linux-hardening - Re: [PATCH v2 2/2] VMCI: Fix memcpy() run-time warning in dg_dispatch_as

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Date: Fri, 12 Jan 2024 08:35:39 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: Kees Cook <keescook@...omium.org>
Cc: "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
	Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>,
	linux-hardening@...r.kernel.org, error27@...il.com,
	gustavoars@...nel.org, Bryan Tan <bryantan@...are.com>,
	Vishnu Dasa <vdasa@...are.com>,
	VMware PV-Drivers Reviewers <pv-drivers@...are.com>,
	Arnd Bergmann <arnd@...db.de>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	linux-kernel@...r.kernel.org, vegard.nossum@...cle.com,
	darren.kenny@...cle.com, syzkaller <syzkaller@...glegroups.com>
Subject: Re: [PATCH v2 2/2] VMCI: Fix memcpy() run-time warning in
 dg_dispatch_as_host()

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