CVE-2026-63816 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: atomic: fix UAF issue on f2fs_inode_info.atomic_inode
- ioctl(F2FS_IOC_GARBAGE_COLLECT_RANGE) - shrink - f2fs_gc - gc_data_segment - ra_data_block(cow_inode) - mapping = F2FS_I(inode)->atomic_inode->i_mapping : f2fs_is_cow_file(cow_inode) is true - f2fs_evict_inode(atomic_inode) - clear_inode_flag(fi->cow_inode, FI_COW_FILE) - F2FS_I(fi->cow_inode)->atomic_inode = NULL ... - truncate_inode_pages_final(atomic_inode) - f2fs_grab_cache_folio(mapping) : create folio in atomic_inode->mapping - clear_inode(atomic_inode) - BUG_ON(atomic_inode->i_data.nrpages)
We need to add a reference on fi->atomic_inode before using its mapping field during garbage collection, otherwise, it will cause UAF issue.
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability identified in the Linux f2fs kernel module represents a use-after-free condition that occurs during garbage collection operations when handling atomic inode structures. This issue specifically manifests when the ioctl(F2FS_IOC_GARBAGE_COLLECT_RANGE) command is executed, triggering a chain of operations that can lead to memory corruption and system instability. The flaw exists in the interaction between garbage collection routines and atomic inode management within the f2fs filesystem implementation.
The technical root cause stems from improper reference counting during garbage collection processes where the system attempts to access the mapping field of an atomic inode without ensuring the inode remains valid throughout the operation sequence. When f2fs_evict_inode is called on a cow_inode, it clears the FI_COW_FILE flag and sets F2FS_I(fi->cow_inode)->atomic_inode = NULL, effectively removing the reference to the atomic inode structure. However, subsequent operations like f2fs_grab_cache_folio may still attempt to access this freed memory through the mapping field, creating a classic use-after-free scenario that can be exploited by malicious actors.
This vulnerability directly relates to CWE-416, which describes the use of freed memory condition, and aligns with ATT&CK technique T1059.003 for command and scripting interpreter usage in exploitation scenarios. The operational impact of this flaw extends beyond simple memory corruption as it can lead to system crashes, data loss, and potentially privilege escalation depending on how the exploit is crafted. Attackers could leverage this vulnerability to cause denial of service conditions or potentially execute arbitrary code within the kernel space.
The fix implementation requires adding proper reference counting mechanisms before accessing the mapping field of fi->atomic_inode during garbage collection operations. This approach ensures that the atomic inode structure remains valid throughout the entire operation sequence, preventing the scenario where a freed memory location is accessed after the clear_inode operations have been completed. The solution must be carefully implemented to maintain performance characteristics while ensuring memory safety during concurrent filesystem operations.
Security implications of this vulnerability are significant as it affects the core filesystem functionality and can potentially be exploited in containerized environments or cloud deployments where f2fs is utilized. The timing of the vulnerability during garbage collection makes it particularly dangerous as it can occur during routine system maintenance operations, making detection difficult and impact more widespread. System administrators should prioritize applying patches that address this specific reference counting issue to prevent potential exploitation and maintain filesystem integrity across affected Linux distributions.