linux-kernel - Re: [PATCH net 2/2] tcp_bpf: fix copied value in tcp_bpf

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Message-ID:
 <MEYP282MB23125E657B3605921535987AC63C2@MEYP282MB2312.AUSP282.PROD.OUTLOOK.COM>
Date: Mon, 9 Dec 2024 19:56:21 +0800
From: Levi Zim <rsworktech@...look.com>
To: John Fastabend <john.fastabend@...il.com>,
 Levi Zim via B4 Relay <devnull+rsworktech.outlook.com@...nel.org>,
 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>,
 Simon Horman <horms@...nel.org>, David Ahern <dsahern@...nel.org>
Cc: netdev@...r.kernel.org, bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net 2/2] tcp_bpf: fix copied value in tcp_bpf_sendmsg

On 2024-12-09 15:02, John Fastabend wrote:
> Levi Zim via B4 Relay wrote:
>> From: Levi Zim <rsworktech@...look.com>
>>
>> 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?
Yes. More specifically only "sockhash::test_txmsg_cork_hangs" but not 
"sockmap::test_txmsg_cork_hangs"
>
>> 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?
I am not familiar enough with neither this part of code nor tcp with bpf 
in general and just
hit this bug when trying to run the bpf kselftests. Then I decided to 
debug it.

In my perspective the buffer(8192) is large enough to hold the data(8192),
but tcp_bpf_sendmsg returns 12289 which is a little surprising for me.

Could you further elaborate why 8192 bytes are not enough? Thanks!

>> This commit changes it to use the real copied bytes.
>>
>> Signed-off-by: Levi Zim <rsworktech@...look.com>
>> ---
>>   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
>>
>>
>