CVE-2025-23150

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

ext4: fix off-by-one error in do_split

Syzkaller detected a use-after-free issue in ext4insertdentry that was caused by out-of-bounds access due to incorrect splitting in do_split.

BUG: KASAN: use-after-free in ext4insertdentry+0x36a/0x6d0 fs/ext4/namei.c:2109 Write of size 251 at addr ffff888074572f14 by task syz-executor335/5847

CPU: 0 UID: 0 PID: 5847 Comm: syz-executor335 Not tainted 6.12.0-rc6-syzkaller-00318-ga9cda7c0ffed #0 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/30/2024 Call Trace: <TASK> __dumpstack lib/dumpstack.c:94 [inline] dump_stacklvl+0x241/0x360 lib/dumpstack.c:120 printaddressdescription mm/kasan/report.c:377 [inline] printreport+0x169/0x550 mm/kasan/report.c:488 kasanreport+0x143/0x180 mm/kasan/report.c:601 kasancheckrange+0x282/0x290 mm/kasan/generic.c:189 __asanmemcpy+0x40/0x70 mm/kasan/shadow.c:106 ext4insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109 adddirenttobuf+0x3d9/0x750 fs/ext4/namei.c:2154 makeindexeddir+0xf98/0x1600 fs/ext4/namei.c:2351 ext4addentry+0x222a/0x25d0 fs/ext4/namei.c:2455 ext4addnondir+0x8d/0x290 fs/ext4/namei.c:2796 ext4symlink+0x920/0xb50 fs/ext4/namei.c:3431 vfssymlink+0x137/0x2e0 fs/namei.c:4615 dosymlinkat+0x222/0x3a0 fs/namei.c:4641 __dosyssymlink fs/namei.c:4662 [inline] __sesyssymlink fs/namei.c:4660 [inline] __x64syssymlink+0x7a/0x90 fs/namei.c:4660 dosyscallx64 arch/x86/entry/common.c:52 [inline] dosyscall64+0xf3/0x230 arch/x86/entry/common.c:83 entrySYSCALL64afterhwframe+0x77/0x7f </TASK>

The following loop is located right above 'if' statement.

for (i = count-1; i >= 0; i--) { /* is more than half of this entry in 2nd half of the block? */ if (size + map[i].size/2 > blocksize/2) break; size += map[i].size; move++; }

'i' in this case could go down to -1, in which case sum of active entries wouldn't exceed half the block size, but previous behaviour would also do split in half if sum would exceed at the very last block, which in case of having too many long name files in a single block could lead to out-of-bounds access and following use-after-free.

Found by Linux Verification Center (linuxtesting.org) with Syzkaller.