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Subject: Re: [PATCH 48/64] drbd: Use struct_group() to zero algs
To:     Kees Cook <keescook@chromium.org>
Cc:     linux-hardening@vger.kernel.org,
        "Gustavo A. R. Silva" <gustavoars@kernel.org>,
        Keith Packard <keithpac@amazon.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        linux-kernel@vger.kernel.org, linux-wireless@vger.kernel.org,
        netdev@vger.kernel.org, dri-devel@lists.freedesktop.org,
        linux-staging@lists.linux.dev, linux-block@vger.kernel.org,
        linux-kbuild@vger.kernel.org, clang-built-linux@googlegroups.com,
        Lars Ellenberg <lars.ellenberg@linbit.com>
References: <20210727205855.411487-1-keescook@chromium.org>
 <20210727205855.411487-49-keescook@chromium.org>
 <1cc74e5e-8d28-6da4-244e-861eac075ca2@acm.org>
 <202107291845.1E1528D@keescook>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <0d71917d-967f-beaa-d83e-a60fa254627c@acm.org>
Date:   Thu, 29 Jul 2021 19:57:47 -0700
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On 7/29/21 7:31 PM, Kees Cook wrote:
> On Wed, Jul 28, 2021 at 02:45:55PM -0700, Bart Van Assche wrote:
>> On 7/27/21 1:58 PM, Kees Cook wrote:
>>> In preparation for FORTIFY_SOURCE performing compile-time and run-time
>>> field bounds checking for memset(), avoid intentionally writing across
>>> neighboring fields.
>>>
>>> Add a struct_group() for the algs so that memset() can correctly reason
>>> about the size.
>>>
>>> Signed-off-by: Kees Cook <keescook@chromium.org>
>>> ---
>>>   drivers/block/drbd/drbd_main.c     | 3 ++-
>>>   drivers/block/drbd/drbd_protocol.h | 6 ++++--
>>>   drivers/block/drbd/drbd_receiver.c | 3 ++-
>>>   3 files changed, 8 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/drivers/block/drbd/drbd_main.c b/drivers/block/drbd/drbd_main.c
>>> index 55234a558e98..b824679cfcb2 100644
>>> --- a/drivers/block/drbd/drbd_main.c
>>> +++ b/drivers/block/drbd/drbd_main.c
>>> @@ -729,7 +729,8 @@ int drbd_send_sync_param(struct drbd_peer_device *peer_device)
>>>   	cmd = apv >= 89 ? P_SYNC_PARAM89 : P_SYNC_PARAM;
>>>   	/* initialize verify_alg and csums_alg */
>>> -	memset(p->verify_alg, 0, 2 * SHARED_SECRET_MAX);
>>> +	BUILD_BUG_ON(sizeof(p->algs) != 2 * SHARED_SECRET_MAX);
>>> +	memset(&p->algs, 0, sizeof(p->algs));
>>>   	if (get_ldev(peer_device->device)) {
>>>   		dc = rcu_dereference(peer_device->device->ldev->disk_conf);
>>> diff --git a/drivers/block/drbd/drbd_protocol.h b/drivers/block/drbd/drbd_protocol.h
>>> index dea59c92ecc1..a882b65ab5d2 100644
>>> --- a/drivers/block/drbd/drbd_protocol.h
>>> +++ b/drivers/block/drbd/drbd_protocol.h
>>> @@ -283,8 +283,10 @@ struct p_rs_param_89 {
>>>   struct p_rs_param_95 {
>>>   	u32 resync_rate;
>>> -	char verify_alg[SHARED_SECRET_MAX];
>>> -	char csums_alg[SHARED_SECRET_MAX];
>>> +	struct_group(algs,
>>> +		char verify_alg[SHARED_SECRET_MAX];
>>> +		char csums_alg[SHARED_SECRET_MAX];
>>> +	);
>>>   	u32 c_plan_ahead;
>>>   	u32 c_delay_target;
>>>   	u32 c_fill_target;
>>> diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
>>> index 1f740e42e457..6df2539e215b 100644
>>> --- a/drivers/block/drbd/drbd_receiver.c
>>> +++ b/drivers/block/drbd/drbd_receiver.c
>>> @@ -3921,7 +3921,8 @@ static int receive_SyncParam(struct drbd_connection *connection, struct packet_i
>>>   	/* initialize verify_alg and csums_alg */
>>>   	p = pi->data;
>>> -	memset(p->verify_alg, 0, 2 * SHARED_SECRET_MAX);
>>> +	BUILD_BUG_ON(sizeof(p->algs) != 2 * SHARED_SECRET_MAX);
>>> +	memset(&p->algs, 0, sizeof(p->algs));
>>
>> Using struct_group() introduces complexity. Has it been considered not to
>> modify struct p_rs_param_95 and instead to use two memset() calls instead of
>> one (one memset() call per member)?
> 
> I went this direction because using two memset()s (or memcpy()s in other
> patches) changes the machine code. It's not much of a change, but it
> seems easier to justify "no binary changes" via the use of struct_group().
> 
> If splitting the memset() is preferred, I can totally do that instead.
> :)

I don't have a strong opinion about this. Lars, do you want to comment
on this patch?

Thanks,

Bart.