f2fs: fix to avoid mapping wrong physical block for swapfile

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

f2fs: fix to avoid mapping wrong physical block for swapfile

Xiaolong Guo reported a f2fs bug in bugzilla [1]

[1] https://bugzilla.kernel.org/show_bug.cgi?id=220951

Quoted:

"When using stress-ng's swap stress test on F2FS filesystem with kernel 6.6+,

the system experiences data corruption leading to either:

1 dm-verity corruption errors and device reboot

2 F2FS node corruption errors and boot hangs

The issue occurs specifically when:

1 Using F2FS filesystem (ext4 is unaffected)

2 Swapfile size is less than F2FS section size (2MB)

3 Swapfile has fragmented physical layout (multiple non-contiguous extents)

4 Kernel version is 6.6+ (6.1 is unaffected)

The root cause is in check_swap_activate() function in fs/f2fs/data.c. When the

first extent of a small swapfile (< 2MB) is not aligned to section boundaries,

the function incorrectly treats it as the last extent, failing to map

subsequent extents. This results in incorrect swap_extent creation where only

the first extent is mapped, causing subsequent swap writes to overwrite wrong

physical locations (other files' data).

Steps to Reproduce

1 Setup a device with F2FS-formatted userdata partition

2 Compile stress-ng from https://github.com/ColinIanKing/stress-ng

3 Run swap stress test: (Android devices)

adb shell "cd /data/stressng; ./stress-ng-64 --metrics-brief --timeout 60

--swap 0"

Log:

1 Ftrace shows in kernel 6.6, only first extent is mapped during second

f2fs_map_blocks call in check_swap_activate():

stress-ng-swap-8990: f2fs_map_blocks: ino=11002, file offset=0, start

blkaddr=0x43143, len=0x1

(Only 4KB mapped, not the full swapfile)

2 in kernel 6.1, both extents are correctly mapped:

stress-ng-swap-5966: f2fs_map_blocks: ino=28011, file offset=0, start

blkaddr=0x13cd4, len=0x1

stress-ng-swap-5966: f2fs_map_blocks: ino=28011, file offset=1, start

blkaddr=0x60c84b, len=0xff

The problematic code is in check_swap_activate():

if ((pblock - SM_I(sbi)->main_blkaddr) % blks_per_sec ||

nr_pblocks % blks_per_sec ||

!f2fs_valid_pinned_area(sbi, pblock)) {

bool last_extent = false;

not_aligned++;

nr_pblocks = roundup(nr_pblocks, blks_per_sec);

if (cur_lblock + nr_pblocks > sis->max)

nr_pblocks -= blks_per_sec;

/* this extent is last one */

if (!nr_pblocks) {

nr_pblocks = last_lblock - cur_lblock;

last_extent = true;

}

ret = f2fs_migrate_blocks(inode, cur_lblock, nr_pblocks);

if (ret) {

if (ret == -ENOENT)

ret = -EINVAL;

goto out;

}

if (!last_extent)

goto retry;

}

When the first extent is unaligned and roundup(nr_pblocks, blks_per_sec)

exceeds sis->max, we subtract blks_per_sec resulting in nr_pblocks = 0. The

code then incorrectly assumes this is the last extent, sets nr_pblocks =

last_lblock - cur_lblock (entire swapfile), and performs migration. After

migration, it doesn't retry mapping, so subsequent extents are never processed.

"

In order to fix this issue, we need to lookup block mapping info after

we migrate all blocks in the tail of swapfile.