ext4: guard against EA inode refcount underflow in xattr update

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.

Reported-by: syzbot+0be4f339a8218d2a5bb1@syzkaller.appspotmail.com
Fixes: https://syzbot.org/bug?extid=0be4f339a8218d2a5bb1
Cc: stable@kernel.org
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>
Message-ID: <20250920021342.45575-1-eraykrdg1@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/fs/ext4/xattr.c b/fs/ext4/xattr.c
index 7743d81b2f213220498a76003f20742a7f125e52..5b680deb1c9b515bec6375ee8c738ec6b10f76f1 100644 (file)
--- a/fs/ext4/xattr.c
+++ b/fs/ext4/xattr.c
@@ -1019,7 +1019,7 @@ static int ext4_xattr_inode_update_ref(handle_t *handle, struct inode *ea_inode,
                                       int ref_change)
 {
        struct ext4_iloc iloc;
-       s64 ref_count;
+       u64 ref_count;
        int ret;
 
        inode_lock_nested(ea_inode, I_MUTEX_XATTR);
@@ -1029,13 +1029,17 @@ static int ext4_xattr_inode_update_ref(handle_t *handle, struct inode *ea_inode,
                goto out;
 
        ref_count = ext4_xattr_inode_get_ref(ea_inode);
+       if ((ref_count == 0 && ref_change < 0) || (ref_count == U64_MAX && ref_change > 0)) {
+               ext4_error_inode(ea_inode, __func__, __LINE__, 0,
+                       "EA inode %lu ref wraparound: ref_count=%lld ref_change=%d",
+                       ea_inode->i_ino, ref_count, ref_change);
+               ret = -EFSCORRUPTED;
+               goto out;
+       }
        ref_count += ref_change;
        ext4_xattr_inode_set_ref(ea_inode, ref_count);
 
        if (ref_change > 0) {
-               WARN_ONCE(ref_count <= 0, "EA inode %lu ref_count=%lld",
-                         ea_inode->i_ino, ref_count);
-
                if (ref_count == 1) {
                        WARN_ONCE(ea_inode->i_nlink, "EA inode %lu i_nlink=%u",
                                  ea_inode->i_ino, ea_inode->i_nlink);
@@ -1044,9 +1048,6 @@ 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",