Re: [PATCH 12/23] bpf: handle the compat string in bpf_trace_copy_string better

Yonghong Song
2020-05-28T17:07:33Z

On 5/27/20 9:39 PM, Christoph Hellwig wrote:
> On Wed, May 27, 2020 at 07:26:30PM -0700, Yonghong Song wrote:
>>> --- a/kernel/trace/bpf_trace.c~xxx
>>> +++ a/kernel/trace/bpf_trace.c
>>> @@ -588,15 +588,22 @@ BPF_CALL_5(bpf_seq_printf, struct seq_fi
>>> }
>>> if (fmt[i] == 's') {
>>> + void *unsafe_ptr;
>>> +
>>> /* try our best to copy */
>>> if (memcpy_cnt >= MAX_SEQ_PRINTF_MAX_MEMCPY) {
>>> err = -E2BIG;
>>> goto out;
>>> }
>>> - err = strncpy_from_unsafe(bufs->buf[memcpy_cnt],
>>> - (void *) (long) args[fmt_cnt],
>>> - MAX_SEQ_PRINTF_STR_LEN);
>>> + unsafe_ptr = (void *)(long)args[fmt_cnt];
>>> + if ((unsigned long)unsafe_ptr < TASK_SIZE) {
>>> + err = strncpy_from_user_nofault(
>>> + bufs->buf[memcpy_cnt], unsafe_ptr,
>>> + MAX_SEQ_PRINTF_STR_LEN);
>>> + } else {
>>> + err = -EFAULT;
>>> + }
>>
>> This probably not right.
>> The pointer stored at args[fmt_cnt] is a kernel pointer,
>> but it could be an invalid address and we do not want to fault.
>> Not sure whether it exists or not, we should use
>> strncpy_from_kernel_nofault()?
>
> If you know it is a kernel pointer with this series it should be
> strncpy_from_kernel_nofault. But even before the series it should have
> been strncpy_from_unsafe_strict.

The use of strncpy_from_unsafe() mimics old bpf_trace_printk()
implementation which just changed to _strict version:
https://lkml.org/lkml/2020/5/18/1309

Agreed that we should change to strncpy_from_unsafe_strict().
I can submit a patch for this.

Thanks!