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Date:   Fri, 6 Aug 2021 15:45:36 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Saeed Mahameed <saeed@...nel.org>
Cc:     Leon Romanovsky <leon@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>, netdev@...r.kernel.org,
        linux-rdma@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH] net/mlx5e: Avoid field-overflowing memcpy()

On Fri, Aug 06, 2021 at 03:17:56PM -0700, Saeed Mahameed wrote:
> On Fri, 2021-08-06 at 14:50 -0700, Kees Cook wrote:
> > [...]
> > So, split the memcpy() so the compiler can reason about the buffer
> > sizes.
> > 
> > "pahole" shows no size nor member offset changes to struct > > mlx5e_tx_wqe
> > nor struct mlx5e_umr_wqe. "objdump -d" shows no meaningful object
> > code changes (i.e. only source line number induced differences and
> > optimizations).
> 
> spiting the memcpy doesn't induce any performance degradation ? extra
> instruction to copy the 1st 2 bytes ? 

Not meaningfully, but strictly speaking, yes, it's a different series of
instructions.

> [...]
> > --- a/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> > +++ b/drivers/net/ethernet/mellanox/mlx5/core/en/xdp.c
> 
> why only here ? mlx5 has at least 3 other places where we use this
> unbound memcpy .. 

Can you point them out? I've been fixing only the ones I've been able to
find through instrumentation (generally speaking, those with fixed
sizes).

-- 
Kees Cook

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