CVE-2026-63820 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: fix missing read bio submission on large folio error
f2fs_read_data_large_folio() can keep a read bio across multiple readahead folios. If a later folio hits an error before any of its blocks are added to the bio, folio_in_bio is false and the current error path returns immediately after ending that folio.
This can leave the bio accumulated for earlier folios unsubmitted. Those folios then never receive read completion, and readers can wait indefinitely on the locked folios.
Route errors through the common out path so any pending bio is submitted before returning. Stop consuming more readahead folios once an error is seen, and only wait on and clear the current folio when it was actually added to the bio.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability in question affects the f2fs filesystem implementation within the Linux kernel, specifically addressing a critical issue in how read bio submissions are handled during large folio operations. This flaw demonstrates a classic case of improper error handling and resource management that can lead to system instability and potential denial of service conditions.
The technical root cause lies in the f2fs_read_data_large_folio() function which maintains read bio objects across multiple readahead folios to optimize I/O operations. When processing large folios, if an error occurs during the processing of a later folio before any of its blocks are added to the accumulated bio, the current error handling path incorrectly returns immediately without submitting the previously accumulated bio data. This creates a scenario where earlier folios remain in an unsubmitted state, causing readers to indefinitely wait on locked folios while the system fails to complete the necessary I/O operations.
This vulnerability directly relates to CWE-457 and CWE-691, representing improper initialization issues and insufficient control flow management respectively. The flaw allows for potential resource exhaustion and system hang conditions where legitimate read requests cannot progress through the I/O stack. From an ATT&CK perspective, this represents a denial of service vector that could be exploited to degrade system performance or availability.
The operational impact of this vulnerability is significant as it can cause indefinite blocking of I/O operations, leading to application timeouts and system unresponsiveness. When readers are left waiting indefinitely on locked folios, the entire I/O subsystem becomes starved of resources, potentially affecting other filesystem operations and system stability. The issue particularly impacts systems heavily reliant on f2fs filesystems where large folio operations are common.
The mitigation strategy involves routing all error conditions through a common out path that ensures any pending bio is properly submitted before returning from the function. This approach prevents the accumulation of unsubmitted I/O operations while maintaining proper resource cleanup. Additionally, the implementation now stops consuming additional readahead folios once an error is detected, and only waits on and clears the current folio when it was actually added to the bio. This change ensures that the system properly handles error conditions without leaving I/O operations in incomplete states, thereby preventing indefinite blocking scenarios while maintaining filesystem consistency and performance characteristics.
The fix addresses fundamental issues in kernel I/O management and demonstrates proper error handling patterns that should be applied across similar subsystems. This vulnerability highlights the importance of careful resource management in kernel code where multiple operations can share state and where error conditions must properly propagate through the system to prevent resource leaks and deadlocks.