Re: [PATCH] ext4: validate readdir offset before accessing dirent

[Zhang Yi <yizhang089@gmail.com>]

On 7/3/2026 9:51 AM, 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 <yaokai34@huawei.com>

Thanks for the fix patch. With Jan's suggestion incorporated, this looks
good to me.

Reviewed-by: Zhang Yi <yi.zhang@huawei.com>

> ---
>   fs/ext4/dir.c | 20 ++++++++++++++++++--
>   1 file changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/fs/ext4/dir.c b/fs/ext4/dir.c
> index 17edd678fa87..5c943d18882a 100644
> --- a/fs/ext4/dir.c
> +++ b/fs/ext4/dir.c
> @@ -138,6 +138,7 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>   	struct buffer_head *bh = NULL;
>   	struct fscrypt_str fstr = FSTR_INIT(NULL, 0);
>   	struct dir_private_info *info = file->private_data;
> +	bool has_csum = ext4_has_feature_metadata_csum(sb);
>   
>   	err = fscrypt_prepare_readdir(inode);
>   	if (err)
> @@ -149,7 +150,7 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>   			return err;
>   
>   		/* Can we just clear INDEX flag to ignore htree information? */
> -		if (!ext4_has_feature_metadata_csum(sb)) {
> +		if (!has_csum) {
>   			/*
>   			 * We don't set the inode dirty flag since it's not
>   			 * critical that it gets flushed back to the disk.
> @@ -235,7 +236,10 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>   		 * dirent right now.  Scan from the start of the block
>   		 * to make sure. */
>   		if (!inode_eq_iversion(inode, info->cookie)) {
> -			for (i = 0; i < sb->s_blocksize && i < offset; ) {
> +			for (i = 0;
> +			     i <= sb->s_blocksize -
> +				  ext4_dir_rec_len(1, has_csum ? NULL : inode) &&
> +			     i < offset;) {
>   				de = (struct ext4_dir_entry_2 *)
>   					(bh->b_data + i);
>   				/* It's too expensive to do a full
> @@ -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;
> +			goto next_block;
> +		}
> +
>   		while (ctx->pos < inode->i_size
>   		       && offset < sb->s_blocksize) {
>   			de = (struct ext4_dir_entry_2 *) (bh->b_data + offset);
> @@ -312,6 +327,7 @@ static int ext4_readdir(struct file *file, struct dir_context *ctx)
>   			ctx->pos += ext4_rec_len_from_disk(de->rec_len,
>   						sb->s_blocksize);
>   		}
> +next_block:
>   		if ((ctx->pos < inode->i_size) && !dir_relax_shared(inode))
>   			goto done;
>   		brelse(bh);