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

On Wed, May 27, 2020 at 07:26:30PM -0700, Yonghong Song wrote:

quoted

--- 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.