[PATCH v2 1/2] binfmt_misc: fix crash when load/unload module

[Tong Zhang <ztong0001@gmail.com>]

We should unregister the table upon module unload otherwise something
horrible will happen when we load binfmt_misc module again. Also note
that we should keep value returned by register_sysctl_mount_point() and
release it later, otherwise it will leak.
Also, per Christian's comment, to fully restore the old behavior that
won't break userspace the check(binfmt_misc_header) should be
eliminated.

reproduce:
modprobe binfmt_misc
modprobe -r binfmt_misc
modprobe binfmt_misc
modprobe -r binfmt_misc
modprobe binfmt_misc

[   18.032038] Call Trace:
[   18.032108]  <TASK>
[   18.032169]  dump_stack_lvl+0x34/0x44
[   18.032273]  __register_sysctl_table+0x6f4/0x720
[   18.032397]  ? preempt_count_sub+0xf/0xb0
[   18.032508]  ? 0xffffffffc0040000
[   18.032600]  init_misc_binfmt+0x2d/0x1000 [binfmt_misc]
[   18.042520] binfmt_misc: Failed to create fs/binfmt_misc sysctl mount point
modprobe: can't load module binfmt_misc (kernel/fs/binfmt_misc.ko): Cannot allocate memory
[   18.063549] binfmt_misc: Failed to create fs/binfmt_misc sysctl mount point
[   18.204779] BUG: unable to handle page fault for address: fffffbfff8004802

Fixes: 3ba442d5331f ("fs: move binfmt_misc sysctl to its own file")
Co-developed-by: Christian Brauner<brauner@kernel.org>
Signed-off-by: Tong Zhang <ztong0001@gmail.com>
---
 fs/binfmt_misc.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/fs/binfmt_misc.c b/fs/binfmt_misc.c
index ddea6acbddde..c07f35719ee3 100644
--- a/fs/binfmt_misc.c
+++ b/fs/binfmt_misc.c
@@ -817,20 +817,20 @@ static struct file_system_type bm_fs_type = {
 };
 MODULE_ALIAS_FS("binfmt_misc");
 
+static struct ctl_table_header *binfmt_misc_header;
+
 static int __init init_misc_binfmt(void)
 {
 	int err = register_filesystem(&bm_fs_type);
 	if (!err)
 		insert_binfmt(&misc_format);
-	if (!register_sysctl_mount_point("fs/binfmt_misc")) {
-		pr_warn("Failed to create fs/binfmt_misc sysctl mount point");
-		return -ENOMEM;
-	}
+	binfmt_misc_header = register_sysctl_mount_point("fs/binfmt_misc");
 	return 0;
 }
 
 static void __exit exit_misc_binfmt(void)
 {
+	unregister_sysctl_table(binfmt_misc_header);
 	unregister_binfmt(&misc_format);
 	unregister_filesystem(&bm_fs_type);
 }
-- 
2.25.1