netdev - Re: [PATCH bpf-next] bpf, test_run: fix alignment problem in bpf_test

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Date:   Wed, 7 Dec 2022 10:16:18 +0800
From:   shaozhengchao <shaozhengchao@...wei.com>
To:     John Fastabend <john.fastabend@...il.com>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <ast@...nel.org>, <daniel@...earbox.net>,
        <andrii@...nel.org>, <davem@...emloft.net>, <edumazet@...gle.com>,
        <kuba@...nel.org>, <pabeni@...hat.com>
CC:     <martin.lau@...ux.dev>, <song@...nel.org>, <yhs@...com>,
        <kpsingh@...nel.org>, <sdf@...gle.com>, <haoluo@...gle.com>,
        <jolsa@...nel.org>, <syzkaller-bugs@...glegroups.com>,
        <weiyongjun1@...wei.com>, <yuehaibing@...wei.com>
Subject: Re: [PATCH bpf-next] bpf, test_run: fix alignment problem in
 bpf_test_init()



On 2022/12/7 3:58, John Fastabend wrote:
> Zhengchao Shao wrote:
>> The problem reported by syz is as follows:
>> BUG: KASAN: slab-out-of-bounds in __build_skb_around+0x230/0x330
>> Write of size 32 at addr ffff88807ec6b2c0 by task bpf_repo/6711
>> Call Trace:
>> <TASK>
>> dump_stack_lvl+0x8e/0xd1
>> print_report+0x155/0x454
>> kasan_report+0xba/0x1f0
>> kasan_check_range+0x35/0x1b0
>> memset+0x20/0x40
>> __build_skb_around+0x230/0x330
>> build_skb+0x4c/0x260
>> bpf_prog_test_run_skb+0x2fc/0x1ce0
>> __sys_bpf+0x1798/0x4b60
>> __x64_sys_bpf+0x75/0xb0
>> do_syscall_64+0x35/0x80
>> entry_SYSCALL_64_after_hwframe+0x46/0xb0
>> </TASK>
>>
>> Allocated by task 6711:
>> kasan_save_stack+0x1e/0x40
>> kasan_set_track+0x21/0x30
>> __kasan_kmalloc+0xa1/0xb0
>> __kmalloc+0x4e/0xb0
>> bpf_test_init.isra.0+0x77/0x100
>> bpf_prog_test_run_skb+0x219/0x1ce0
>> __sys_bpf+0x1798/0x4b60
>> __x64_sys_bpf+0x75/0xb0
>> do_syscall_64+0x35/0x80
>> entry_SYSCALL_64_after_hwframe+0x46/0xb0
>>
>> The process is as follows:
>> bpf_prog_test_run_skb()
>> 	bpf_test_init()
>> 		data = kzalloc()	//The length of input is 576.
>> 					//The actual allocated memory
>> 					//size is 1024.
>> 	build_skb()
>> 		__build_skb_around()
>> 			size = ksize(data)//size = 1024
>> 			size -= SKB_DATA_ALIGN(
>> 					sizeof(struct skb_shared_info));
>> 					//size = 704
>> 			skb_set_end_offset(skb, size);
>> 			shinfo = skb_shinfo(skb);//shinfo = data + 704
>> 			memset(shinfo...)	//Write out of bounds
>>
>> In bpf_test_init(), the accessible space allocated to data is 576 bytes,
>> and the memory allocated to data is 1024 bytes. In __build_skb_around(),
>> shinfo indicates the offset of 704 bytes of data, which triggers the issue
>> of writing out of bounds.
>>
>> Fixes: 1cf1cae963c2 ("bpf: introduce BPF_PROG_TEST_RUN command")
>> Reported-by: syzbot+fda18eaa8c12534ccb3b@...kaller.appspotmail.com
>> Signed-off-by: Zhengchao Shao <shaozhengchao@...wei.com>
>> ---
>>   net/bpf/test_run.c | 10 ++++++++++
>>   1 file changed, 10 insertions(+)
>>
>> diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c
>> index fcb3e6c5e03c..fbd5337b8f68 100644
>> --- a/net/bpf/test_run.c
>> +++ b/net/bpf/test_run.c
>> @@ -766,6 +766,8 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
>>   			   u32 size, u32 headroom, u32 tailroom)
>>   {
>>   	void __user *data_in = u64_to_user_ptr(kattr->test.data_in);
>> +	unsigned int true_size;
>> +	void *true_data;
>>   	void *data;
>>   
>>   	if (size < ETH_HLEN || size > PAGE_SIZE - headroom - tailroom)
>> @@ -779,6 +781,14 @@ static void *bpf_test_init(const union bpf_attr *kattr, u32 user_size,
>>   	if (!data)
>>   		return ERR_PTR(-ENOMEM);
>>   
>> +	true_size = ksize(data);
>> +	if (size + headroom + tailroom < true_size) {
>> +		true_data = krealloc(data, true_size, GFP_USER | __GFP_ZERO);
> 
> This comes from a kzalloc, should we zero realloc'd memory as well?
> 
>> +			if (!true_data)
>> +				return ERR_PTR(-ENOMEM);
> 
> I think its worth fixing the extra tab here.
> 

Hi John:
	Thank you for your review. Your suggestion looks good to me. And I 
found Kees Cook also focus on this issue.
https://patchwork.kernel.org/project/netdevbpf/patch/20221206231659.never.929-kees@kernel.org/
Perhaps his solution will be more common?

Zhengchao Shao
>> +		data = true_data;
>> +	}
>> +
>>   	if (copy_from_user(data + headroom, data_in, user_size)) {
>>   		kfree(data);
>>   		return ERR_PTR(-EFAULT);
>> -- 
>> 2.17.1
>>