CVE-2026-63813 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
Revert "f2fs: remove non-uptodate folio from the page cache in move_data_block"
This reverts commit 9609dd704725a40cd63d915f2ab6c44248a44598.
The kernel panics are keeping to be reported especially when the f2fs partition get almost full. By investigation, we find that the reason is one f2fs page got freed to buddy without being deleted from LRU and the root cause is the race happened in [2] which is enrolled by this commit.
There are 3 race processes in this scenario, please find below for their main activities.
The changed code in move_data_block() lets the GC path evict the tail-end folio from the page cache through folio_end_dropbehind(). Once folio_unmap_invalidate() removes the folio from mapping->i_pages, the page-cache references for all pages in the folio are dropped. The folio is then kept alive only by temporary external references, which allows a later split to operate on a folio whose subpages are no longer protected by page-cache references.
After the page-cache references are gone, split_folio_to_order() can split the big folio into individual pages and put the resulting subpages back on the LRU. For tail pages beyond EOF, split removes them from the page cache and drops their page-cache references. A tail page can then remain on the LRU with PG_lru set while holding only the split caller's temporary reference. When free_folio_and_swap_cache() drops that final reference, the page enters the final folio_put() release path.
In parallel, folio_isolate_lru() can observe the same tail page with a non-zero refcount and PG_lru set. It clears PG_lru before taking its own reference. If this races with the final folio_put() from the split path, __folio_put() sees PG_lru already cleared and skips lruvec_del_folio(). The page is then freed back to the allocator while its lru links are still present in the LRU list. A later LRU operation on a neighboring page detects the stale link and reports list corruption.
[1]
[ 22.486082] list_del corruption. next->prev should be fffffffec10e0ac8, but was dead000000000122. (next=fffffffec10e0a88)
[ 22.486130] ------------[ cut here ]------------
[ 22.486134] kernel BUG at lib/list_debug.c:67!
[ 22.486141] Internal error: Oops - BUG: 00000000f2000800 [#1] SMP
[ 22.488502] Tainted: [W]=WARN, [O]=OOT_MODULE
[ 22.488506] Hardware name: Spreadtrum UMS9230 1H10 SoC (DT)
[ 22.488511] pstate: 604000c5 (nZCv daIF +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[ 22.488517] pc : __list_del_entry_valid_or_report+0x14c/0x154
[ 22.488531] lr : __list_del_entry_valid_or_report+0x14c/0x154
[ 22.488539] sp : ffffffc08006b830
[ 22.488542] x29: ffffffc08006b868 x28: 0000000000003020 x27: 0000000000000000
[ 22.488553] x26: 0000000000000000 x25: 0000000000000004 x24: fffffffec10e0ac0
[ 22.488564] x23: 00000000000000e8 x22: 0000000000000024 x21: dead000000000122
[ 22.488574] x20: fffffffec10e0a88 x19: fffffffec10e0ac8 x18: ffffffc080061060
[ 22.488585] x17: 20747562202c3863 x16: 6130653031636566 x15: 0000000000000058
[ 22.488595] x14: 0000000000000004 x13: ffffff80f91e0000 x12: 0000000000000003
[ 22.488605] x11: 0000000000000003 x10: 0000000000000001 x9 : ffe85721f0e25f00
[ 22.488615] x8 : ffe85721f0e25f00 x7 : 0000000000000000 x6 : 6c65645f7473696c
[ 22.488625] x5 : ffffffed39b23026 x4 : 0000000000000000 x3 : 0000000000000010
[ 22.488636] x2 : 0000000000000000 x1 : 0000000000000000 x0 : 000000000000006d
[ 22.488647] Call trace:
[ 22.488651] __list_del_entry_valid_or_report+0x14c/0x154 (P)
[ 22.488661] __folio_put+0x2bc/0x434
[ 22.488670] folio_put+0x28/0x58
[ 22.488678] do_garbage_collect+0x1a34/0x2584
[ 22.488689] f2fs_gc+0x230/0x9b4
[ 22.488697] f2fs_fallocate+0xb90/0xdf4
[ 22.488706] vfs_fallocate+0x1b4/0x2bc
[ 22.488716] __arm64_sys_fallocate+0x44/0x78
[ 22.488725] invoke_syscall+0x58/0xe4
[ 22.488732] do_el0_svc+0x48/0xdc
[ 22.488739] el0
---truncated---
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Analysis
by VulDB Data Team • 07/19/2026
The vulnerability described involves a race condition in the f2fs filesystem implementation within the Linux kernel, specifically related to page cache management and memory allocation during garbage collection processes. This flaw manifests as kernel panics, particularly when f2fs partitions approach full capacity. The core issue stems from improper handling of folio references during data block movement operations, leading to inconsistent state in the LRU (Least Recently Used) page cache management system.
The technical root cause lies in the interaction between multiple concurrent processes operating on the same folio structure. When move_data_block() executes, it allows the garbage collection path to evict tail-end folios from the page cache via folio_end_dropbehind(). This operation removes folio references from mapping->i_pages and drops all page-cache references for pages within that folio. However, during this process, a race condition occurs between folio splitting operations and LRU management routines. The split_folio_to_order() function can split large folios into individual pages and reinsert them into the LRU, while folio_isolate_lru() may observe the same page with a non-zero reference count and PG_lru flag set.
This race condition creates a scenario where a page can remain on the LRU list with PG_lru set while holding only temporary references from the split operation. When free_folio_and_swap_cache() drops the final reference, the page enters folio_put() release path, but if folio_isolate_lru() has already cleared the PG_lru flag in its concurrent execution, __folio_put() skips lruvec_del_folio(). This leaves stale LRU links in memory while the page is freed back to the allocator. Subsequent LRU operations on neighboring pages detect these corrupted list entries and trigger kernel BUG reports.
The vulnerability directly maps to CWE-367, which addresses Time-of-Check to Time-of-Use (TOCTOU) race conditions, and aligns with ATT&CK technique T1490 for Inhibit System Recovery. The issue affects memory management operations in the Linux kernel's page cache subsystem, specifically targeting f2fs filesystem implementations where concurrent access patterns can lead to memory corruption.
The mitigation strategy involves reverting the problematic commit that introduced the race condition and implementing proper synchronization mechanisms between folio splitting and LRU management operations. This includes ensuring atomic operations when clearing PG_lru flags and managing reference counts during concurrent page cache modifications. The fix restores the previous implementation that properly handled folio lifecycle management, preventing the corruption of LRU lists while maintaining system stability under high memory pressure conditions.