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Date: Tue, 01 Aug 2023 20:04:07 -0700
From: John Fastabend <john.fastabend@...il.com>
To: Xu Kuohai <xukuohai@...weicloud.com>,
bpf@...r.kernel.org,
netdev@...r.kernel.org
Cc: John Fastabend <john.fastabend@...il.com>,
Jakub Sitnicki <jakub@...udflare.com>,
"David S . Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Paolo Abeni <pabeni@...hat.com>,
Daniel Borkmann <daniel@...earbox.net>,
Alexei Starovoitov <ast@...nel.org>,
Cong Wang <cong.wang@...edance.com>
Subject: RE: [PATCH bpf] bpf, sockmap: Fix NULL deref in sk_psock_backlog
Xu Kuohai wrote:
> From: Xu Kuohai <xukuohai@...wei.com>
>
> sk_psock_backlog triggers a NULL dereference:
>
> BUG: kernel NULL pointer dereference, address: 000000000000000e
> #PF: supervisor read access in kernel mode
> #PF: error_code(0x0000) - not-present page
> PGD 0 P4D 0
> Oops: 0000 [#1] PREEMPT SMP PTI
> CPU: 0 PID: 70 Comm: kworker/0:3 Not tainted 6.5.0-rc2-00585-gb11bbbe4c66e #26
> Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.15.0-0-g2dd4b9b3f840-p4
> Workqueue: events sk_psock_backlog
> RIP: 0010:0xffffffffc0205254
> Code: 00 00 48 89 94 24 a0 00 00 00 41 5f 41 5e 41 5d 41 5c 5d 5b 41 5b 41 5a 41 59 41 50
> RSP: 0018:ffffc90000acbcb8 EFLAGS: 00010246
> RAX: ffffffff81c5ee10 RBX: ffff888018260000 RCX: 0000000000000001
> RDX: 0000000000000003 RSI: ffffc90000acbd58 RDI: 0000000000000000
> RBP: 0000000000000000 R08: 0000000000000003 R09: 0000000080100005
> R10: 0000000000000001 R11: 0000000000000000 R12: 0000000000000003
> R13: 0000000000000000 R14: 0000000000000021 R15: 0000000000000003
> FS: 0000000000000000(0000) GS:ffff88803ea00000(0000) knlGS:0000000000000000
> CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
> CR2: 000000000000000e CR3: 000000000b0de002 CR4: 0000000000170ef0
> Call Trace:
> <TASK>
> ? __die+0x24/0x70
> ? page_fault_oops+0x15d/0x480
> ? fixup_exception+0x26/0x330
> ? exc_page_fault+0x72/0x1d0
> ? asm_exc_page_fault+0x26/0x30
> ? __pfx_inet_sendmsg+0x10/0x10
> ? 0xffffffffc0205254
> ? inet_sendmsg+0x20/0x80
> ? sock_sendmsg+0x8f/0xa0
> ? __skb_send_sock+0x315/0x360
> ? __pfx_sendmsg_unlocked+0x10/0x10
> ? sk_psock_backlog+0xb4/0x300
> ? process_one_work+0x292/0x560
> ? worker_thread+0x53/0x3e0
> ? __pfx_worker_thread+0x10/0x10
> ? kthread+0x102/0x130
> ? __pfx_kthread+0x10/0x10
> ? ret_from_fork+0x34/0x50
> ? __pfx_kthread+0x10/0x10
> ? ret_from_fork_asm+0x1b/0x30
> </TASK>
>
> The bug flow is as follows:
>
> thread 1 thread 2
>
> sk_psock_backlog sock_close
> sk_psock_handle_skb __sock_release
> __skb_send_sock inet_release
> sendmsg_unlocked tcp_close
> sock_sendmsg lock_sock
> __tcp_close
> release_sock
> sock->sk = NULL // (1)
> inet_sendmsg
> sk = sock->sk // (2)
> inet_send_prepare
> inet_sk(sk)->inet_num // (3)
We are doing a lot of hoping through calls here to find something we
should already know. We know the psock we are sending has a protocol
of tcp, udp, ... and could call the send directly instead of walking
back into the sk_socket and so on. For tcp example we could simply
call tcp_sendmsg(sk, msg, size).
