[PATCH v3] erofs: fix the out-of-bounds nameoff handling for trailing dirents

[Chao Yu]

On 4/16/2026 5:44 PM, Gao Xiang wrote:
> Currently we already have boundary-checks for nameoffs, but the trailing
> dirents are special since the namelens are calculated with strnlen()
> with unchecked nameoffs.
> 
> If a crafted EROFS has a trailing dirent with nameoff >= maxsize,
> maxsize - nameoff can underflow, causing strnlen() to read past the
> directory block.
> 
> nameoff0 should also be verified to be a multiple of
> `sizeof(struct erofs_dirent)` as well [1].
> 
> [1] https://sashiko.dev/#/patchset/20260416063511.3173774-1-hsiangkao%40linux.alibaba.com
> Fixes: 3aa8ec716e52 ("staging: erofs: add directory operations")
> Fixes: 33bac912840f ("staging: erofs: keep corrupted fs from crashing kernel in erofs_readdir()")
> Reported-by: Yuhao Jiang <danisjiang at gmail.com>
> Reported-by: Junrui Luo <moonafterrain at outlook.com>
> Closes: https://lore.kernel.org/r/A0FD7E0F-7558-49B0-8BC8-EB1ECDB2479A@outlook.com
> Cc: stable at vger.kernel.org
> Signed-off-by: Gao Xiang <hsiangkao at linux.alibaba.com>
> ---
> v3:
>   - Disallow unaligned nameoff0 to avoid petential oob reads as well.
> 
>   fs/erofs/dir.c | 29 ++++++++++++++++-------------
>   1 file changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/fs/erofs/dir.c b/fs/erofs/dir.c
> index e5132575b9d3..d074fded1577 100644
> --- a/fs/erofs/dir.c
> +++ b/fs/erofs/dir.c
> @@ -19,20 +19,18 @@ static int erofs_fill_dentries(struct inode *dir, struct dir_context *ctx,
>   		const char *de_name = (char *)dentry_blk + nameoff;
>   		unsigned int de_namelen;
>   
> -		/* the last dirent in the block? */
> -		if (de + 1 >= end)
> -			de_namelen = strnlen(de_name, maxsize - nameoff);
> -		else
> +		/* non-trailing dirent in the directory block? */
> +		if (de + 1 < end)
>   			de_namelen = le16_to_cpu(de[1].nameoff) - nameoff;
> +		else if (maxsize <= nameoff)
> +			goto err_bogus;
> +		else
> +			de_namelen = strnlen(de_name, maxsize - nameoff);
>   
> -		/* a corrupted entry is found */
> -		if (nameoff + de_namelen > maxsize ||
> -		    de_namelen > EROFS_NAME_LEN) {
> -			erofs_err(dir->i_sb, "bogus dirent @ nid %llu",
> -				  EROFS_I(dir)->nid);
> -			DBG_BUGON(1);
> -			return -EFSCORRUPTED;
> -		}
> +		/* a corrupted entry is found (including negative namelen) */
> +		if (!in_range32(de_namelen, 1, EROFS_NAME_LEN) ||
> +		    nameoff + de_namelen > maxsize)
> +			goto err_bogus;
>   
>   		if (!dir_emit(ctx, de_name, de_namelen,
>   			      erofs_nid_to_ino64(EROFS_SB(dir->i_sb),
> @@ -42,6 +40,10 @@ static int erofs_fill_dentries(struct inode *dir, struct dir_context *ctx,
>   		ctx->pos += sizeof(struct erofs_dirent);
>   	}
>   	return 0;
> +err_bogus:
> +	erofs_err(dir->i_sb, "bogus dirent @ nid %llu", EROFS_I(dir)->nid);
> +	DBG_BUGON(1);
> +	return -EFSCORRUPTED;
>   }
>   
>   static int erofs_readdir(struct file *f, struct dir_context *ctx)
> @@ -88,7 +90,8 @@ static int erofs_readdir(struct file *f, struct dir_context *ctx)
>   		}
>   
>   		nameoff = le16_to_cpu(de->nameoff);
> -		if (nameoff < sizeof(struct erofs_dirent) || nameoff >= bsz) {

You mean?

if (!nameoff || nameoff >= bsz || nameoff % sizeof(struct erofs_dirent))

Thanks,

> +		if (nameoff < sizeof(struct erofs_dirent) || nameoff >= bsz ||
> +		    (nameoff % sizeof(struct erofs_dirent))) {
>   			erofs_err(sb, "invalid de[0].nameoff %u @ nid %llu",
>   				  nameoff, EROFS_I(dir)->nid);
>   			err = -EFSCORRUPTED;