linux-kernel - Re: [PATCH] Drivers: hv: vmbus: Split memcpy of flex-array

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Date:   Tue, 27 Sep 2022 14:23:59 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Wei Liu <wei.liu@...nel.org>
Cc:     "K. Y. Srinivasan" <kys@...rosoft.com>,
        "Gustavo A. R. Silva" <gustavoars@...nel.org>,
        Haiyang Zhang <haiyangz@...rosoft.com>,
        Stephen Hemminger <sthemmin@...rosoft.com>,
        Dexuan Cui <decui@...rosoft.com>, linux-hyperv@...r.kernel.org,
        Nathan Chancellor <nathan@...nel.org>,
        linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org
Subject: Re: [PATCH] Drivers: hv: vmbus: Split memcpy of flex-array

On Sat, Sep 24, 2022 at 10:34:12AM +0000, Wei Liu wrote:
> Hi Kees
> 
> On Fri, Sep 23, 2022 at 09:22:55PM -0700, Kees Cook wrote:
> > On Fri, Sep 23, 2022 at 10:42:38PM -0500, Gustavo A. R. Silva wrote:
> > > On Fri, Sep 23, 2022 at 08:07:41PM -0700, Kees Cook wrote:
> > > > To work around a misbehavior of the compiler's ability to see into
> > > > composite flexible array structs (as detailed in the coming memcpy()
> > > > hardening series[1]), split the memcpy() of the header and the payload
> > > > so no false positive run-time overflow warning will be generated. As it
> > > > turns out, this appears to actually reduce the text size:
> > 
> > Er, actually, I can't read/math. ;) It _does_ grow the text size. (That's
> > 2_3_ not 22 at the start of the text size...) On examination, it appears
> > to unroll the already inlined memcpy further.
> 
> Can you provide an updated commit message? No need to resend.

Since I got more testing from Nathan (and the original warning message),
I figured a full v2 respin would easier. Now sent. :) Thanks!

-Kees

-- 
Kees Cook