CVE-2025-68772

In the Linux kernel, the following vulnerability has been resolved:

f2fs: fix to avoid updating compression context during writeback

Bai, Shuangpeng sjb7183@psu.edu reported a bug as below:

Oops: divide error: 0000 [#1] SMP KASAN PTI CPU: 0 UID: 0 PID: 11441 Comm: syz.0.46 Not tainted 6.17.0 #1 PREEMPT(full) Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.15.0-1 04/01/2014 RIP: 0010:f2fs_all_cluster_page_ready+0x106/0x550 fs/f2fs/compress.c:857 Call Trace: f2fs_write_cache_pages fs/f2fs/data.c:3078 [inline] __f2fs_write_data_pages fs/f2fs/data.c:3290 [inline] f2fs_write_data_pages+0x1c19/0x3600 fs/f2fs/data.c:3317 do_writepages+0x38e/0x640 mm/page-writeback.c:2634 filemap_fdatawrite_wbc mm/filemap.c:386 [inline] __filemap_fdatawrite_range mm/filemap.c:419 [inline] file_write_and_wait_range+0x2ba/0x3e0 mm/filemap.c:794 f2fs_do_sync_file+0x6e6/0x1b00 fs/f2fs/file.c:294 generic_write_sync include/linux/fs.h:3043 [inline] f2fs_file_write_iter+0x76e/0x2700 fs/f2fs/file.c:5259 new_sync_write fs/read_write.c:593 [inline] vfs_write+0x7e9/0xe00 fs/read_write.c:686 ksys_write+0x19d/0x2d0 fs/read_write.c:738 do_syscall_x64 arch/x86/entry/syscall_64.c:63 [inline] do_syscall_64+0xf7/0x470 arch/x86/entry/syscall_64.c:94 entry_SYSCALL_64_after_hwframe+0x77/0x7f

The bug was triggered w/ below race condition:

fsync setattr ioctl

• f2fs_do_sync_file
• file_write_and_wait_range
• f2fs_write_cache_pages : inode is non-compressed : cc.cluster_size = F2FS_I(inode)->i_cluster_size = 0
• tag_pages_for_writeback - f2fs_setattr - truncate_setsize - f2fs_truncate - f2fs_fileattr_set - f2fs_setflags_common - set_compress_context : F2FS_I(inode)->i_cluster_size = 4 : set_inode_flag(inode, FI_COMPRESSED_FILE)
• f2fs_compressed_file : return true
• f2fs_all_cluster_page_ready : pgidx % cc->cluster_size trigger dividing 0 issue

Lets change as below to fix this issue:

• introduce a new atomic type variable .writeback in structure f2fs_inode_info to track the number of threads which calling f2fs_write_cache_pages().
• use .i_sem lock to protect .writeback update.
• check .writeback before update compression context in f2fs_setflags_common() to avoid race w/ ->writepages.

Affected Software

References

• https://git.kernel.org/stable/c/0bf1a02494c7eb5bd43445de4c83c8592e02c4bf
• https://git.kernel.org/stable/c/0df713a9c082a474c8b0bcf670edc8e98461d5a0
• https://git.kernel.org/stable/c/10b591e7fb7cdc8c1e53e9c000dc0ef7069aaa76
• https://git.kernel.org/stable/c/ad26bfbc085c939b5dca77ff8c14798c06d151c4
• https://git.kernel.org/stable/c/bcd0086ee5a2e88c1224ff2ec1e4a43c83efe5a0