FAILED: patch "[PATCH] f2fs: prevent creating duplicate encrypted filenames" failed to apply to 4.9-stable tree

[<gregkh@linuxfoundation.org>]

The patch below does not apply to the 4.9-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From bfc2b7e8518999003a61f91c1deb5e88ed77b07d Mon Sep 17 00:00:00 2001
From: Eric Biggers <ebiggers@google.com>
Date: Tue, 17 Nov 2020 23:56:07 -0800
Subject: [PATCH] f2fs: prevent creating duplicate encrypted filenames

As described in "fscrypt: add fscrypt_is_nokey_name()", it's possible to
create a duplicate filename in an encrypted directory by creating a file
concurrently with adding the directory's encryption key.

Fix this bug on f2fs by rejecting no-key dentries in f2fs_add_link().

Note that the weird check for the current task in f2fs_do_add_link()
seems to make this bug difficult to reproduce on f2fs.

Fixes: 9ea97163c6da ("f2fs crypto: add filename encryption for f2fs_add_link")
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20201118075609.120337-4-ebiggers@kernel.org
Signed-off-by: Eric Biggers <ebiggers@google.com>

diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
index cb700d797296..9a321c52face 100644
--- a/fs/f2fs/f2fs.h
+++ b/fs/f2fs/f2fs.h
@@ -3251,6 +3251,8 @@ bool f2fs_empty_dir(struct inode *dir);
 
 static inline int f2fs_add_link(struct dentry *dentry, struct inode *inode)
 {
+	if (fscrypt_is_nokey_name(dentry))
+		return -ENOKEY;
 	return f2fs_do_add_link(d_inode(dentry->d_parent), &dentry->d_name,
 				inode, inode->i_ino, inode->i_mode);
 }