Subject: RE: [PATCH net 2/2] tcp_bpf: fix copied value in tcp_bpf_sendmsg

From: John Fastabend <[email-masked]>
Date: Sun, 08 Dec 2024 23:02:09 -0800

Levi Zim via B4 Relay wrote:
> From: Levi Zim <[email-masked]>
> 
> bpf kselftest sockhash::test_txmsg_cork_hangs in test_sockmap.c triggers a
> kernel NULL pointer dereference:

Is it just the cork test that causes issue?

> 
> BUG: kernel NULL pointer dereference, address: 0000000000000008
>  ? __die_body+0x6e/0xb0
>  ? __die+0x8b/0xa0
>  ? page_fault_oops+0x358/0x3c0
>  ? local_clock+0x19/0x30
>  ? lock_release+0x11b/0x440
>  ? kernelmode_fixup_or_oops+0x54/0x60
>  ? __bad_area_nosemaphore+0x4f/0x210
>  ? mmap_read_unlock+0x13/0x30
>  ? bad_area_nosemaphore+0x16/0x20
>  ? do_user_addr_fault+0x6fd/0x740
>  ? prb_read_valid+0x1d/0x30
>  ? exc_page_fault+0x55/0xd0
>  ? asm_exc_page_fault+0x2b/0x30
>  ? splice_to_socket+0x52e/0x630
>  ? shmem_file_splice_read+0x2b1/0x310
>  direct_splice_actor+0x47/0x70
>  splice_direct_to_actor+0x133/0x300
>  ? do_splice_direct+0x90/0x90
>  do_splice_direct+0x64/0x90
>  ? __ia32_sys_tee+0x30/0x30
>  do_sendfile+0x214/0x300
>  __se_sys_sendfile64+0x8e/0xb0
>  __x64_sys_sendfile64+0x25/0x30
>  x64_sys_call+0xb82/0x2840
>  do_syscall_64+0x75/0x110
>  entry_SYSCALL_64_after_hwframe+0x4b/0x53
> 
> This is caused by tcp_bpf_sendmsg() returning a larger value(12289) than
> size (8192), which causes the while loop in splice_to_socket() to release
> an uninitialized pipe buf.
> 
> The underlying cause is that this code assumes sk_msg_memcopy_from_iter()
> will copy all bytes upon success but it actually might only copy part of
> it.

The intent was to ensure we allocate a buffer large enough to fit the
data. I guess the cork + send here is not allocating enough bytes? 

> 
> This commit changes it to use the real copied bytes.
> 
> Signed-off-by: Levi Zim <[email-masked]>
> ---
>  net/ipv4/tcp_bpf.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/net/ipv4/tcp_bpf.c b/net/ipv4/tcp_bpf.c
> index 370993c03d31363c0f82a003d9e5b0ca3bbed721..8e46c4d618cbbff0d120fe4cd917624e5d5cae15 100644
> --- a/net/ipv4/tcp_bpf.c
> +++ b/net/ipv4/tcp_bpf.c
> @@ -496,7 +496,7 @@ static int tcp_bpf_send_verdict(struct sock *sk, struct sk_psock *psock,
>  static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>  {
>  	struct sk_msg tmp, *msg_tx = NULL;
> -	int copied = 0, err = 0;
> +	int copied = 0, err = 0, ret = 0;
>  	struct sk_psock *psock;
>  	long timeo;
>  	int flags;
> @@ -539,14 +539,14 @@ static int tcp_bpf_sendmsg(struct sock *sk, struct msghdr *msg, size_t size)
>  			copy = msg_tx->sg.size - osize;
>  		}
>  
> -		err = sk_msg_memcopy_from_iter(sk, &msg->msg_iter, msg_tx,
> +		ret = sk_msg_memcopy_from_iter(sk, &msg->msg_iter, msg_tx,
>  					       copy);
> -		if (err < 0) {
> +		if (ret < 0) {
>  			sk_msg_trim(sk, msg_tx, osize);
>  			goto out_err;
>  		}
>  
> -		copied += copy;
> +		copied += ret;
>  		if (psock->cork_bytes) {
>  			if (size > psock->cork_bytes)
>  				psock->cork_bytes = 0;
> 
> -- 
> 2.47.1
> 
>