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Date: Sun, 25 Sep 2022 19:39:23 -0700 From: Kees Cook <keescook@...omium.org> To: Eric Dumazet <edumazet@...gle.com> Cc: Jamal Hadi Salim <jhs@...atatu.com>, syzbot <syzbot+a2c4601efc75848ba321@...kaller.appspotmail.com>, David Miller <davem@...emloft.net>, Jiri Pirko <jiri@...nulli.us>, Jakub Kicinski <kuba@...nel.org>, LKML <linux-kernel@...r.kernel.org>, netdev <netdev@...r.kernel.org>, Paolo Abeni <pabeni@...hat.com>, syzkaller-bugs <syzkaller-bugs@...glegroups.com>, Cong Wang <xiyou.wangcong@...il.com> Subject: Re: [syzbot] WARNING in u32_change On Sun, Sep 25, 2022 at 10:34:37AM -0700, Eric Dumazet wrote: > Sure, please look at: > > commit 54d9469bc515dc5fcbc20eecbe19cea868b70d68 > Author: Kees Cook <keescook@...omium.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
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