Bug 2414732 (CVE-2025-40190)

Summary: CVE-2025-40190 kernel: ext4: guard against EA inode refcount underflow in xattr update
Product: [Other] Security Response Reporter: OSIDB Bzimport <bzimport>
Component: vulnerabilityAssignee: Product Security DevOps Team <prodsec-dev>
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Version: unspecifiedKeywords: Security
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OS: Linux   
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A flaw was found in the ext4 filesystem implementation in the Linux kernel. The function ext4_xattr_inode_update_ref() may read an EA-inode reference count that is already non-positive and then apply a negative change (-1), causing the reference count to underflow and the filesystem to treat the inode as corrupted. This may lead to unexpected filesystem errors or denial of service (e.g., the filesystem remounting read-only).
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Description OSIDB Bzimport 2025-11-12 23:03:19 UTC 
In the Linux kernel, the following vulnerability has been resolved:

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.