Fixed #20347 -- Allowed customizing the maximum number of instantiated forms in formsets.

Co-authored-by: ethurgood <ethurgood@gmail.com>

8 files changed, 202 insertions, 19 deletions

diff --git a/django/forms/formsets.py b/django/forms/formsets.py
index 2b013dcafb..6f819bd696 100644
--- a/django/forms/formsets.py
+++ b/django/forms/formsets.py
@@ -433,16 +433,21 @@ class BaseFormSet:
 
 def formset_factory(form, formset=BaseFormSet, extra=1, can_order=False,
                     can_delete=False, max_num=None, validate_max=False,
-                    min_num=None, validate_min=False):
+                    min_num=None, validate_min=False, absolute_max=None):
     """Return a FormSet for the given form class."""
     if min_num is None:
         min_num = DEFAULT_MIN_NUM
     if max_num is None:
         max_num = DEFAULT_MAX_NUM
-    # hard limit on forms instantiated, to prevent memory-exhaustion attacks
-    # limit is simply max_num + DEFAULT_MAX_NUM (which is 2*DEFAULT_MAX_NUM
-    # if max_num is None in the first place)
-    absolute_max = max_num + DEFAULT_MAX_NUM
+    # absolute_max is a hard limit on forms instantiated, to prevent
+    # memory-exhaustion attacks. Default to max_num + DEFAULT_MAX_NUM
+    # (which is 2 * DEFAULT_MAX_NUM if max_num is None in the first place).
+    if absolute_max is None:
+        absolute_max = max_num + DEFAULT_MAX_NUM
+    if max_num > absolute_max:
+        raise ValueError(
+            "'absolute_max' must be greater or equal to 'max_num'."
+        )
     attrs = {
         'form': form,
         'extra': extra,
diff --git a/django/forms/models.py b/django/forms/models.py
index 0f6608a4bc..9f21156329 100644
--- a/django/forms/models.py
+++ b/django/forms/models.py
@@ -862,7 +862,8 @@ def modelformset_factory(model, form=ModelForm, formfield_callback=None,
                          can_order=False, max_num=None, fields=None, exclude=None,
                          widgets=None, validate_max=False, localized_fields=None,
                          labels=None, help_texts=None, error_messages=None,
-                         min_num=None, validate_min=False, field_classes=None):
+                         min_num=None, validate_min=False, field_classes=None,
+                         absolute_max=None):
     """Return a FormSet class for the given Django model class."""
     meta = getattr(form, 'Meta', None)
     if (getattr(meta, 'fields', fields) is None and
@@ -879,7 +880,8 @@ def modelformset_factory(model, form=ModelForm, formfield_callback=None,
                              error_messages=error_messages, field_classes=field_classes)
     FormSet = formset_factory(form, formset, extra=extra, min_num=min_num, max_num=max_num,
                               can_order=can_order, can_delete=can_delete,
-                              validate_min=validate_min, validate_max=validate_max)
+                              validate_min=validate_min, validate_max=validate_max,
+                              absolute_max=absolute_max)
     FormSet.model = model
     return FormSet
 
@@ -1048,7 +1050,8 @@ def inlineformset_factory(parent_model, model, form=ModelForm,
                           can_delete=True, max_num=None, formfield_callback=None,
                           widgets=None, validate_max=False, localized_fields=None,
                           labels=None, help_texts=None, error_messages=None,
-                          min_num=None, validate_min=False, field_classes=None):
+                          min_num=None, validate_min=False, field_classes=None,
+                          absolute_max=None):
     """
     Return an ``InlineFormSet`` for the given kwargs.
 
@@ -1078,6 +1081,7 @@ def inlineformset_factory(parent_model, model, form=ModelForm,
         'help_texts': help_texts,
         'error_messages': error_messages,
         'field_classes': field_classes,
+        'absolute_max': absolute_max,
     }
     FormSet = modelformset_factory(model, **kwargs)
     FormSet.fk = fk
diff --git a/docs/ref/forms/formsets.txt b/docs/ref/forms/formsets.txt
index 778ae20db3..e145eeb103 100644
--- a/docs/ref/forms/formsets.txt
+++ b/docs/ref/forms/formsets.txt
@@ -11,8 +11,12 @@ Formset API reference. For introductory material about formsets, see the
 ``formset_factory``
 ===================
 
-.. function:: formset_factory(form, formset=BaseFormSet, extra=1, can_order=False, can_delete=False, max_num=None, validate_max=False, min_num=None, validate_min=False)
+.. function:: formset_factory(form, formset=BaseFormSet, extra=1, can_order=False, can_delete=False, max_num=None, validate_max=False, min_num=None, validate_min=False, absolute_max=None)
 
     Returns a ``FormSet`` class for the given ``form`` class.
 
