linux-kernel - Re: [PATCH] ovl: avoid possible NULL dereference

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Message-ID: <b1a6134d-f976-ed9d-aac0-06f3c93fc1c6@nfschina.com>
Date:   Thu, 28 Sep 2023 09:12:01 +0800
From:   Su Hui <suhui@...china.com>
To:     Dan Carpenter <dan.carpenter@...aro.org>,
        Amir Goldstein <amir73il@...il.com>
Cc:     miklos@...redi.hu, linux-unionfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] ovl: avoid possible NULL dereference

On 2023/9/27 22:39, Dan Carpenter wrote:
> On Wed, Sep 27, 2023 at 05:02:26PM +0300, Amir Goldstein wrote:
>> On Mon, Sep 25, 2023 at 7:52 AM Su Hui <suhui@...china.com> wrote:
>>> smatch warn:
>>> fs/overlayfs/copy_up.c:450 ovl_set_origin() warn:
>>> variable dereferenced before check 'fh' (see line 449)
>>>
>>> If 'fh' is NULL, passing NULL instead of 'fh->buf'.
>>>
>>> Signed-off-by: Su Hui <suhui@...china.com>
>>> ---
>>>   fs/overlayfs/copy_up.c | 2 +-
>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
>>> index d1761ec5866a..086f9176b4d4 100644
>>> --- a/fs/overlayfs/copy_up.c
>>> +++ b/fs/overlayfs/copy_up.c
>>> @@ -446,7 +446,7 @@ int ovl_set_origin(struct ovl_fs *ofs, struct dentry *lower,
>>>          /*
>>>           * Do not fail when upper doesn't support xattrs.
>>>           */
>>> -       err = ovl_check_setxattr(ofs, upper, OVL_XATTR_ORIGIN, fh->buf,
>>> +       err = ovl_check_setxattr(ofs, upper, OVL_XATTR_ORIGIN, fh ? fh->buf : NULL,
>>>                                   fh ? fh->fb.len : 0, 0);
>>>          kfree(fh);
>>>
>>> --
>>> 2.30.2
>> After discussing this with Dan Carpenter, this is not a kernel bug,
>> it is a smatch bug.
> Yeah.  Sorry about that, Su Hui.  The ->buf struct member is not a
> pointer, it's an array.  So this isn't really a dereference, it's just
> pointer math and foo = fh->buf won't crash even if fh is NULL.
Got it, I'm so careless that make this wrong patch.
Really thanks for your reminder!

Su Hui

>
> I have written a fix for this in Smatch.  I'll test it a bit before I
> push it.
>
> regards,
> dan carpenter
>