From c7df4a1ecb8579838ec8c56b2bb6a6716e974f37 Mon Sep 17 00:00:00 2001

From: Theodore Ts'o <tytso@mit.edu>

Date: Mon, 11 Nov 2019 22:18:13 -0500

Subject: [PATCH] ext4: work around deleting a file with i_nlink == 0 safely

Git-commit: c7df4a1ecb8579838ec8c56b2bb6a6716e974f37

Patch-mainline: v5.5-rc1

References: bsc#1158819 CVE-2019-19447

If the file system is corrupted such that a file's i_links_count is

too small, then it's possible that when unlinking that file, i_nlink

will already be zero.  Previously we were working around this kind of

corruption by forcing i_nlink to one; but we were doing this before

trying to delete the directory entry --- and if the file system is

corrupted enough that ext4_delete_entry() fails, then we exit with

i_nlink elevated, and this causes the orphan inode list handling to be

FUBAR'ed, such that when we unmount the file system, the orphan inode

list can get corrupted.

A better way to fix this is to simply skip trying to call drop_nlink()

if i_nlink is already zero, thus moving the check to the place where

it makes the most sense.

https://bugzilla.kernel.org/show_bug.cgi?id=205433

Link: https://lore.kernel.org/r/20191112032903.8828-1-tytso@mit.edu

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

Cc: stable@kernel.org

Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Acked-by: Jan Kara <jack@suse.cz>

---

 fs/ext4/namei.c | 11 +++++------

 1 file changed, 5 insertions(+), 6 deletions(-)

diff --git a/fs/ext4/namei.c b/fs/ext4/namei.c

index a67cae3c8ff5..a856997d87b5 100644

--- a/fs/ext4/namei.c

+++ b/fs/ext4/namei.c

@@ -3196,18 +3196,17 @@ static int ext4_unlink(struct inode *dir, struct dentry *dentry)

 	if (IS_DIRSYNC(dir))

 		ext4_handle_sync(handle);

-	if (inode->i_nlink == 0) {

-		ext4_warning_inode(inode, "Deleting file '%.*s' with no links",

-				   dentry->d_name.len, dentry->d_name.name);

-		set_nlink(inode, 1);

-	}

 	retval = ext4_delete_entry(handle, dir, de, bh);

 	if (retval)

 		goto end_unlink;

 	dir->i_ctime = dir->i_mtime = current_time(dir);

 	ext4_update_dx_flag(dir);

 	ext4_mark_inode_dirty(handle, dir);

-	drop_nlink(inode);

+	if (inode->i_nlink == 0)

+		ext4_warning_inode(inode, "Deleting file '%.*s' with no links",

+				   dentry->d_name.len, dentry->d_name.name);

+	else

+		drop_nlink(inode);

 	if (!inode->i_nlink)

 		ext4_orphan_add(handle, inode);

 	inode->i_ctime = current_time(inode);

-- 

2.16.4