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Subject: Re: [PATCH -next 3/5] ceph: fix possible null-ptr-deref when parsing
 param
To:     Hawkins Jiawei <yin31149@gmail.com>,
        Ilya Dryomov <idryomov@gmail.com>,
        Jeff Layton <jlayton@kernel.org>
Cc:     18801353760@163.com, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, ceph-devel@vger.kernel.org
References: <20221023163945.39920-1-yin31149@gmail.com>
 <20221023163945.39920-4-yin31149@gmail.com>
From:   Xiubo Li <xiubli@redhat.com>
Message-ID: <aa3f35c1-1550-a322-956f-1837cb2389a9@redhat.com>
Date:   Mon, 24 Oct 2022 08:55:13 +0800
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On 24/10/2022 00:39, Hawkins Jiawei wrote:
> According to commit "vfs: parse: deal with zero length string value",
> kernel will set the param->string to null pointer in vfs_parse_fs_string()
> if fs string has zero length.
>
> Yet the problem is that, ceph_parse_mount_param() will dereferences the
> param->string, without checking whether it is a null pointer, which may
> trigger a null-ptr-deref bug.
>
> This patch solves it by adding sanity check on param->string
> in ceph_parse_mount_param().
>
> Signed-off-by: Hawkins Jiawei <yin31149@gmail.com>
> ---
>   fs/ceph/super.c | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/fs/ceph/super.c b/fs/ceph/super.c
> index 3fc48b43cab0..341e23fe29eb 100644
> --- a/fs/ceph/super.c
> +++ b/fs/ceph/super.c
> @@ -417,6 +417,9 @@ static int ceph_parse_mount_param(struct fs_context *fc,
>   		param->string = NULL;
>   		break;
>   	case Opt_mds_namespace:
> +		if (!param->string)
> +			return invalfc(fc, "Bad value '%s' for mount option '%s'\n",
> +				       param->string, param->key);
>   		if (!namespace_equals(fsopt, param->string, strlen(param->string)))
>   			return invalfc(fc, "Mismatching mds_namespace");
>   		kfree(fsopt->mds_namespace);

BTW, did you hit any crash issue when testing this ?

$ ./bin/mount.ceph :/ /mnt/kcephfs -o mds_namespace=

<5>[  375.535442] ceph: module verification failed: signature and/or 
required key missing - tainting kernel
<6>[  375.698145] ceph: loaded (mds proto 32)
<3>[  375.801621] ceph: Bad value for 'mds_namespace'

 From my test, the 'fsparam_string()' has already make sure it won't 
trigger the null-ptr-deref bug.

But it will always make sense to fix it in ceph code with your patch.

- Xiubo