set_fs/get_fs in addition to handling
+        * oopses handles kernel threads that run as set_fs(KERNEL_DS) by
+        * default.
+        */
+       force_uaccess_begin();
+
        profile_task_exit(tsk);
        kcov_task_exit(tsk);
 
@@ -862,15 +876,6 @@ void __noreturn make_task_dead(int signr)
        if (unlikely(!tsk->pid))
                panic("Attempted to kill the idle task!");
 
-       /*
-        * If make_task_dead is called because this processes oopsed, it's possible
-        * that get_fs() was left as KERNEL_DS, so reset it to USER_DS before
-        * continuing. Amongst other possible reasons, this is to prevent
-        * mm_release()->clear_child_tid() from writing to a user-controlled
-        * kernel address.
-        */
-       force_uaccess_begin();
-
        if (unlikely(in_atomic())) {
                pr_info("note: %s[%d] exited with preempt_count %d\n",
                        current->comm, task_pid_nr(current),