Re: [syzbot] WARNING in u32_change

On Sun, Sep 25, 2022 at 10:13 AM Jamal Hadi Salim [off-list ref] wrote:

To be clear, that splat didnt happen for me.
Is there something else syzbot does to activate it?

Sure, please look at:

commit 54d9469bc515dc5fcbc20eecbe19cea868b70d68
Author: Kees Cook [off-list ref]
Date:   Thu Jun 24 15:39:26 2021 -0700

    fortify: Add run-time WARN for cross-field memcpy()

cheers,
jamal

On Sun, Sep 25, 2022 at 1:08 PM Jamal Hadi Salim [off-list ref] wrote:

quoted

Yes, after testing i realize there is nothing wrong here.
What warning was i supposed to see from running the reproducer?

We will still add the test will multiple keys later

cheers,
jamal

On Sun, Sep 25, 2022 at 12:29 PM Eric Dumazet [off-list ref] wrote:

quoted

On Sun, Sep 25, 2022 at 9:14 AM Jamal Hadi Salim [off-list ref] wrote:

quoted

On Sun, Sep 25, 2022 at 11:38 AM Jamal Hadi Salim [off-list ref] wrote:

quoted

Is there a way to tell the boat "looking into it?"

I guess I have to swim across to it to get the message;->

I couldnt see the warning message  but it is obvious by inspection that
the memcpy is broken. We should add more test coverage.
This should fix it. Will send a formal patch later:

diff --git a/net/sched/cls_u32.c b/net/sched/cls_u32.c
index 4d27300c2..591cbbf27 100644
--- a/net/sched/cls_u32.c
+++ b/net/sched/cls_u32.c

@@ -1019,7 +1019,7 @@ static int u32_change(struct net *net, struct

sk_buff *in_skb,
        }

        s = nla_data(tb[TCA_U32_SEL]);
-       sel_size = struct_size(s, keys, s->nkeys);
+       sel_size = struct_size(s, keys, s->nkeys) + sizeof(n->sel);
        if (nla_len(tb[TCA_U32_SEL]) < sel_size) {
                err = -EINVAL;
                goto erridr;

This patch is not needed, please look at struct_size() definition.

Here, we might switch to unsafe_memcpy() instead of memcpy()