Received-SPF: pass (google.com: domain of linux-kernel+bounces-24316-linux.lists.archive=gmail.com@vger.kernel.org designates 147.75.199.223 as permitted sender) client-ip=147.75.199.223;
Date: Fri, 12 Jan 2024 08:35:39 +0300
From: Dan Carpenter <dan.carpenter@linaro.org>
To: Kees Cook <keescook@chromium.org>
Cc: "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
	Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>,
	linux-hardening@vger.kernel.org, error27@gmail.com,
	gustavoars@kernel.org, Bryan Tan <bryantan@vmware.com>,
	Vishnu Dasa <vdasa@vmware.com>,
	VMware PV-Drivers Reviewers <pv-drivers@vmware.com>,
	Arnd Bergmann <arnd@arndb.de>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	linux-kernel@vger.kernel.org, vegard.nossum@oracle.com,
	darren.kenny@oracle.com, syzkaller <syzkaller@googlegroups.com>
Subject: Re: [PATCH v2 2/2] VMCI: Fix memcpy() run-time warning in
 dg_dispatch_as_host()
Message-ID: <9d71d005-0007-4983-b5dc-251bb4bc17bd@moroto.mountain>
References: <20240105164001.2129796-1-harshit.m.mogalapalli@oracle.com>
 <20240105164001.2129796-2-harshit.m.mogalapalli@oracle.com>
 <202401081430.9DAB37B46@keescook>
 <9c742547-0021-464b-b7a8-7af46b0a4afa@embeddedor.com>
 <202401101601.30ED61A1A3@keescook>
 <8e527ade-fe49-4697-8e36-589775c63354@moroto.mountain>
 <202401111001.78284D8F@keescook>
Precedence: bulk
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <202401111001.78284D8F@keescook>

On Thu, Jan 11, 2024 at 10:13:44AM -0800, Kees Cook wrote:
> On Thu, Jan 11, 2024 at 10:15:40AM +0300, Dan Carpenter wrote:
> > On Wed, Jan 10, 2024 at 04:03:28PM -0800, Kees Cook wrote:
> > (I wrote a patch for this a few months back but didn't send it because
> > of the merge window.  I forgot about it until now that we're in a merge
> > window again...  :P)
> 
> 			memcpy(&ivi->vlan, &bulletin->vlan, VLAN_HLEN);
> 
> #define VLAN_HLEN 4
> ivi->vlan is u32
> bulletin has:
> 	u16 vlan;
> 	u8 vlan_padding[6];
> 
> yeah, ew. Should it even be reading padding? i.e. should this be:
> 
> 		ivi->vlan = bulletin->vlan << 16;
> 

That seems reasonable.  We don't care about endianness?

> ?
> 
> Or should bulletin be:
> 
> 	union {
> 		struct {
> 			u16 vlan;
> 			u8 vlan_padding[6];
> 		};
> 		struct {
> 			u32 vlan_header;
> 			u8 vlan_header_padding[4];
> 		};
> 	};
> 
> with:
> 
> 		ivi->vlan = bulletin->vlan_header;
> 
> ?

My patch used a struct group.

	struct_group(vlan_header,
		u16 vlan;
		u16 vlan_padding;
	);

We don't need 6 bytes of padding, only 2.  Using a shift seems like
possibly a better approach.  Have to think about that.


> 
> I've been finding that almost all memcpy()s and memset()s into non-array
> types are better just rewritten as a direct assignment. :P

That seems like a good rule of thumb, thanks.

regards,
dan carpenter