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

From: John Fastabend <[email-masked]>
Date: Tue, 02 Jul 2024 18:03:06 -0700

Geliang Tang wrote:
> From: Geliang Tang <[email-masked]>
> 
> 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 <[email-masked]>
> ---
>  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.

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.

Maybe a better fix assuming my assumption about fin being skb->len=0 is
correct?

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