maple_tree: fix mas_empty_area_rev() null pointer dereference

Currently the code calls mas_start() followed by mas_data_end() if the
maple state is MA_START, but mas_start() may return with the maple state
node == NULL.  This will lead to a null pointer dereference when checking
information in the NULL node, which is done in mas_data_end().

Avoid setting the offset if there is no node by waiting until after the
maple state is checked for an empty or single entry state.

A user could trigger the events to cause a kernel oops by unmapping all
vmas to produce an empty maple tree, then mapping a vma that would cause
the scenario described above.

Link: https://lkml.kernel.org/r/20240422203349.2418465-1-Liam.Howlett@oracle.com
Fixes: 54a611b6 ("Maple Tree: add new data structure")
Signed-off-by: Liam R. Howlett <Liam.Howlett@oracle.com>
Reported-by: Marius Fleischer <fleischermarius@gmail.com>
Closes: https://lore.kernel.org/lkml/CAJg=8jyuSxDL6XvqEXY_66M20psRK2J53oBTP+fjV5xpW2-R6w@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAJg=8jyuSxDL6XvqEXY_66M20psRK2J53oBTP+fjV5xpW2-R6w@mail.gmail.com/Tested-by: Marius Fleischer <fleischermarius@gmail.com>
Tested-by: Sidhartha Kumar <sidhartha.kumar@oracle.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

lib/maple_tree.c

@@ -5109,18 +5109,18 @@ int mas_empty_area_rev(struct ma_state *mas, unsigned long min,
 	if (size == 0 || max - min < size - 1)
 		return -EINVAL;
 
-	if (mas_is_start(mas)) {
+	if (mas_is_start(mas))
 		mas_start(mas);
-		mas->offset = mas_data_end(mas);
-	} else if (mas->offset >= 2) {
-		mas->offset -= 2;
-	} else if (!mas_rewind_node(mas)) {
+	else if ((mas->offset < 2) && (!mas_rewind_node(mas)))
 		return -EBUSY;
-	}
 
-	/* Empty set. */
-	if (mas_is_none(mas) || mas_is_ptr(mas))
+	if (unlikely(mas_is_none(mas) || mas_is_ptr(mas)))
 		return mas_sparse_area(mas, min, max, size, false);
+	else if (mas->offset >= 2)
+		mas->offset -= 2;
+	else
+		mas->offset = mas_data_end(mas);
+
 
 	/* The start of the window can only be within these values. */
 	mas->index = min;