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]), use unsafe_memcpy(), as the sizing,
bounds-checking, and allocation are all very tightly coupled here.
This silences the false-positive reported by syzbot:

  memcpy: detected field-spanning write (size 80) of single field "&n->sel" at net/sched/cls_u32.c:1043 (size 16)

[1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org

Cc: Jamal Hadi Salim <jhs@mojatatu.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Cong Wang <xiyou.wangcong@gmail.com>
Cc: Jiri Pirko <jiri@resnulli.us>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: netdev@vger.kernel.org
Reported-by: syzbot+a2c4601efc75848ba321@syzkaller.appspotmail.com
Link: https://lore.kernel.org/lkml/000000000000a96c0b05e97f0444@google.com/
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 net/sched/cls_u32.c | 6 +++++-
 1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 4d27300c287c..5f33472aad36 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c
@@ -1040,7 +1040,11 @@ 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 with struct_size(),
+		       * bounds-checked against nla_len(), and allocated
+		       * above. */);
 	RCU_INIT_POINTER(n->ht_up, ht);
 	n->handle = handle;
 	n->fshift = s->hmask ? ffs(ntohl(s->hmask)) - 1 : 0;
-- 
2.34.1

Hello:

This patch was applied to netdev/net-next.git (master)
by Jakub Kicinski <kuba@kernel.org>:

On Tue, 27 Sep 2022 08:37:01 -0700 you 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]), use unsafe_memcpy(), as the sizing,
> bounds-checking, and allocation are all very tightly coupled here.
> This silences the false-positive reported by syzbot:
> 
>   memcpy: detected field-spanning write (size 80) of single field "&n->sel" at net/sched/cls_u32.c:1043 (size 16)
> 
> [...]

Here is the summary with links:
  - net: sched: cls_u32: Avoid memcpy() false-positive warning
    https://git.kernel.org/netdev/net-next/c/7cba18332e36

You are awesome, thank you!
-- 
Deet-doot-dot, I am a bot.
https://korg.docs.kernel.org/patchwork/pwbot.html

On Tue, Sep 27, 2022 at 11:37 AM Kees Cook <keescook@chromium.org> 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]), use unsafe_memcpy(), as the sizing,
> bounds-checking, and allocation are all very tightly coupled here.
> This silences the false-positive reported by syzbot:
>
>   memcpy: detected field-spanning write (size 80) of single field "&n->sel" at net/sched/cls_u32.c:1043 (size 16)
>
> [1] https://lore.kernel.org/linux-hardening/20220901065914.1417829-2-keescook@chromium.org
>
> Cc: Jamal Hadi Salim <jhs@mojatatu.com>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Cong Wang <xiyou.wangcong@gmail.com>
> Cc: Jiri Pirko <jiri@resnulli.us>
> Cc: "David S. Miller" <davem@davemloft.net>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Paolo Abeni <pabeni@redhat.com>
> Cc: netdev@vger.kernel.org
> Reported-by: syzbot+a2c4601efc75848ba321@syzkaller.appspotmail.com
> Link: https://lore.kernel.org/lkml/000000000000a96c0b05e97f0444@google.com/
> Signed-off-by: Kees Cook <keescook@chromium.org>

Reviewed-by: Jamal Hadi Salim <jhs@mojatatu.com>

cheers,
jamal