[PATCH net-next V2] net: fix kfree_skb_list use of skb_mark_not_on_list

[PATCH net-next V2] net: fix kfree_skb_list use of skb_mark_not_on_list

[Jesper Dangaard Brouer]

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().

In this patch we choose to remove the skb_mark_not_on_list() call as it
isn't necessary. It would be possible and correct to call
skb_mark_not_on_list() only when __kfree_skb_reason() returns true,
meaning the SKB is ready to be free'ed, as it calls/check skb_unref().

This fix is needed as kfree_skb_list() is also invoked on skb_shared_info
frag_list (skb_drop_fraglist() calling kfree_skb_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().

Reported-by: syzbot+c8a2e66e37eee553c4fd@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+c8a2e66e37eee553c4fd@syzkaller.appspotmail.com
Fixes: eedade12f4cb ("net: kfree_skb_list use kmem_cache_free_bulk")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
---
 net/core/skbuff.c |    2 --
 1 file changed, 2 deletions(-)

diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 4e73ab3482b8..180df58e85c7 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);
 

Re: [PATCH net-next V2] net: fix kfree_skb_list use of skb_mark_not_on_list

[Eric Dumazet]

On Fri, Jan 20, 2023 at 11:34 AM Jesper Dangaard Brouer
<brouer@redhat.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().
>
> In this patch we choose to remove the skb_mark_not_on_list() call as it
> isn't necessary. It would be possible and correct to call
> skb_mark_not_on_list() only when __kfree_skb_reason() returns true,
> meaning the SKB is ready to be free'ed, as it calls/check skb_unref().
>
> This fix is needed as kfree_skb_list() is also invoked on skb_shared_info
> frag_list (skb_drop_fraglist() calling kfree_skb_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().
>
> Reported-by: syzbot+c8a2e66e37eee553c4fd@syzkaller.appspotmail.com
> Reported-and-tested-by: syzbot+c8a2e66e37eee553c4fd@syzkaller.appspotmail.com
> Fixes: eedade12f4cb ("net: kfree_skb_list use kmem_cache_free_bulk")
> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>

Reviewed-by: Eric Dumazet <edumazet@google.com>

Re: [PATCH net-next V2] net: fix kfree_skb_list use of skb_mark_not_on_list

[patchwork-bot+netdevbpf]

Hello:

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

On Fri, 20 Jan 2023 11:34:44 +0100 you 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().
> 
> In this patch we choose to remove the skb_mark_not_on_list() call as it
> isn't necessary. It would be possible and correct to call
> skb_mark_not_on_list() only when __kfree_skb_reason() returns true,
> meaning the SKB is ready to be free'ed, as it calls/check skb_unref().
> 
> [...]

Here is the summary with links:
  - [net-next,V2] net: fix kfree_skb_list use of skb_mark_not_on_list
    https://git.kernel.org/netdev/net-next/c/f72ff8b81ebc

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