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ext4: guard against EA inode refcount underflow in xattr update

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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>
This commit is contained in:
Ahmet Eray Karadag
2025-09-20 05:13:43 +03:00
committed by Theodore Ts'o
parent 04a91570ac
commit 57295e8354
1 changed files with 8 additions and 7 deletions
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15
fs/ext4/xattr.c
View File

@@ -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",