Re: [PATCH] net: kcm: fix memory leak in kcm_sendmsg

[Cong Wang <xiyou.wangcong@gmail.com>]

On Wed, Jun 2, 2021 at 12:29 PM Pavel Skripkin <paskripkin@gmail.com> wrote:
>
> Syzbot reported memory leak in kcm_sendmsg()[1].
> The problem was in non-freed frag_list in case of error.
>
> In the while loop:
>
>         if (head == skb)
>                 skb_shinfo(head)->frag_list = tskb;
>         else
>                 skb->next = tskb;
>
> frag_list filled with skbs, but nothing was freeing them.

What do you mean by "nothing was freeing them"?

I am sure kfree_skb() will free those in frag_list:

 654 static void skb_release_data(struct sk_buff *skb)
 655 {
 656         struct skb_shared_info *shinfo = skb_shinfo(skb);
 657         int i;
...
 669         if (shinfo->frag_list)
 670                 kfree_skb_list(shinfo->frag_list);


>
> backtrace:
>   [<0000000094c02615>] __alloc_skb+0x5e/0x250 net/core/skbuff.c:198
>   [<00000000e5386cbd>] alloc_skb include/linux/skbuff.h:1083 [inline]
>   [<00000000e5386cbd>] kcm_sendmsg+0x3b6/0xa50 net/kcm/kcmsock.c:967 [1]
>   [<00000000f1613a8a>] sock_sendmsg_nosec net/socket.c:652 [inline]
>   [<00000000f1613a8a>] sock_sendmsg+0x4c/0x60 net/socket.c:672
>
> Reported-and-tested-by: syzbot+b039f5699bd82e1fb011@syzkaller.appspotmail.com
> Fixes: ab7ac4eb9832 ("kcm: Kernel Connection Multiplexor module")
> Cc: stable@vger.kernel.org
> Signed-off-by: Pavel Skripkin <paskripkin@gmail.com>
> ---
>  net/kcm/kcmsock.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/net/kcm/kcmsock.c b/net/kcm/kcmsock.c
> index 6201965bd822..1c572c8daced 100644
> --- a/net/kcm/kcmsock.c
> +++ b/net/kcm/kcmsock.c
> @@ -1066,6 +1066,11 @@ static int kcm_sendmsg(struct socket *sock, struct msghdr *msg, size_t len)
>                 goto partial_message;
>         }
>
> +       if (skb_has_frag_list(head)) {
> +               kfree_skb_list(skb_shinfo(head)->frag_list);
> +               skb_shinfo(head)->frag_list = NULL;
> +       }
> +
>         if (head != kcm->seq_skb)
>                 kfree_skb(head);

This exact kfree_skb() should free those in frag_list. If the above
if condition does not meet for some reason, then fix that condition?

Thanks.