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Subject: Re: [Linux-kernel-mentees] [PATCH net] rds: Prevent kernel-infoleak
 in rds_notify_queue_get()
From:   =?utf-8?Q?H=C3=A5kon_Bugge?= <haakon.bugge@oracle.com>
In-Reply-To: <20200731115909.GA1649637@kroah.com>
Date:   Fri, 31 Jul 2020 14:03:06 +0200
Cc:     Dan Carpenter <dan.carpenter@oracle.com>,
        Leon Romanovsky <leon@kernel.org>,
        Peilin Ye <yepeilin.cs@gmail.com>,
        Santosh Shilimkar <santosh.shilimkar@oracle.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Arnd Bergmann <arnd@arndb.de>,
        linux-kernel-mentees@lists.linuxfoundation.org,
        netdev@vger.kernel.org,
        OFED mailing list <linux-rdma@vger.kernel.org>,
        rds-devel@oss.oracle.com, linux-kernel@vger.kernel.org
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References: <20200730192026.110246-1-yepeilin.cs@gmail.com>
 <20200731045301.GI75549@unreal> <20200731095943.GI5493@kadam>
 <81B40AF5-EBCA-4628-8CF6-687C12134552@oracle.com>
 <20200731115909.GA1649637@kroah.com>
To:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Sender: linux-kernel-owner@vger.kernel.org
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> On 31 Jul 2020, at 13:59, Greg Kroah-Hartman =
<gregkh@linuxfoundation.org> wrote:
>=20
> On Fri, Jul 31, 2020 at 01:14:09PM +0200, H=C3=A5kon Bugge wrote:
>>=20
>>=20
>>> On 31 Jul 2020, at 11:59, Dan Carpenter <dan.carpenter@oracle.com> =
wrote:
>>>=20
>>> On Fri, Jul 31, 2020 at 07:53:01AM +0300, Leon Romanovsky wrote:
>>>> On Thu, Jul 30, 2020 at 03:20:26PM -0400, Peilin Ye wrote:
>>>>> rds_notify_queue_get() is potentially copying uninitialized kernel =
stack
>>>>> memory to userspace since the compiler may leave a 4-byte hole at =
the end
>>>>> of `cmsg`.
>>>>>=20
>>>>> In 2016 we tried to fix this issue by doing `=3D { 0 };` on =
`cmsg`, which
>>>>> unfortunately does not always initialize that 4-byte hole. Fix it =
by using
>>>>> memset() instead.
>>>>=20
>>>> Of course, this is the difference between "{ 0 }" and "{}" =
initializations.
>>>>=20
>>>=20
>>> No, there is no difference.  Even struct assignments like:
>>>=20
>>> 	foo =3D *bar;
>>>=20
>>> can leave struct holes uninitialized.  Depending on the compiler the
>>> assignment can be implemented as a memset() or as a series of struct
>>> member assignments.
>>=20
>> What about:
>>=20
>> struct rds_rdma_notify {
>> 	__u64                      user_token;
>> 	__s32                      status;
>> } __attribute__((packed));
>=20
> Why is this still a discussion at all?
>=20
> Try it and see, run pahole and see if there are holes in this =
structure
> (odds are no), you don't need us to say what is happening here...

An older posting had this:

$ pahole -C "rds_rdma_notify" net/rds/recv.o
struct rds_rdma_notify {
	__u64                      user_token;           /*     0     8 =
*/
	__s32                      status;               /*     8     4 =
*/

	/* size: 16, cachelines: 1, members: 2 */
	/* padding: 4 */
	/* last cacheline: 16 bytes */
};


Thxs, H=C3=A5kon