CVE-2025-37785 in Linux
Summary
by MITRE • 04/18/2025
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]
[ 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
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Analysis
by VulDB Data Team • 02/16/2026
The vulnerability described in CVE-2025-37785 represents a critical out-of-bounds read condition within the Linux kernel's ext4 filesystem implementation. This flaw occurs during the processing of corrupted filesystems where directory entries contain specific malformed structures that bypass normal validation checks. The issue manifests when a filesystem contains a directory with a '.' entry whose rec_len parameter equals the block size, typically 4KB in ext4 implementations. This specific configuration allows an attacker to manipulate directory structures in a way that leads to memory access violations.
The technical root cause lies within the ext4_empty_dir() function which makes assumptions about directory entry layouts that break down under certain corrupted conditions. The function first loads the '.' directory entry and performs sanity checks through ext4_check_dir_entry(), but fails to properly validate entries that precisely fill an entire block. When the rec_len of the '.' entry equals the block size, the pointer calculation in ext4_next_entry() results in a pointer that points exactly past the allocated memory block. Subsequent dereference operations on this out-of-bounds pointer trigger the memory access violation that KASAN reports as a use-after-free error, though the actual issue is an out-of-bounds read.
This vulnerability has significant operational impact as it can be exploited to cause kernel crashes or potentially enable privilege escalation in scenarios where malicious filesystems are mounted. The flaw affects any system running the Linux kernel with ext4 filesystem support and is particularly concerning because it can be triggered through normal filesystem operations like directory removal. The vulnerability was discovered through automated fuzzing using the syzkaller tool, which systematically tests kernel interfaces and reveals edge cases in filesystem handling. The call trace shows the execution path leading to the issue through ext4_empty_dir, ext4_rmdir, and ultimately to the problematic __ext4_check_dir_entry function.
The fix implemented addresses the core issue by extending the __ext4_check_dir_entry() function to properly handle '.' directory entries that reach the end of data blocks. The solution specifically checks for entries where name_len is non-zero to properly ignore phantom directory entries that would otherwise be processed incorrectly. This approach aligns with established security practices for memory safety and follows the principle of defensive programming. The fix demonstrates adherence to CWE-129: Improper Validation of Array Indices and CWE-787: Out-of-bounds Write, as it prevents both over-indexing and improper memory access patterns. The mitigation strategy prevents the exploitation of this vulnerability by ensuring that directory entry validation properly accounts for edge cases where entries fill entire blocks, thereby maintaining kernel stability and preventing potential denial-of-service attacks against systems with ext4 filesystems.