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

Christoph Hellwig
2020-05-28T04:40:05Z

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.