CVE-2025-37785

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

ext4: fix OOB read when checking dotdot dir

Mounting a corrupted filesystem with directory which contains '.' dir

entry with rec_len == block size results in out-of-bounds read (later

on, when the corrupted directory is removed).

ext4_empty_dir() assumes every ext4 directory contains at least '.'

and '..' as directory entries in the first data block. It first loads

the '.' dir entry, performs sanity checks by calling ext4_check_dir_entry()

and then uses its rec_len member to compute the location of '..' dir

entry (in ext4_next_entry). It assumes the '..' dir entry fits into the

same data block.

If the rec_len of '.' is precisely one block (4KB), it slips through the

sanity checks (it is considered the last directory entry in the data

block) and leaves "struct ext4_dir_entry_2 *de" point exactly past the

memory slot allocated to the data block. The following call to

ext4_check_dir_entry() on new value of de then dereferences this pointer

which results in out-of-bounds mem access.

Fix this by extending __ext4_check_dir_entry() to check for '.' dir

entries that reach the end of data block. Make sure to ignore the phony

dir entries for checksum (by checking name_len for non-zero).

Note: This is reported by KASAN as use-after-free in case another

structure was recently freed from the slot past the bound, but it is

really an OOB read.

This issue was found by syzkaller tool.

Call Trace:

[ 38.594108] BUG: KASAN: slab-use-after-free in __ext4_check_dir_entry+0x67e/0x710

[ 38.594649] Read of size 2 at addr ffff88802b41a004 by task syz-executor/5375

[ 38.595158]

[ 38.595288] CPU: 0 UID: 0 PID: 5375 Comm: syz-executor Not tainted 6.14.0-rc7 #1

[ 38.595298] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.16.3-0-ga6ed6b701f0a-prebuilt.qemu.org 04/01/2014

[ 38.595304] Call Trace:

[ 38.595308] <TASK>

[ 38.595311] dump_stack_lvl+0xa7/0xd0

[ 38.595325] print_address_description.constprop.0+0x2c/0x3f0

[ 38.595339] ? __ext4_check_dir_entry+0x67e/0x710

[ 38.595349] print_report+0xaa/0x250

[ 38.595359] ? __ext4_check_dir_entry+0x67e/0x710

[ 38.595368] ? kasan_addr_to_slab+0x9/0x90

[ 38.595378] kasan_report+0xab/0xe0

[ 38.595389] ? __ext4_check_dir_entry+0x67e/0x710

[ 38.595400] __ext4_check_dir_entry+0x67e/0x710

[ 38.595410] ext4_empty_dir+0x465/0x990

[ 38.595421] ? __pfx_ext4_empty_dir+0x10/0x10

[ 38.595432] ext4_rmdir.part.0+0x29a/0xd10

[ 38.595441] ? __dquot_initialize+0x2a7/0xbf0

[ 38.595455] ? __pfx_ext4_rmdir.part.0+0x10/0x10

[ 38.595464] ? __pfx___dquot_initialize+0x10/0x10

[ 38.595478] ? down_write+0xdb/0x140

[ 38.595487] ? __pfx_down_write+0x10/0x10

[ 38.595497] ext4_rmdir+0xee/0x140

[ 38.595506] vfs_rmdir+0x209/0x670

[ 38.595517] ? lookup_one_qstr_excl+0x3b/0x190

[ 38.595529] do_rmdir+0x363/0x3c0

[ 38.595537] ? __pfx_do_rmdir+0x10/0x10

[ 38.595544] ? strncpy_from_user+0x1ff/0x2e0

[ 38.595561] __x64_sys_unlinkat+0xf0/0x130

[ 38.595570] do_syscall_64+0x5b/0x180

[ 38.595583] entry_SYSCALL_64_after_hwframe+0x76/0x7e