ext4: fix off-by-one error in do_split

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 ext4_insert_dentry that was

caused by out-of-bounds access due to incorrect splitting in do_split.

BUG: KASAN: use-after-free in ext4_insert_dentry+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>

__dump_stack lib/dump_stack.c:94 [inline]

dump_stack_lvl+0x241/0x360 lib/dump_stack.c:120

print_address_description mm/kasan/report.c:377 [inline]

print_report+0x169/0x550 mm/kasan/report.c:488

kasan_report+0x143/0x180 mm/kasan/report.c:601

kasan_check_range+0x282/0x290 mm/kasan/generic.c:189

__asan_memcpy+0x40/0x70 mm/kasan/shadow.c:106

ext4_insert_dentry+0x36a/0x6d0 fs/ext4/namei.c:2109

add_dirent_to_buf+0x3d9/0x750 fs/ext4/namei.c:2154

make_indexed_dir+0xf98/0x1600 fs/ext4/namei.c:2351

ext4_add_entry+0x222a/0x25d0 fs/ext4/namei.c:2455

ext4_add_nondir+0x8d/0x290 fs/ext4/namei.c:2796

ext4_symlink+0x920/0xb50 fs/ext4/namei.c:3431

vfs_symlink+0x137/0x2e0 fs/namei.c:4615

do_symlinkat+0x222/0x3a0 fs/namei.c:4641

__do_sys_symlink fs/namei.c:4662 [inline]

__se_sys_symlink fs/namei.c:4660 [inline]

__x64_sys_symlink+0x7a/0x90 fs/namei.c:4660

do_syscall_x64 arch/x86/entry/common.c:52 [inline]

do_syscall_64+0xf3/0x230 arch/x86/entry/common.c:83

entry_SYSCALL_64_after_hwframe+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.