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

Chao Yu
2026-04-21T07:26:53Z

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@gmail.com>
> Reported-by: Junrui Luo <moonafterrain@outlook.com>
> Closes: https://lore.kernel.org/r/A0FD7E0F-7558-49B0-8BC8-EB1ECDB2479A@outlook.com
> Cc: stable@vger.kernel.org
> Signed-off-by: Gao Xiang <hsiangkao@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;