[PATCH] fs/exec.c: Avoid a race in formats

[Levi Yun <ppbuk5246@gmail.com>]

Suppose a module registers its own binfmt (custom) and formats is like:

+---------+    +----------+    +---------+
| custom  | -> |  format1 | -> | format2 |
+---------+    +----------+    +---------+

and try to call unregister_binfmt with custom NOT in __exit stage.

In that situation, below race scenario can happen.

CPU 0						CPU1
search_binary_handler				...
	read_lock				unregister_binfmt(custom)
	list_for_each_entry			< wait >
	(get custom binfmt)			...
	read_unlock				...
	...					list_del
	custom binfmt return -ENOEXEC
	get next fmt entry (LIST_POISON1)

Because CPU1 set the fmt->lh.next as LIST_POISON1,
CPU 0 get next binfmt as LIST_POISON1.
In that situation, CPU0 try to dereference LIST_POISON1 address and
makes PANIC.

To avoid this situation, check the fmt is valid.
And if it isn't valid, return -EAGAIN.

Signed-off-by: Levi Yun <ppbuk5246@gmail.com>
---
 fs/exec.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/fs/exec.c b/fs/exec.c
index 79f2c9483302..2042a1232656 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -1720,6 +1720,12 @@ static int search_binary_handler(struct linux_binprm *bprm)
  retry:
 	read_lock(&binfmt_lock);
 	list_for_each_entry(fmt, &formats, lh) {
+		if (fmt == LIST_POISON1) {
+			read_unlock(&binfmt_lock);
+			retval = -EAGAIN;
+			break;
+		}
+
 		if (!try_module_get(fmt->module))
 			continue;
 		read_unlock(&binfmt_lock);
-- 
2.34.1