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

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Date:   Wed, 27 Sep 2023 17:02:26 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     Su Hui <suhui@...china.com>
Cc:     miklos@...redi.hu, linux-unionfs@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-janitors@...r.kernel.org,
        Dan Carpenter <dan.carpenter@...aro.org>
Subject: Re: [PATCH] ovl: avoid possible NULL dereference

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.

The value being passed to setxattr is (void *)OVL_FH_FID_OFFSET,
which is just as arbitrary as NULL when size is 0.

Thanks,
Amir.