[PATCH v2 -next 2/2] security: Fix call security_backing_file_free second time

[Cai Xinchen <caixinchen1@huawei.com>]

I found the following path:

alloc_empty_backing-file
    init_file(&ff->file, xxx)
        -> file_ref_init(&f->f_ref, 1); // only 1
    error = init_backing_file
        -> security_backing_file_alloc
        -> rc = call_int_hook(backing_file_alloc, ...)
           if (unlikely(rc))
           	security_backing_file_free(backing_file); // first call
    if (unlikely(error)) {
        fput(&ff->file);
         -> if (unlikely(file_ref_put(&file->f_ref))) // zero
                __fput_deferred(file);
                 -> ____fput -> __fput -> file_free(file);
                 -> backing_file_free(backing_file(f));
                 -> security_backing_file_free(&ff->file); // second call

Currently, only SELinux has the lsm backing_file_alloc hook, and it always
return 0. When security_backing_file_free is called for the first time,
the blobs pointer is set to NULL. Therefore, double free will not occur in
the code.

Fixes: 6af36aeb147a ("lsm: add backing_file LSM hooks")
Signed-off-by: Cai Xinchen <caixinchen1@huawei.com>
---
 security/security.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/security/security.c b/security/security.c
index 71aea8fdf014..bec2f4ebea34 100644
--- a/security/security.c
+++ b/security/security.c
@@ -2486,9 +2486,8 @@ void security_backing_file_free(struct file *backing_file)
 {
 	void *blob = backing_file_security(backing_file);
 
-	call_void_hook(backing_file_free, backing_file);
-
 	if (blob) {
+		call_void_hook(backing_file_free, backing_file);
 		backing_file_set_security(backing_file, NULL);
 		kmem_cache_free(lsm_backing_file_cache, blob);
 	}
-- 
2.34.1