Subject: RE: [PATCH net v5] skmsg: skip zero length skb in sk_msg_recvmsg

From: John Fastabend <[email-masked]>
Date: Mon, 08 Jul 2024 17:33:00 -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 implementations depending on the
> architecture, page_address(NULL) will trigger a panic on Loongarch
> platform but not on X86 platform. So this bug was hidden on X86 platform
> for a while, but now it is exposed on Loongarch platform.
> 
> The root cause is a zero length skb (skb->len == 0) is put on the queue.
> 
> This zero length skb is a TCP FIN packet, which is sent by shutdown(),
> invoked in test_sockmap_skb_verdict_shutdown():
> 
> 	shutdown(p1, SHUT_WR);
> 
> 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(), which passes it
> to kmap_local_page() and to page_address(), then kernel panics.
> 
> To solve this, we should skip this zero length skb. So in sk_msg_recvmsg(),
> if copy is zero, that means it's a zero length skb, skip invoking
> copy_page_to_iter(). We are using the EFAULT return triggered by
> copy_page_to_iter to check for is_fin in tcp_bpf.c.
> 
> Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
> Suggested-by: John Fastabend <[email-masked]>
> Signed-off-by: Geliang Tang <[email-masked]>
> ---
> v5:
>  - update v5 as John suggested.
>  - skmsg: skip zero length skb in sk_msg_recvmsg
> 
> v4:
>  - skmsg: skip empty sge in sk_msg_recvmsg
> 
> v3:
>  - skmsg: prevent empty ingress skb from enqueuing
>    
> v2:
>  - skmsg: null check for sg_page in sk_msg_recvmsg
> ---
>  net/core/skmsg.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)

I don't have any better ideas so lets use this.

Reviewed-by: John Fastabend <[email-masked]>