     See :doc:`formsets </topics/forms/formsets>` for example usage.
+
+    .. versionchanged:: 3.2
+
+        The ``absolute_max`` argument was added.
diff --git a/docs/ref/forms/models.txt b/docs/ref/forms/models.txt
index c5ddf99011..e18660b3c2 100644
--- a/docs/ref/forms/models.txt
+++ b/docs/ref/forms/models.txt
@@ -52,7 +52,7 @@ Model Form API reference. For introductory material about model forms, see the
 ``modelformset_factory``
 ========================
 
-.. function:: modelformset_factory(model, form=ModelForm, formfield_callback=None, formset=BaseModelFormSet, extra=1, can_delete=False, can_order=False, max_num=None, fields=None, exclude=None, widgets=None, validate_max=False, localized_fields=None, labels=None, help_texts=None, error_messages=None, min_num=None, validate_min=False, field_classes=None)
+.. function:: modelformset_factory(model, form=ModelForm, formfield_callback=None, formset=BaseModelFormSet, extra=1, can_delete=False, can_order=False, max_num=None, fields=None, exclude=None, widgets=None, validate_max=False, localized_fields=None, labels=None, help_texts=None, error_messages=None, min_num=None, validate_min=False, field_classes=None, absolute_max=None)
 
     Returns a ``FormSet`` class for the given ``model`` class.
 
@@ -62,16 +62,20 @@ Model Form API reference. For introductory material about model forms, see the
     through to :func:`~django.forms.models.modelform_factory`.
 
     Arguments ``formset``, ``extra``, ``max_num``, ``can_order``,
-    ``can_delete`` and ``validate_max`` are passed through to
-    :func:`~django.forms.formsets.formset_factory`. See :doc:`formsets
+    ``can_delete``, ``validate_max``, and ``absolute_max`` are passed through
+    to :func:`~django.forms.formsets.formset_factory`. See :doc:`formsets
     </topics/forms/formsets>` for details.
 
     See :ref:`model-formsets` for example usage.
 
+    .. versionchanged:: 3.2
+
+        The ``absolute_max`` argument was added.
+
 ``inlineformset_factory``
 =========================
 
-.. function:: inlineformset_factory(parent_model, model, form=ModelForm, formset=BaseInlineFormSet, fk_name=None, fields=None, exclude=None, extra=3, can_order=False, can_delete=True, max_num=None, formfield_callback=None, widgets=None, validate_max=False, localized_fields=None, labels=None, help_texts=None, error_messages=None, min_num=None, validate_min=False, field_classes=None)
+.. function:: inlineformset_factory(parent_model, model, form=ModelForm, formset=BaseInlineFormSet, fk_name=None, fields=None, exclude=None, extra=3, can_order=False, can_delete=True, max_num=None, formfield_callback=None, widgets=None, validate_max=False, localized_fields=None, labels=None, help_texts=None, error_messages=None, min_num=None, validate_min=False, field_classes=None, absolute_max=None)
 
     Returns an ``InlineFormSet`` using :func:`modelformset_factory` with
     defaults of ``formset=``:class:`~django.forms.models.BaseInlineFormSet`,
@@ -81,3 +85,7 @@ Model Form API reference. For introductory material about model forms, see the
     the ``parent_model``, you must specify a ``fk_name``.
 
     See :ref:`inline-formsets` for example usage.
+
+    .. versionchanged:: 3.2
+
+        The ``absolute_max`` argument was added.
diff --git a/docs/releases/3.2.txt b/docs/releases/3.2.txt
index 2ff09c69b0..03f485b224 100644
--- a/docs/releases/3.2.txt
+++ b/docs/releases/3.2.txt
@@ -136,7 +136,10 @@ File Uploads
 Forms
 ~~~~~
 
-* ...
+* The new ``absolute_max`` argument for :func:`.formset_factory`,
+  :func:`.inlineformset_factory`, and :func:`.modelformset_factory` allows
+  customizing the maximum number of forms that can be instantiated when
+  supplying ``POST`` data. See :ref:`formsets-absolute-max` for more details.
 
 Generic Views
 ~~~~~~~~~~~~~
diff --git a/docs/topics/forms/formsets.txt b/docs/topics/forms/formsets.txt
index 3b5775cabb..b3e696ed8c 100644
--- a/docs/topics/forms/formsets.txt
+++ b/docs/topics/forms/formsets.txt
@@ -126,6 +126,38 @@ affect validation.  If ``validate_max=True`` is passed to the
 :func:`~django.forms.formsets.formset_factory`, then ``max_num`` will affect
 validation.  See :ref:`validate_max`.
 
