[PATCH] Fix unserialized task->files changing

Fixed race on put_files_struct on exec with proc.  Restoring files on
current on error path may lead to proc having a pointer to already kfree-d
files_struct.

->files changing at exit.c and khtread.c are safe as exit_files() makes all
things under lock.

Found during OpenVZ stress testing.

[akpm@osdl.org: add export]
Signed-off-by: Pavel Emelianov <xemul@openvz.org>
Signed-off-by: Kirill Korotaev <dev@openvz.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

fs/binfmt_elf.c

@@ -1038,10 +1038,8 @@ out_free_interp:
 out_free_file:
 	sys_close(elf_exec_fileno);
 out_free_fh:
-	if (files) {
-		put_files_struct(current->files);
-		current->files = files;
-	}
+	if (files)
+		reset_files_struct(current, files);
 out_free_ph:
 	kfree(elf_phdata);
 	goto out;

fs/binfmt_misc.c

@@ -215,10 +215,8 @@ _error:
 	bprm->interp_flags = 0;
 	bprm->interp_data = 0;
 _unshare:
-	if (files) {
-		put_files_struct(current->files);
-		current->files = files;
-	}
+	if (files)
+		reset_files_struct(current, files);
 	goto _ret;
 }
 

fs/exec.c

@@ -898,8 +898,7 @@ int flush_old_exec(struct linux_binprm * bprm)
 	return 0;
 
 mmap_failed:
-	put_files_struct(current->files);
-	current->files = files;
+	reset_files_struct(current, files);
 out:
 	return retval;
 }

include/linux/file.h

@@ -112,5 +112,6 @@ struct task_struct;
 
 struct files_struct *get_files_struct(struct task_struct *);
 void FASTCALL(put_files_struct(struct files_struct *fs));
+void reset_files_struct(struct task_struct *, struct files_struct *);
 
 #endif /* __LINUX_FILE_H */

kernel/exit.c

@@ -487,6 +487,18 @@ void fastcall put_files_struct(struct files_struct *files)
 
 EXPORT_SYMBOL(put_files_struct);
 
+void reset_files_struct(struct task_struct *tsk, struct files_struct *files)
+{
+	struct files_struct *old;
+
+	old = tsk->files;
+	task_lock(tsk);
+	tsk->files = files;
+	task_unlock(tsk);
+	put_files_struct(old);
+}
+EXPORT_SYMBOL(reset_files_struct);
+
 static inline void __exit_files(struct task_struct *tsk)
 {
 	struct files_struct * files = tsk->files;