Re: [PATCH] ext4: validate readdir offset before accessing dirent
From: Jan Kara <jack@suse.cz>
Date: 2026-07-03 15:43:27
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On Fri 03-07-26 09:51:06, Yao Kai wrote:
A corrupted directory can trigger the following KASAN report when
ext4_readdir() resumes from an invalid position:
BUG: KASAN: use-after-free in __ext4_check_dir_entry+0x5ef/0x820
Read of size 2 at addr ffff88810a646000 by task repro_linear/509
Call Trace:
<TASK>
dump_stack_lvl+0x53/0x70
print_report+0xd0/0x630
kasan_report+0xce/0x100
__ext4_check_dir_entry+0x5ef/0x820
ext4_readdir+0xcde/0x2b70
iterate_dir+0x1a1/0x520
__x64_sys_getdents64+0x12b/0x220
do_syscall_64+0xf9/0x540
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>
KASAN reports use-after-free because the out-of-bounds access lands in an
adjacent freed page. The directory buffer itself is still referenced.
ext4_dir_llseek() invalidates the directory cookie so that ext4_readdir()
rescans directory entries from the start of the block. The rescan checks
only the lower bound of rec_len before advancing. A corrupted rec_len can
therefore place the offset where the block has insufficient space for a
complete directory entry. The rescan itself may dereference that truncated
entry, or the main loop may pass it to __ext4_check_dir_entry(). The latter
reads de->rec_len before validating the range. For example:
block offset 0 4092 4096
|---- de1.rec_len = 4092 -----|----|
de2.inode
| de2.rec_len
^ OOB, reported as UAF
de2 starts at offset 4092 in this 4 KiB block. Its four-byte inode fits in
the block, but its rec_len starts at offset 4096 and crosses the boundary.
The minimum safe length is inode-dependent. Encrypted and casefolded
directory entries need eight additional hash bytes, while a valid metadata
checksum tail is only 12 bytes.
Cache the metadata checksum feature state and derive the minimum directory
entry length from the on-disk format. Use it to bound both the rescan and
the offset passed to the main loop. Report an offset in a truncated block
tail and skip the remainder of the block, while continuing to accept an
offset exactly at the block boundary.
Reported-by: syzbot+5322c5c260eb44d209ed@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=5322c5c260eb44d209ed
Fixes: ac27a0ec112a ("[PATCH] ext4: initial copy of files from ext3")
Signed-off-by: Yao Kai <redacted>Just one nit below. Feel free to add: Reviewed-by: Jan Kara <jack@suse.cz>
quoted hunk ↗ jump to hunk
@@ -257,6 +261,17 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx) info->cookie = inode_query_iversion(inode); } + if (unlikely(offset < sb->s_blocksize && + offset > sb->s_blocksize - + ext4_dir_rec_len(1, has_csum ? NULL : inode))) { + EXT4_ERROR_FILE(file, bh->b_blocknr, + "bad entry in directory: %s - offset=%u, size=%lu", + "directory entry too close to block end", + offset, sb->s_blocksize); + ctx->pos = (ctx->pos | (sb->s_blocksize - 1)) + 1;
^^^ this is round_up(ctx->pos, sb->s_blocksize)
+ goto next_block;
+ }
+
while (ctx->pos < inode->i_size
&& offset < sb->s_blocksize) {
de = (struct ext4_dir_entry_2 *) (bh->b_data + offset);Honza -- Jan Kara [off-list ref] SUSE Labs, CR