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

[Yonghong Song]

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!