Re: [PATCH bpf-next v2] skmsg: Fix wrong last sg check in sk_msg_recvmsg()

[Jakub Sitnicki <jakub@cloudflare.com>]

On Tue, Aug 09, 2022 at 05:49 PM +08, Liu Jian wrote:
> Fix one kernel NULL pointer dereference as below:
>
> [  224.462334] Call Trace:
> [  224.462394]  __tcp_bpf_recvmsg+0xd3/0x380
> [  224.462441]  ? sock_has_perm+0x78/0xa0
> [  224.462463]  tcp_bpf_recvmsg+0x12e/0x220
> [  224.462494]  inet_recvmsg+0x5b/0xd0
> [  224.462534]  __sys_recvfrom+0xc8/0x130
> [  224.462574]  ? syscall_trace_enter+0x1df/0x2e0
> [  224.462606]  ? __do_page_fault+0x2de/0x500
> [  224.462635]  __x64_sys_recvfrom+0x24/0x30
> [  224.462660]  do_syscall_64+0x5d/0x1d0
> [  224.462709]  entry_SYSCALL_64_after_hwframe+0x65/0xca
>
> In commit 9974d37ea75f ("skmsg: Fix invalid last sg check in
> sk_msg_recvmsg()"), we change last sg check to sg_is_last(),
> but in sockmap redirection case (without stream_parser/stream_verdict/
> skb_verdict), we did not mark the end of the scatterlist. Check the
> sk_msg_alloc, sk_msg_page_add, and bpf_msg_push_data functions, they all
> do not mark the end of sg. They are expected to use sg.end for end
> judgment. So the judgment of '(i != msg_rx->sg.end)' is added back here.
>
> Fixes: 9974d37ea75f ("skmsg: Fix invalid last sg check in sk_msg_recvmsg()")
> Signed-off-by: Liu Jian <liujian56@huawei.com>
> ---
> v1->v2: change fix commit info in fixes tag and commit message.
>  net/core/skmsg.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
>
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 81627892bdd4..385ae23580a5 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -462,7 +462,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
>  
>  			if (copied == len)
>  				break;
> -		} while (!sg_is_last(sge));
> +		} while ((i != msg_rx->sg.end) && !sg_is_last(sge));
>  
>  		if (unlikely(peek)) {
>  			msg_rx = sk_psock_next_msg(psock, msg_rx);
> @@ -472,7 +472,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
>  		}
>  
>  		msg_rx->sg.start = i;
> -		if (!sge->length && sg_is_last(sge)) {
> +		if (!sge->length && (i == msg_rx->sg.end || sg_is_last(sge))) {
>  			msg_rx = sk_psock_dequeue_msg(psock);
>  			kfree_sk_msg(msg_rx);
>  		}

Looks to me like I sk_msg_alloc does mark the end of a scatterlist:

sk_msg_alloc
  sg_unmark_end
  sg_set_page
    sg_assign_page <- sets SG_END bit

Why do we unmark the SG end in sk_msg_page_add and bpf_msg_push_data?

John, do you recall?


As it might take a moment to figure it out, and we want to fix the
regression introduced by 9974d37ea75f ("skmsg: Fix invalid last sg check
in sk_msg_recvmsg()"):

Acked-by: Jakub Sitnicki <jakub@cloudflare.com>