Re: [PATCH net v3 1/2] skmsg: prevent empty ingress skb from enqueuing

[Geliang Tang <geliang@kernel.org>]

Hi John,

On Wed, 2024-07-03 at 09:54 +0800, Geliang Tang wrote:
> On Tue, 2024-07-02 at 18:03 -0700, John Fastabend wrote:
> > Geliang Tang wrote:
> > > From: Geliang Tang <tanggeliang@kylinos.cn>
> > > 
> > > Run this BPF selftests (./test_progs -t sockmap_basic) on a
> > > Loongarch
> > > platform, a Kernel panic occurs:
> > > 
> > > '''
> > > Oops[#1]:
> > > CPU: 22 PID: 2824 Comm: test_progs Tainted: G           OE 
> > > 6.10.0-
> > > rc2+ #18
> > > Hardware name: LOONGSON Dabieshan/Loongson-TC542F0, BIOS
> > > Loongson-
> > > UDK2018
> > >    ... ...
> > >    ra: 90000000048bf6c0 sk_msg_recvmsg+0x120/0x560
> > >   ERA: 9000000004162774 copy_page_to_iter+0x74/0x1c0
> > >  CRMD: 000000b0 (PLV0 -IE -DA +PG DACF=CC DACM=CC -WE)
> > >  PRMD: 0000000c (PPLV0 +PIE +PWE)
> > >  EUEN: 00000007 (+FPE +SXE +ASXE -BTE)
> > >  ECFG: 00071c1d (LIE=0,2-4,10-12 VS=7)
> > > ESTAT: 00010000 [PIL] (IS= ECode=1 EsubCode=0)
> > >  BADV: 0000000000000040
> > >  PRID: 0014c011 (Loongson-64bit, Loongson-3C5000)
> > > Modules linked in: bpf_testmod(OE) xt_CHECKSUM xt_MASQUERADE
> > > xt_conntrack
> > > Process test_progs (pid: 2824, threadinfo=0000000000863a31,
> > > task=...)
> > > Stack : ...
> > >         ...
> > > Call Trace:
> > > [<9000000004162774>] copy_page_to_iter+0x74/0x1c0
> > > [<90000000048bf6c0>] sk_msg_recvmsg+0x120/0x560
> > > [<90000000049f2b90>] tcp_bpf_recvmsg_parser+0x170/0x4e0
> > > [<90000000049aae34>] inet_recvmsg+0x54/0x100
> > > [<900000000481ad5c>] sock_recvmsg+0x7c/0xe0
> > > [<900000000481e1a8>] __sys_recvfrom+0x108/0x1c0
> > > [<900000000481e27c>] sys_recvfrom+0x1c/0x40
> > > [<9000000004c076ec>] do_syscall+0x8c/0xc0
> > > [<9000000003731da4>] handle_syscall+0xc4/0x160
> > > 
> > > Code: ...
> > > 
> > > ---[ end trace 0000000000000000 ]---
> > > Kernel panic - not syncing: Fatal exception
> > > Kernel relocated by 0x3510000
> > >  .text @ 0x9000000003710000
> > >  .data @ 0x9000000004d70000
> > >  .bss  @ 0x9000000006469400
> > > ---[ end Kernel panic - not syncing: Fatal exception ]---
> > > '''
> > > 
> > > This crash happens every time when running
> > > sockmap_skb_verdict_shutdown
> > > subtest in sockmap_basic.
> > > 
> > > This crash is because a NULL pointer is passed to page_address()
> > > in
> > > sk_msg_recvmsg(). Due to the difference in architecture,
> > > page_address(0)
> > > will not trigger a panic on the X86 platform but will panic on
> > > the
> > > Loogarch platform. So this bug was hidden on the x86 platform,
> > > but
> > > now
> > > it is exposed on the Loogarch platform.
> > > 
> > > The root cause is an empty skb (skb->len == 0) is put on the
> > > queue.
> > > 
> > > In this case, in sk_psock_skb_ingress_enqueue(), num_sge is zero,
> > > and no
> > > page is put to this sge (see sg_set_page in sg_set_page), but
> > > this
> > > empty
> > > sge is queued into ingress_msg list.
> > > 
> > > And in sk_msg_recvmsg(), this empty sge is used, and a NULL page
> > > is
> > > got by sg_page(sge). Pass this NULL-page to copy_page_to_iter(),
> > > it
> > > passed to kmap_local_page() and page_address(), then kernel
> > > panics.
> > > 
> > > To solve this, we should prevent empty skb from putting on the
> > > queue. So
> > > in sk_psock_verdict_recv(), if the skb->len is zero, drop this
> > > skb.
> > > 
> > > Fixes: ef5659280eb1 ("bpf, sockmap: Allow skipping sk_skb parser
> > > program")
> > > Signed-off-by: Geliang Tang <tanggeliang@kylinos.cn>
> > > ---
> > >  net/core/skmsg.c | 2 +-
> > >  1 file changed, 1 insertion(+), 1 deletion(-)
> > > 
> > > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > > index fd20aae30be2..44952cdd1425 100644
> > > --- a/net/core/skmsg.c
> > > +++ b/net/core/skmsg.c
> > > @@ -1184,7 +1184,7 @@ static int sk_psock_verdict_recv(struct
> > > sock
> > > *sk, struct sk_buff *skb)
> > >  
> > >  	rcu_read_lock();
> > >  	psock = sk_psock(sk);
> > > -	if (unlikely(!psock)) {
> > > +	if (unlikely(!psock || !len)) {
> > >  		len = 0;
> > >  		tcp_eat_skb(sk, skb);
> > >  		sock_drop(sk, skb);
> > 
> > The skb->len == 0 here is the FIN pkt right? We are using the
> > EFAULT
> > return
> > triggered by copy_page_to_iter to check for is_fin in tcp_bpf.c.

