CVE-2026-92500 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
ext4: use fsdata to track inline data write state and fix race
Instead of checking the live inode state (ext4_has_inline_data(inode) and ext4_test_inode_state(inode, EXT4_STATE_MAY_INLINE_DATA)) in the write_end handlers, use the fsdata parameter of the address space operations to explicitly pass down the state in which write_begin prepared the write.
A concurrent thread (such as ext4_page_mkwrite()) can convert the inline data to an extent between write_begin and write_end. If this happens, the write_end handlers would previously miss the inline write_end path and fall through to extent-based write_end logic. However, since block buffers were never allocated in write_begin, this resulted in NULL pointer dereferences or data loss because folio_buffers(folio) was NULL.
Define EXT4_WRITE_DATA_INLINE (4) as a bit flag (Bit 2), treating fsdata as bitwise flags rather than mutually exclusive enums to keep states of the write path independent. Communicate this state via fsdata: 1) ext4_write_begin() and ext4_da_write_begin() set the EXT4_WRITE_DATA_INLINE bit in *fsdata via bitwise OR when an inline write is successfully prepared. 2) On entry, ext4_write_begin() clears the EXT4_WRITE_DATA_INLINE bit to safely handle VFS retries (where generic_perform_write() bypasses the fsdata initialization on its retry jump). 3) The write_end handlers perform a bitwise AND to check if the EXT4_WRITE_DATA_INLINE bit is set and invoke the inline write_end helper accordingly.
Furthermore, during a buffered write, ext4_write_inline_data_end() acquires the xattr lock after preparing the write. If a concurrent page fault (ext4_page_mkwrite()) converts the inline data to an extent after the write_end handlers check the state but before ext4_write_inline_data_end() acquires the xattr write lock, the subsequent check will trigger a kernel panic via BUG_ON(!ext4_has_inline_data(inode)).
To keep git history working and bisectability clean, replace the BUG_ON check in ext4_write_inline_data_end() with a graceful error- handling retry path in this same commit. If the inline data is cleared after locking the xattr, we safely release all resources (releasing iloc.bh, unlocking/putting the folio, stopping the active journal transaction handle) and return 0 (VFS retry) to let the generic write path retry the operation safely.
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Analysis
by VulDB Data Team • 09/18/2026
The Linux kernel ext4 filesystem driver contained a critical race condition vulnerability related to the handling of inline data during buffered writes. This flaw stemmed from an improper reliance on live inode state checks within the write_end handlers rather than tracking the specific preparation state established by write_begin operations. Specifically, the code previously used functions like ext4_has_inline_data and ext4_test_inode_state to determine how to finalize a write operation. However, this approach was susceptible to concurrency issues where another thread could modify the file's data structure between the initial write setup and its completion. The vulnerability is categorized under CWE-362, which describes concurrent execution using shared resources with improper synchronization, as well as CWE-415 for double free or use-after-free scenarios that can arise from inconsistent state handling leading to memory corruption.
The operational impact of this race condition was severe, potentially resulting in NULL pointer dereferences and data loss. When a concurrent thread, such as ext4_page_mkwrite triggered by a page fault, converted inline data into extent-based storage between the write_begin and write_end phases, the write_end handler would fail to recognize that it should handle an inline write. Instead, it proceeded with logic designed for extent-based writes. Since block buffers are not allocated during the preparation phase of an inline write, attempting to access folio_buffers on a NULL pointer caused a kernel panic or corrupted data structures. This represents a significant stability risk and potential denial-of-service vector against systems relying on ext4 filesystems, particularly under high-concurrency workloads involving memory-mapped I/O operations.
To resolve this issue, the fix introduces a mechanism to explicitly track the write state using the fsdata parameter of address space operations. A new flag, EXT4_WRITE_DATA_INLINE, was defined as bit two in an enum treated as bitwise flags rather than mutually exclusive states. This allows ext4_write_begin and ext4_da_write_begin to set this bit when successfully preparing an inline write. The implementation ensures that on entry, the write_begin function clears this bit to safely handle Virtual File System retries where generic_perform_write might bypass fsdata initialization during retry jumps. During the write_end phase, handlers perform a bitwise AND operation to check for the presence of the EXT4_WRITE_DATA_INLINE flag before invoking inline-specific finalization helpers. This approach decouples the state tracking from live inode checks, ensuring that the logic follows the path intended by the initial preparation regardless of concurrent modifications.
A secondary aspect of this vulnerability involved race conditions during buffered writes where ext4_write_inline_data_end acquired the xattr lock after preparing the write. If a concurrent page fault converted inline data to extents after the state check but before the lock was acquired, subsequent checks would trigger kernel panics via BUG_ON assertions due to missing expected inline data structures. The remediation replaces these fatal assertion failures with graceful error-handling retry paths. If the inline data is cleared after locking the xattr attribute, the code now safely releases all associated resources, including releasing inode locks and buffers, unlocking folios, and stopping active journal transaction handles. It then returns a value indicating that the Virtual File System should retry the operation, thereby preventing crashes while maintaining system integrity and ensuring git history bisectability remains clean for future debugging efforts.