CVE-2025-40138

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

f2fs: fix to avoid NULL pointer dereference in f2fscheckquota_consistency()

syzbot reported a f2fs bug as below:

Oops: gen[ 107.736417][ T5848] Oops: general protection fault, probably for non-canonical address 0xdffffc0000000000: 0000 [#1] SMP KASAN PTI KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] CPU: 1 UID: 0 PID: 5848 Comm: syz-executor263 Tainted: G W 6.17.0-rc1-syzkaller-00014-g0e39a731820a #0 PREEMPT{RT,(full)} RIP: 0010:strcmp+0x3c/0xc0 lib/string.c:284 Call Trace: <TASK> f2fscheckquotaconsistency fs/f2fs/super.c:1188 [inline] f2fscheckoptconsistency+0x1378/0x2c10 fs/f2fs/super.c:1436 _f2fsremount fs/f2fs/super.c:2653 [inline] f2fsreconfigure+0x482/0x1770 fs/f2fs/super.c:5297 reconfiguresuper+0x224/0x890 fs/super.c:1077 doremount fs/namespace.c:3314 [inline] pathmount+0xd18/0xfe0 fs/namespace.c:4112 domount fs/namespace.c:4133 [inline] _dosysmount fs/namespace.c:4344 [inline] _sesysmount+0x317/0x410 fs/namespace.c:4321 dosyscallx64 arch/x86/entry/syscall64.c:63 [inline] dosyscall64+0xfa/0x3b0 arch/x86/entry/syscall64.c:94 entrySYSCALL64afterhwframe+0x77/0x7f

The direct reason is f2fscheckquota_consistency() may suffer null-ptr-deref issue in strcmp().

The bug can be reproduced w/ below scripts: mkfs.f2fs -f /dev/vdb mount -t f2fs -o usrquota /dev/vdb /mnt/f2fs quotacheck -uc /mnt/f2fs/ umount /mnt/f2fs mount -t f2fs -o usrjquota=aquota.user,jqfmt=vfsold /dev/vdb /mnt/f2fs mount -t f2fs -o remount,usrjquota=,jqfmt=vfsold /dev/vdb /mnt/f2fs umount /mnt/f2fs

So, before oldqname and newqname comparison, we need to check whether they are all valid pointers, fix it.