netdev - Re: [PATCH net-next] net: fix kfree_skb_list use of skb_mark_not_on

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Date:   Thu, 19 Jan 2023 19:04:57 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     netdev@...r.kernel.org, Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>, pabeni@...hat.com,
        syzbot+c8a2e66e37eee553c4fd@...kaller.appspotmail.com
Subject: Re: [PATCH net-next] net: fix kfree_skb_list use of skb_mark_not_on_list

On Thu, Jan 19, 2023 at 6:50 PM Jesper Dangaard Brouer
<brouer@...hat.com> wrote:
>
> A bug was introduced by commit eedade12f4cb ("net: kfree_skb_list use
> kmem_cache_free_bulk"). It unconditionally unlinked the SKB list via
> invoking skb_mark_not_on_list().
>
> The skb_mark_not_on_list() should only be called if __kfree_skb_reason()
> returns true, meaning the SKB is ready to be free'ed, as it calls/check
> skb_unref().
>
> This is needed as kfree_skb_list() is also invoked on skb_shared_info
> frag_list. A frag_list can have SKBs with elevated refcnt due to cloning
> via skb_clone_fraglist(), which takes a reference on all SKBs in the
> list. This implies the invariant that all SKBs in the list must have the
> same refcnt, when using kfree_skb_list().

Yeah, or more precisely skb_drop_fraglist() calling kfree_skb_list()

>
> Reported-by: syzbot+c8a2e66e37eee553c4fd@...kaller.appspotmail.com
> Reported-and-tested-by: syzbot+c8a2e66e37eee553c4fd@...kaller.appspotmail.com
> Fixes: eedade12f4cb ("net: kfree_skb_list use kmem_cache_free_bulk")
> Signed-off-by: Jesper Dangaard Brouer <brouer@...hat.com>
> ---
>  net/core/skbuff.c |    6 +++---
>  1 file changed, 3 insertions(+), 3 deletions(-)
>
> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> index 4e73ab3482b8..1bffbcbe6087 100644
> --- a/net/core/skbuff.c
> +++ b/net/core/skbuff.c
> @@ -999,10 +999,10 @@ kfree_skb_list_reason(struct sk_buff *segs, enum skb_drop_reason reason)
>         while (segs) {
>                 struct sk_buff *next = segs->next;
>
> -               skb_mark_not_on_list(segs);
> -
> -               if (__kfree_skb_reason(segs, reason))
> +               if (__kfree_skb_reason(segs, reason)) {
> +                       skb_mark_not_on_list(segs);

Real question is : Why do we need to set/change/dirt skb->next ?

I would remove this completely, and save extra cache lines dirtying.

Before your patch, we were not calling skb_mark_not_on_list(segs),
so why bother ?

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4e73ab3482b87d81371cff266627dab646d3e84c..180df58e85c72eaa16f5cb56b56d181a379b8921
100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -999,8 +999,6 @@ kfree_skb_list_reason(struct sk_buff *segs, enum
skb_drop_reason reason)
        while (segs) {
                struct sk_buff *next = segs->next;

-               skb_mark_not_on_list(segs);
-
                if (__kfree_skb_reason(segs, reason))
                        kfree_skb_add_bulk(segs, &sa, reason);