[PATCH] Fix: ext4: guard against EA inode refcount underflow in xattr update

[Ahmet Eray Karadag <eraykrdg1@gmail.com>]

syzkaller found a path where ext4_xattr_inode_update_ref() reads an EA
inode refcount that is already <= 0 and then applies ref_change (often
-1). That lets the refcount underflow and we proceed with a bogus value,
triggering errors like:

  EXT4-fs error: EA inode <n> ref underflow: ref_count=-1 ref_change=-1
  EXT4-fs warning: ea_inode dec ref err=-117

Make the invariant explicit: if the current refcount is non-positive,
treat this as on-disk corruption, emit EXT4_ERROR_INODE(), and fail the
operation with -EFSCORRUPTED instead of updating the refcount. Delete the
WARN_ONCE() as negative refcounts are now impossible; keep error reporting
in ext4_error_inode().

This prevents the underflow and the follow-on orphan/cleanup churn.

Fixes: https://syzbot.org/bug?extid=0be4f339a8218d2a5bb1
Co-developed-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Albin Babu Varghese <albinbabuvarghese20@gmail.com>
Signed-off-by: Ahmet Eray Karadag <eraykrdg1@gmail.com>
---
 fs/ext4/xattr.c | 10 +++++++---
 1 file changed, 7 insertions(+), 3 deletions(-)

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 5a6fe1513fd2..a056f98579c3 100644
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -1030,6 +1030,13 @@ static int ext4_xattr_inode_update_ref(handle_t *handle, struct inode *ea_inode,
 
 	ref_count = ext4_xattr_inode_get_ref(ea_inode);
 	ref_count += ref_change;
+	if (ref_count < 0) {
+		ext4_error_inode(ea_inode, __func__, __LINE__, 0,
+				"EA inode %lu ref underflow: ref_count=%lld ref_change=%d",
+				ea_inode->i_ino, ref_count, ref_change);
+		ret = -EFSCORRUPTED;
+		goto out;
+	}
 	ext4_xattr_inode_set_ref(ea_inode, ref_count);
 
 	if (ref_change > 0) {
@@ -1044,9 +1051,6 @@ static int ext4_xattr_inode_update_ref(handle_t *handle, struct inode *ea_inode,
 			ext4_orphan_del(handle, ea_inode);
 		}
 	} else {
-		WARN_ONCE(ref_count < 0, "EA inode %lu ref_count=%lld",
-			  ea_inode->i_ino, ref_count);
-
 		if (ref_count == 0) {
 			WARN_ONCE(ea_inode->i_nlink != 1,
 				  "EA inode %lu i_nlink=%u",
-- 
2.34.1