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

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Message-ID: <79ca2f85-1f8d-4a12-aa5a-09137033605f@moroto.mountain>
Date: Tue, 9 Jan 2024 12:07:58 +0300
From: Dan Carpenter <dan.carpenter@...aro.org>
To: "Gustavo A. R. Silva" <gustavo@...eddedor.com>
Cc: Kees Cook <keescook@...omium.org>,
	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 Mon, Jan 08, 2024 at 08:05:38PM -0600, Gustavo A. R. Silva wrote:
> > > Gustavo quoted:
> > > "Under FORTIFY_SOURCE we should not copy data across multiple members
> > > in a structure."
> > > 
> > > Reported-by: syzkaller <syzkaller@...glegroups.com>
> > > Suggested-by: Vegard Nossum <vegard.nossum@...cle.com>
> > > Suggested-by: Gustavo A. R. Silva <gustavoars@...nel.org>
> > > Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@...cle.com>
> > 
> > Thanks for getting this fixed!
> > 
> > Yeah, it's a "false positive" in the sense that the code was expecting
> 
> It's a false positive _bug_, and a legitimate _warning_ coming from fortified
> memcpy().

It really feels like you're trying to sell the cost of this as a good
thing...  We've already merged fortify so why are you still fighting
about this?  Now that it's merged, let's just all admit that false
positives are bad.

I feel like once we recognize that actually false positives are bad as
opposed to good then that's when we start looking for solutions.  In
Smatch, I have code that silences warnings about cross member writes
because it was a common source of false positives...

The Kconfig entry does not mention the risk of false positives at all
and it doesn't say anything about turning on fortify along with
CONFIG_PANIC_ON_OOPS is probably bad.  There are simple things we could
do to make this less risky.

regards,
dan carpenter