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From:   Jamal Hadi Salim <jhs@mojatatu.com>
Date:   Tue, 27 Sep 2022 07:23:29 -0400
Message-ID: <CAM0EoMm91Gmt6M7AVqp+OKvTH0Y0vCxQzpLndhwHzE-tke7J3g@mail.gmail.com>
Subject: Re: [syzbot] WARNING in u32_change
To:     Kees Cook <keescook@chromium.org>
Cc:     Eric Dumazet <edumazet@google.com>,
        syzbot <syzbot+a2c4601efc75848ba321@syzkaller.appspotmail.com>,
        David Miller <davem@davemloft.net>,
        Jiri Pirko <jiri@resnulli.us>,
        Jakub Kicinski <kuba@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>, Paolo Abeni <pabeni@redhat.com>,
        syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
        Cong Wang <xiyou.wangcong@gmail.com>
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Do you want to submit that patch then?
FWIW, I could not recreate what syzbot saw even after setting
CONFIG_FORTIFY_SOURCE=y

cheers,
jamal

On Sun, Sep 25, 2022 at 10:39 PM Kees Cook <keescook@chromium.org> wrote:
>
> On Sun, Sep 25, 2022 at 10:34:37AM -0700, Eric Dumazet wrote:
> > Sure, please look at:
> >
> > commit 54d9469bc515dc5fcbc20eecbe19cea868b70d68
> > Author: Kees Cook <keescook@chromium.org>
> > Date:   Thu Jun 24 15:39:26 2021 -0700
> >
> >     fortify: Add run-time WARN for cross-field memcpy()
> > [...]
> > Here, we might switch to unsafe_memcpy() instead of memcpy()
>
> I would tend to agree. Something like:
>
> diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
> index 4d27300c287c..21e0e6206ecc 100644
> --- a/net/sched/cls_u32.c
> +++ b/net/sched/cls_u32.c
> @@ -1040,7 +1040,9 @@ static int u32_change(struct net *net, struct sk_buff *in_skb,
>         }
>  #endif
>
> -       memcpy(&n->sel, s, sel_size);
> +       unsafe_memcpy(&n->sel, s, sel_size,
> +                     /* A composite flex-array structure destination,
> +                      * which was correctly sized and allocated above. */);
>         RCU_INIT_POINTER(n->ht_up, ht);
>         n->handle = handle;
>         n->fshift = s->hmask ? ffs(ntohl(s->hmask)) - 1 : 0;
>
> This alloc/partial-copy pattern is relatively common in the kernel, so
> I've been considering adding a helper for it. It'd be like kmemdup(),
> but more like kmemdup_offset(), which only the object from a certainly
> point is copied.
>
> --
> Kees Cook