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

[Chao Yu]

On 4/21/2026 3:38 PM, Gao Xiang wrote:
> On 2026/4/21 15:26, Chao Yu wrote:
> > 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@xxxxxxxxx>
> > > Reported-by: Junrui Luo <moonafterrain@xxxxxxxxxxx>
> > > Closes: https://lore.kernel.org/r/A0FD7E0F-7558-49B0-8BC8-EB1ECDB2479A@xxxxxxxxxxx
> > > Cc: stable@xxxxxxxxxxxxxxx
> > > Signed-off-by: Gao Xiang <hsiangkao@xxxxxxxxxxxxxxxxx>
> > > ---
> > > 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))
>
> The explanation can be seen as:
> https://sashiko.dev/#/patchset/20260416063511.3173774-1-hsiangkao%40linux.alibaba.com
>
> But I think `nameoff < sizeof(struct erofs_dirent)` is also fine?

Yes, it's fine to use "nameoff < sizeof(struct erofs_dirent)", it's a minor
cleanup to use "!nameof".

Thanks,

> I could also switch to your suggested version.
>
> Thanks,
> Gao Xiang