CVE-2026-63814 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
f2fs: validate ACL entry sizes in f2fs_acl_from_disk()
f2fs_acl_count() only validates the aggregate ACL xattr length. A malformed ACL can still place ACL_USER or ACL_GROUP in a slot that only contains struct f2fs_acl_entry_short bytes, and f2fs_acl_from_disk() then reads entry->e_id before verifying that a full entry fits.
Require a short entry before reading e_tag and e_perm, and require a full entry before reading e_id for ACL_USER and ACL_GROUP. Return -EFSCORRUPTED from these new truncated-entry checks, while keeping the pre-existing -EINVAL paths unchanged.
Validation reproduced this kernel report: KASAN slab-out-of-bounds in __f2fs_get_acl+0x6fb/0x7e0 RIP: 0033:0x7f4b835ea7aa The buggy address belongs to the object at ffff888114589960 which belongs to the cache kmalloc-8 of size 8 The buggy address is located 0 bytes to the right of allocated 8-byte region [ffff888114589960, ffff888114589968)
Read of size 4 Call trace: dump_stack_lvl+0x66/0xa0 (?:?) print_report+0xce/0x630 (?:?) __f2fs_get_acl+0x6fb/0x7e0 (fs/f2fs/acl.c:169) srso_alias_return_thunk+0x5/0xfbef5 (?:?) __virt_addr_valid+0x224/0x430 (?:?) kasan_report+0xe0/0x110 (?:?) __f2fs_get_acl+0x5/0x7e0 (fs/f2fs/acl.c:169) __get_acl+0x281/0x380 (?:?) vfs_get_acl+0x10b/0x190 (?:?) do_get_acl+0x2a/0x410 (?:?) do_get_acl+0x9/0x410 (?:?) do_getxattr+0xe8/0x260 (?:?) filename_getxattr+0xd1/0x140 (?:?) do_getname+0x2d/0x2d0 (?:?) path_getxattrat+0x16c/0x200 (?:?) lock_release+0xc8/0x290 (?:?) cgroup_update_frozen+0x9d/0x320 (?:?) lockdep_hardirqs_on_prepare+0xea/0x1a0 (?:?) trace_hardirqs_on+0x1a/0x170 (?:?) _raw_spin_unlock_irq+0x28/0x50 (?:?) do_syscall_64+0x115/0x6a0 (arch/x86/entry/syscall_64.c:87) entry_SYSCALL_64_after_hwframe+0x77/0x7f (?:?)
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability in the Linux f2fs file system kernel module stems from inadequate validation of Access Control List (ACL) entry sizes during the processing of ACL extended attributes. The core issue resides in the f2fs_acl_from_disk() function which fails to properly verify that ACL entries contain sufficient data before attempting to read specific fields. This flaw allows attackers to craft malformed ACLs that place ACL_USER or ACL_GROUP entries in slots containing only struct f2fs_acl_entry_short bytes, leading to out-of-bounds memory access patterns.
The technical implementation problem occurs because f2fs_acl_count() function only validates the total length of the ACL extended attribute without ensuring individual entry structures contain sufficient data. When processing these malformed entries, the code reads entry->e_id before verifying that a complete entry structure fits within the allocated memory region. This violation results in reading beyond the bounds of allocated kernel memory, specifically triggering a slab-out-of-bounds error as demonstrated by the KASAN report showing access to an 8-byte allocation region at address ffff888114589960.
This vulnerability directly maps to CWE-129 Improper Validation of Array Index and CWE-787 Out-of-bounds Write, representing a classic buffer overflow scenario where insufficient bounds checking leads to memory corruption. The operational impact extends beyond simple denial of service, as this flaw could potentially be exploited to achieve privilege escalation or arbitrary code execution within the kernel context. The attack surface is particularly concerning since file system ACL operations are frequently accessed during normal system operations, making this a persistent threat vector.
The fix implemented requires that short entries be validated before reading e_tag and e_perm fields, while ensuring full entries contain sufficient data before reading e_id for ACL_USER and ACL_GROUP types. This approach follows the principle of defensive programming by validating structure integrity before field access. The solution maintains backward compatibility with existing valid ACLs while introducing new error handling through -EFSCORRUPTED return codes for truncated entry detection, preserving existing -EINVAL paths for other validation failures. This remediation aligns with ATT&CK technique T1068 Exploitation for Privilege Escalation by addressing kernel-level memory corruption vulnerabilities that could otherwise be leveraged for system compromise.
The memory access pattern identified in the call trace demonstrates a clear path to exploitation where __f2fs_get_acl attempts to read a 4-byte value from an 8-byte allocated region, triggering KASAN memory safety checks. The vulnerability affects systems using f2fs file systems and could be particularly impactful in environments where ACL-based access controls are heavily utilized. Security researchers should prioritize patching this vulnerability as it represents a direct path to kernel memory corruption that could be chained with other exploits for more severe outcomes.