pyverbs: Protect the write and read methods of MR

For the implicit MR registrations the internal MR's buffer
doesn't have to be allocated (controlled by the length argument).
Therefore we should protect the write and read methods of MR object
by checking before the operation if the buf attribute and the
passed arguments are valid.

Signed-off-by: Itay Aveksis <itayav@mellanox.com>
Signed-off-by: Edward Srouji <edwards@mellanox.com>

pyverbs/mr.pyx

@@ -6,7 +6,8 @@ import logging
 
 from posix.mman cimport mmap, munmap, MAP_PRIVATE, PROT_READ, PROT_WRITE, \
     MAP_ANONYMOUS, MAP_HUGETLB
-from pyverbs.pyverbs_error import PyverbsError, PyverbsRDMAError
+from pyverbs.pyverbs_error import PyverbsError, PyverbsRDMAError, \
+    PyverbsUserError
 from libc.stdint cimport uintptr_t, SIZE_MAX
 from pyverbs.base import PyverbsRDMAErrno
 from posix.stdlib cimport posix_memalign
@@ -135,6 +136,9 @@ cdef class MR(PyverbsCM):
         :param length: Length of the data to write
         :return: None
         """
+        if not self.buf or length < 0:
+            raise PyverbsUserError('The MR buffer isn\'t allocated or length'
+                                   f' {length} is invalid')
         # If data is a string, cast it to bytes as Python3 doesn't
         # automatically convert it.
         if isinstance(data, str):
@@ -151,6 +155,11 @@ cdef class MR(PyverbsCM):
         cdef char *data
         cdef int off = offset # we can't use offset in the next line, as it is
                               # a Python object and not C
+        if offset < 0:
+            raise PyverbsUserError(f'Invalid offset {offset}')
+        if not self.buf or length < 0:
+            raise PyverbsUserError('The MR buffer isn\'t allocated or length'
+                                   f' {length} is invalid')
         data = <char*>(self.buf + off)
         return data[:length]