[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 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))
> 
> 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
>