+.. _formsets-absolute-max:
+
+Limiting the maximum number of instantiated forms
+=================================================
+
+.. versionadded:: 3.2
+
+The ``absolute_max`` parameter to :func:`.formset_factory` allows limiting the
+number of forms that can be instantiated when supplying ``POST`` data. This
+protects against memory exhaustion attacks using forged ``POST`` requests::
+
+    >>> from django.forms.formsets import formset_factory
+    >>> from myapp.forms import ArticleForm
+    >>> ArticleFormSet = formset_factory(ArticleForm, absolute_max=1500)
+    >>> data = {
+    ...     'form-TOTAL_FORMS': '1501',
+    ...     'form-INITIAL_FORMS': '0',
+    ...     'form-MAX_NUM_FORMS': '',
+    ... }
+    >>> formset = ArticleFormSet(data)
+    >>> len(formset.forms)
+    1500
+    >>> formset.is_valid()
+    False
+    >>> formset.non_form_errors()
+    ['Please submit 1000 or fewer forms.']
+
+When ``absolute_max`` is None, it defaults to ``max_num + 1000``. (If
+``max_num`` is ``None``, it defaults to ``2000``).
+
+If ``absolute_max`` is less than ``max_num``, a ``ValueError`` will be raised.
+
 Formset validation
 ==================
 
@@ -348,11 +380,11 @@ excessive.
 .. note::
 
     Regardless of ``validate_max``, if the number of forms in a data set
-    exceeds ``max_num`` by more than 1000, then the form will fail to validate
-    as if ``validate_max`` were set, and additionally only the first 1000
-    forms above ``max_num`` will be validated.  The remainder will be
-    truncated entirely.  This is to protect against memory exhaustion attacks
-    using forged POST requests.
+    exceeds ``absolute_max``, then the form will fail to validate as if
+    ``validate_max`` were set, and additionally only the first ``absolute_max``
+    forms will be validated. The remainder will be truncated entirely. This is
+    to protect against memory exhaustion attacks using forged POST requests.
+    See :ref:`formsets-absolute-max`.
 
 ``validate_min``
 ----------------
diff --git a/tests/forms_tests/tests/test_formsets.py b/tests/forms_tests/tests/test_formsets.py
index 117b50fa7c..e44b347802 100644
--- a/tests/forms_tests/tests/test_formsets.py
+++ b/tests/forms_tests/tests/test_formsets.py
@@ -892,6 +892,55 @@ class FormsFormsetTestCase(SimpleTestCase):
         )
         self.assertEqual(formset.absolute_max, 2000)
 
