CVE-2024-53220 is a critical vulnerability found and fixed in the Linux kernel's F2FS filesystem code. The bug could cause a full kernel panic (system crash) during regular filesystem activity in rare but realistic edge cases. The root issue stems from incorrect dirty data calculation when estimating how much free space is needed during certain operations. Here’s a breakdown that anyone can understand.

What Is F2FS?

F2FS stands for Flash-Friendly File System. It's optimized for NAND flash memory-based storage, such as SSDs, eMMCs, and SD cards. Because F2FS is used in Android phones, Linux servers, and embedded devices, any filesystem bug can have a large impact.

The Heart of the Bug

The vulnerability happens in the logic that accounts for dirty data (data modified in memory but not yet written to disk). F2FS needs to compute how many free segments (large blocks of storage) it will require in certain write and checkpoint scenarios. Due to a bug, it could underestimate the amount needed, especially when:

LFS mode (Log-structured File System mode) is active.

In these conditions, the filesystem might run out of space unexpectedly, leading to out-of-bounds memory writes or a forced kernel crash—something you definitely don't want.

Here’s an actual kernel trace (simplified for readability)

------------[ cut here ]------------
kernel BUG at fs/f2fs/segment.c:2752!
RIP: 001:new_curseg+xc81/x211
Call Trace:
 f2fs_allocate_data_block+x1c91/x454
 do_write_page+x163/xdf
 f2fs_outplace_write_data+x1aa/x340
 ...
 file_write_and_wait_range+xa1/x110
 f2fs_do_sync_file+x26f/x1c50
 f2fs_sync_file+x12b/x1d
 ...

This bug triggers a kernel panic—your PC or server just halts—during common filesystem actions if the problem scenario is met.

Why Did It Happen?

F2FS uses a combination of "segments" and "pages" to track what parts of the disk are in use. When both checkpointing is disabled and LFS mode is on, F2FS tries to outplace (allocate new space for) all overwritten data—costing more free space than expected.

If the filesystem underestimates how many free segments are needed, it tries to allocate more than available, causing panic. What should happen is that the kernel returns ENOSPC (no space left on device) _earlier_ to prevent over-allocation.

The Patch

The fix is small but important: F2FS now properly accounts for dirty data in __get_secs_required(), guaranteeing that it doesn't go on to allocate more segments than are actually free, and thus safely returns an error before the situation gets critical.

Patch snippet

diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
index ... 
@@ -XYZ,7 +XYZ,8 @@
 /* ... removed for brevity ... */
-    /* Old logic: Misaccounted dirty data */
+    /* New logic: Properly count dirty data */
+    dirty_segments += get_dirty_data_count();
     if (free_segments < dirty_segments)
         return -ENOSPC;

*Note: This is a synthetic example for illustration; see the real change upstream.*

Exploit Details

While this isn't directly exploitable as a privilege escalation or arbitrary code execution, it is a denial of service (DoS) vulnerability. Anyone with write permissions to an F2FS filesystem could crash the whole system by triggering low-space writes in the right conditions (with checkpoint disabled and LFS mode on).

The original report and a reproducing testcase are here

- Upstream kernel list: f2fs OPU crash testcase

Advice for Users

- Upgrade: If you use F2FS anywhere, update to a Linux kernel with this fix applied. The patch was accepted into mainline after June 2024.
- Mitigate: Avoid using both checkpoint disabling AND LFS mode on the same F2FS mount until patched.

References

- Original bug report and patch discussion
- Commit fixing the bug (when available)
- F2FS documentation (kernel.org)

Summary

CVE-2024-53220 is a Linux kernel bug that could crash whole systems due to a subtle miscount in how F2FS calculates free space during certain write operations. If you use F2FS—especially on servers, embedded devices, or Android—be sure to get this fix, or risk sudden reboots and unscheduled downtime!

Timeline

Published on: 12/27/2024 14:15:30 UTC
Last modified on: 05/04/2025 09:56:16 UTC