[PATCH] configfs: fix a race in configfs_lookup()

[sishuaigong]

When configfs_lookup() is executing list_for_each_entry(),
it is possible that configfs_dir_lseek() is calling list_del().
Some unfortunate interleavings of them can cause a kernel NULL
pointer dereference error

Thread 1                  Thread 2
//configfs_dir_lseek()    //configfs_lookup()
list_del(&cursor->s_sibling);
                          list_for_each_entry(sd, ...)

Fix this bug by using list_for_each_entry_safe() instead.

Reported-by: Sishuai Gong <sishuai@xxxxxxxxxx>
Signed-off-by: sishuaigong <sishuai@xxxxxxxxxx>
---
 fs/configfs/dir.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/fs/configfs/dir.c b/fs/configfs/dir.c
index ac5e0c0e9181..8f5d0309fb4a 100644
--- a/fs/configfs/dir.c
+++ b/fs/configfs/dir.c
@@ -452,7 +452,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
 				       unsigned int flags)
 {
 	struct configfs_dirent * parent_sd = dentry->d_parent->d_fsdata;
-	struct configfs_dirent * sd;
+	struct configfs_dirent *sd, *tmp;
 	int found = 0;
 	int err;
 
@@ -468,7 +468,7 @@ static struct dentry * configfs_lookup(struct inode *dir,
 	if (!configfs_dirent_is_ready(parent_sd))
 		goto out;
 
-	list_for_each_entry(sd, &parent_sd->s_children, s_sibling) {
+	list_for_each_entry_safe(sd, tmp, &parent_sd->s_children, s_sibling) {
 		if (sd->s_type & CONFIGFS_NOT_PINNED) {
 			const unsigned char * name = configfs_get_name(sd);
 
-- 
2.17.1