+    def test_absolute_max(self):
+        data = {
+            'form-TOTAL_FORMS': '2001',
+            'form-INITIAL_FORMS': '0',
+            'form-MAX_NUM_FORMS': '0',
+        }
+        AbsoluteMaxFavoriteDrinksFormSet = formset_factory(
+            FavoriteDrinkForm,
+            absolute_max=3000,
+        )
+        formset = AbsoluteMaxFavoriteDrinksFormSet(data=data)
+        self.assertIs(formset.is_valid(), True)
+        self.assertEqual(len(formset.forms), 2001)
+        # absolute_max provides a hard limit.
+        data['form-TOTAL_FORMS'] = '3001'
+        formset = AbsoluteMaxFavoriteDrinksFormSet(data=data)
+        self.assertIs(formset.is_valid(), False)
+        self.assertEqual(len(formset.forms), 3000)
+        self.assertEqual(
+            formset.non_form_errors(),
+            ['Please submit 1000 or fewer forms.'],
+        )
+
+    def test_absolute_max_with_max_num(self):
+        data = {
+            'form-TOTAL_FORMS': '1001',
+            'form-INITIAL_FORMS': '0',
+            'form-MAX_NUM_FORMS': '0',
+        }
+        LimitedFavoriteDrinksFormSet = formset_factory(
+            FavoriteDrinkForm,
+            max_num=30,
+            absolute_max=1000,
+        )
+        formset = LimitedFavoriteDrinksFormSet(data=data)
+        self.assertIs(formset.is_valid(), False)
+        self.assertEqual(len(formset.forms), 1000)
+        self.assertEqual(
+            formset.non_form_errors(),
+            ['Please submit 30 or fewer forms.'],
+        )
+
+    def test_absolute_max_invalid(self):
+        msg = "'absolute_max' must be greater or equal to 'max_num'."
+        for max_num in [None, 31]:
+            with self.subTest(max_num=max_num):
+                with self.assertRaisesMessage(ValueError, msg):
+                    formset_factory(FavoriteDrinkForm, max_num=max_num, absolute_max=30)
+
     def test_more_initial_form_result_in_one(self):
         """
         One form from initial and extra=3 with max_num=2 results in the one
diff --git a/tests/model_formsets/tests.py b/tests/model_formsets/tests.py
index 90e5cb1bd8..8c6d87e59c 100644
--- a/tests/model_formsets/tests.py
+++ b/tests/model_formsets/tests.py
@@ -1838,3 +1838,81 @@ class TestModelFormsetOverridesTroughFormMeta(TestCase):
         form = BookFormSet.form(data={'title': 'Foo ' * 30, 'author': author.id})
         self.assertIs(Book._meta.get_field('title').__class__, models.CharField)
         self.assertIsInstance(form.fields['title'], forms.SlugField)
+
+    def test_modelformset_factory_absolute_max(self):
+        AuthorFormSet = modelformset_factory(Author, fields='__all__', absolute_max=1500)
+        data = {
+            'form-TOTAL_FORMS': '1501',
+            'form-INITIAL_FORMS': '0',
+            'form-MAX_NUM_FORMS': '0',
+        }
+        formset = AuthorFormSet(data=data)
+        self.assertIs(formset.is_valid(), False)
+        self.assertEqual(len(formset.forms), 1500)
+        self.assertEqual(
+            formset.non_form_errors(),
+            ['Please submit 1000 or fewer forms.'],
+        )
+
+    def test_modelformset_factory_absolute_max_with_max_num(self):
+        AuthorFormSet = modelformset_factory(
+            Author,
+            fields='__all__',
+            max_num=20,
+            absolute_max=100,
+        )
+        data = {
+            'form-TOTAL_FORMS': '101',
+            'form-INITIAL_FORMS': '0',
+            'form-MAX_NUM_FORMS': '0',
+        }
+        formset = AuthorFormSet(data=data)
+        self.assertIs(formset.is_valid(), False)
+        self.assertEqual(len(formset.forms), 100)
+        self.assertEqual(
+            formset.non_form_errors(),
+            ['Please submit 20 or fewer forms.'],
+        )
+
+    def test_inlineformset_factory_absolute_max(self):
+        author = Author.objects.create(name='Charles Baudelaire')
+        BookFormSet = inlineformset_factory(
+            Author,
+            Book,
+            fields='__all__',
+            absolute_max=1500,
+        )
+        data = {
+            'book_set-TOTAL_FORMS': '1501',
+            'book_set-INITIAL_FORMS': '0',
+            'book_set-MAX_NUM_FORMS': '0',
+        }
+        formset = BookFormSet(data, instance=author)
+        self.assertIs(formset.is_valid(), False)
+        self.assertEqual(len(formset.forms), 1500)
+        self.assertEqual(
+            formset.non_form_errors(),
+            ['Please submit 1000 or fewer forms.'],
+        )
+
+    def test_inlineformset_factory_absolute_max_with_max_num(self):
+        author = Author.objects.create(name='Charles Baudelaire')
+        BookFormSet = inlineformset_factory(
+            Author,
+            Book,
+            fields='__all__',
+            max_num=20,
+            absolute_max=100,
+        )
+        data = {
+            'book_set-TOTAL_FORMS': '101',
+            'book_set-INITIAL_FORMS': '0',
+            'book_set-MAX_NUM_FORMS': '0',
+        }
+        formset = BookFormSet(data, instance=author)
+        self.assertIs(formset.is_valid(), False)
+        self.assertEqual(len(formset.forms), 100)
+        self.assertEqual(
+            formset.non_form_errors(),
+            ['Please submit 20 or fewer forms.'],
+        )