[PATCH] ovl: use a dedicated semaphore for dir upperfile caching

[Icenowy Zheng <icenowy@aosc.io>]

The function ovl_dir_real_file() currently uses the semaphore of the
inode to synchronize write to the upperfile cache field.

However, this function will get called by ovl_ioctl_set_flags(), which
utilizes the inode semaphore too. In this case ovl_dir_real_file() will
try to claim a lock that is owned by a function in its call stack, which
won't get released before ovl_dir_real_file() returns.

Define a dedicated semaphore for the upperfile cache, so that the
deadlock won't happen.

Fixes: 61536bed2149 ("ovl: support [S|G]ETFLAGS and FS[S|G]ETXATTR ioctls for directories")
Cc: stable@vger.kernel.org # v5.10
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
---
 fs/overlayfs/readdir.c | 6 ++++--
 1 file changed, 4 insertions(+), 2 deletions(-)

diff --git a/fs/overlayfs/readdir.c b/fs/overlayfs/readdir.c
index 01620ebae1bd..f10701aabb71 100644
--- a/fs/overlayfs/readdir.c
+++ b/fs/overlayfs/readdir.c
@@ -56,6 +56,7 @@ struct ovl_dir_file {
 	struct list_head *cursor;
 	struct file *realfile;
 	struct file *upperfile;
+	struct semaphore upperfile_sem;
 };
 
 static struct ovl_cache_entry *ovl_cache_entry_from_node(struct rb_node *n)
@@ -883,7 +884,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 			ovl_path_upper(dentry, &upperpath);
 			realfile = ovl_dir_open_realfile(file, &upperpath);
 
-			inode_lock(inode);
+			down(&od->upperfile_sem);
 			if (!od->upperfile) {
 				if (IS_ERR(realfile)) {
 					inode_unlock(inode);
@@ -896,7 +897,7 @@ struct file *ovl_dir_real_file(const struct file *file, bool want_upper)
 					fput(realfile);
 				realfile = od->upperfile;
 			}
-			inode_unlock(inode);
+			up(&od->upperfile_sem);
 		}
 	}
 
@@ -959,6 +960,7 @@ static int ovl_dir_open(struct inode *inode, struct file *file)
 	od->realfile = realfile;
 	od->is_real = ovl_dir_is_real(file->f_path.dentry);
 	od->is_upper = OVL_TYPE_UPPER(type);
+	sema_init(&od->upperfile_sem, 1);
 	file->private_data = od;
 
 	return 0;
-- 
2.28.0