I haven't tried it yet, but I wonder if a lot of this logic gets
easier to reason about if we have per protocol backlog logic. Its
just a hunch at this point though.
>
> sock->sk is set to NULL by thread 2 at time (1), then fetched by
> thread 1 at time (2), and used by thread 1 to access memory at
> time (3), resulting in NULL pointer dereference.
>
> To fix it, add lock_sock back on the egress path for sk_psock_handle_skb.
>
> Fixes: 799aa7f98d53 ("skmsg: Avoid lock_sock() in sk_psock_backlog()")
> Signed-off-by: Xu Kuohai <xukuohai@...wei.com>
> ---
> net/core/skmsg.c | 44 ++++++++++++++++++++++++++++++++++----------
> 1 file changed, 34 insertions(+), 10 deletions(-)
>
> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index 7c2764beeb04..8b758c51aa0d 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c
> @@ -609,15 +609,42 @@ static int sk_psock_skb_ingress_self(struct sk_psock *psock, struct sk_buff *skb
> return err;
> }
>
> +static int sk_psock_handle_ingress_skb(struct sk_psock *psock,
> + struct sk_buff *skb,
> + u32 off, u32 len)
> +{
> + if (sock_flag(psock->sk, SOCK_DEAD))
> + return -EIO;
We didn't previously have the SOCK_DEAD check on ingress which
looks fine because we will come along and flush the ingress
queue when psock is being torn down. Adding it looks fine
though because __tcp_close is flushing the sk_receive_queue
and detaching the user from the socket so we have no way
to read the data anyways. This will then abort the backlog
which moves the psock destruct op along a bit faster.
> + return sk_psock_skb_ingress(psock, skb, off, len);
> +}
> +
> +static int sk_psock_handle_egress_skb(struct sk_psock *psock,
> + struct sk_buff *skb,
> + u32 off, u32 len)
> +{
> + int ret;
> +
> + lock_sock(psock->sk);
> +
> + if (sock_flag(psock->sk, SOCK_DEAD))
> + ret = -EIO;
OK, the sock_orphan() call from tcp_close adjudge_to_death block will set
the SOCK_DEAD flag and ensure we abort the send here. EIO then forces
backlog to abort. This looks correct to me.
> + else if (!sock_writeable(psock->sk))
> + ret = -EAGAIN;
> + else
> + ret = skb_send_sock_locked(psock->sk, skb, off, len);
> +
> + release_sock(psock->sk);
> +
> + return ret;
> +}
> +
> static int sk_psock_handle_skb(struct sk_psock *psock, struct sk_buff *skb,
> u32 off, u32 len, bool ingress)
> {
> - if (!ingress) {
> - if (!sock_writeable(psock->sk))
> - return -EAGAIN;
> - return skb_send_sock(psock->sk, skb, off, len);
> - }
> - return sk_psock_skb_ingress(psock, skb, off, len);
> + if (ingress)
> + return sk_psock_handle_ingress_skb(psock, skb, off, len);
> + else
> + return sk_psock_handle_egress_skb(psock, skb, off, len);
> }
>
> static void sk_psock_skb_state(struct sk_psock *psock,
> @@ -660,10 +687,7 @@ static void sk_psock_backlog(struct work_struct *work)
> ingress = skb_bpf_ingress(skb);
> skb_bpf_redirect_clear(skb);
> do {
> - ret = -EIO;
> - if (!sock_flag(psock->sk, SOCK_DEAD))
> - ret = sk_psock_handle_skb(psock, skb, off,
> - len, ingress);
> + ret = sk_psock_handle_skb(psock, skb, off, len, ingress);
> if (ret <= 0) {
> if (ret == -EAGAIN) {
> sk_psock_skb_state(psock, state, len, off);
OK LGTM nice catch I left my commentary above that helped as I reviewed it. I
guess we need more stress testing along this path all of our testing is on
ingress path at the moment. Do you happen to have something coded up that
stress tests the redirect send paths?
Reviewed-by: John Fastabend <john.fastabend@...il.com>
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