I added some logs for debugging and found that this FIN packet do hit
is_fin check in tcp_bpf.c.

> > 
> > The concern I have here is if we don't have the skb fin pkt on the
> > recv
> > queue we might go into wait_data and block instead of return to
> > user
> > when
> > rcvmsg() is called from user. I wonder if we can write a test for
> > this if
> > we don't already have one we probably should create one.

In test_sockmap_skb_verdict_shutdown(), the FIN packet is sent by

	shutdown(p1, SHUT_WR);

and received by

	n = recv(c1, &b, 1, SOCK_NONBLOCK);
	ASSERT_EQ(n, 0, "recv_timeout(fin)");

I think this test has covered the FIN packet scenario already. No need
to add a new one. WDYT?

> > 
> > Maybe a better fix assuming my assumption about fin being skb-
> > >len=0
> > is
> > correct?
> 
> Thanks John. Your fix is much better than mine. I'll use this as v5
> and
> update the commit log. I'll add your "Suggested-by" tag in it.

Anyway, this v5 (skmsg: skip zero length skb in sk_msg_recvmsg) seems
ready to be merged.

Thanks,
-Geliang

> 
> -Geliang
> 
> > 
> > diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> > index fd20aae30be2..bbf40b999713 100644
> > --- a/net/core/skmsg.c
> > +++ b/net/core/skmsg.c
> > @@ -434,7 +434,8 @@ int sk_msg_recvmsg(struct sock *sk, struct
> > sk_psock *psock, struct msghdr *msg,
> >                         page = sg_page(sge);
> >                         if (copied + copy > len)
> >                                 copy = len - copied;
> > -                       copy = copy_page_to_iter(page, sge->offset,
> > copy, iter);
> > +                       if (copy)
> > +                               copy = copy_page_to_iter(page, sge-
> > > offset, copy, iter);
> >                         if (!copy) {
> >                                 copied = copied ? copied : -EFAULT;
> >                                 goto out;
> > 
> > Thanks,
> > John
> 
>