Re: [syzbot] WARNING in u32_change

[Kees Cook <keescook@chromium.org>]

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