[3.1.x] Fixed #32718 -- Relaxed file name validation in FileField.

- Validate filename returned by FileField.upload_to() not a filename
  passed to the FileField.generate_filename() (upload_to() may
  completely ignored passed filename).
- Allow relative paths (without dot segments) in the generated filename.

Thanks to Jakub Kleň for the report and review.
Thanks to all folks for checking this patch on existing projects.
Thanks Florian Apolloner and Markus Holtermann for the discussion and
implementation idea.

Regression in 0b79eb36915d178aef5c6a7bbce71b1e76d376d3.

Backport of b55699968fc9ee985384c64e37f6cc74a0a23683 from main.

7 files changed, 134 insertions, 16 deletions

diff --git a/django/core/files/utils.py b/django/core/files/utils.py
index f83cb1a3cf..f28cea1077 100644
--- a/django/core/files/utils.py
+++ b/django/core/files/utils.py
@@ -1,16 +1,26 @@
 import os
+import pathlib
 
 from django.core.exceptions import SuspiciousFileOperation
 
 
-def validate_file_name(name):
-    if name != os.path.basename(name):
-        raise SuspiciousFileOperation("File name '%s' includes path elements" % name)
-
+def validate_file_name(name, allow_relative_path=False):
     # Remove potentially dangerous names
-    if name in {'', '.', '..'}:
+    if os.path.basename(name) in {'', '.', '..'}:
         raise SuspiciousFileOperation("Could not derive file name from '%s'" % name)
 
+    if allow_relative_path:
+        # Use PurePosixPath() because this branch is checked only in
+        # FileField.generate_filename() where all file paths are expected to be
+        # Unix style (with forward slashes).
+        path = pathlib.PurePosixPath(name)
+        if path.is_absolute() or '..' in path.parts:
+            raise SuspiciousFileOperation(
+                "Detected path traversal attempt in '%s'" % name
+            )
+    elif name != os.path.basename(name):
+        raise SuspiciousFileOperation("File name '%s' includes path elements" % name)
+
     return name
 
 
diff --git a/django/db/models/fields/files.py b/django/db/models/fields/files.py
index b16fd05f5b..28d476224c 100644
--- a/django/db/models/fields/files.py
+++ b/django/db/models/fields/files.py
@@ -319,12 +319,12 @@ class FileField(Field):
         Until the storage layer, all file paths are expected to be Unix style
         (with forward slashes).
         """
-        filename = validate_file_name(filename)
         if callable(self.upload_to):
             filename = self.upload_to(instance, filename)
         else:
             dirname = datetime.datetime.now().strftime(str(self.upload_to))
             filename = posixpath.join(dirname, filename)
+        filename = validate_file_name(filename, allow_relative_path=True)
         return self.storage.generate_filename(filename)
 
     def save_form_data(self, instance, data):
diff --git a/docs/releases/2.2.23.txt b/docs/releases/2.2.23.txt
new file mode 100644
index 0000000000..6c39361e5f
--- /dev/null
+++ b/docs/releases/2.2.23.txt
@@ -0,0 +1,15 @@
+===========================
+Django 2.2.23 release notes
+===========================
+
+*May 13, 2021*
+
+Django 2.2.23 fixes a regression in 2.2.21.
+
+Bugfixes
+========
+
+* Fixed a regression in Django 2.2.21 where saving ``FileField`` would raise a
+  ``SuspiciousFileOperation`` even when a custom
+  :attr:`~django.db.models.FileField.upload_to` returns a valid file path
+  (:ticket:`32718`).
diff --git a/docs/releases/3.1.11.txt b/docs/releases/3.1.11.txt
new file mode 100644
index 0000000000..d5fb537466
--- /dev/null
+++ b/docs/releases/3.1.11.txt
@@ -0,0 +1,15 @@
+===========================
+Django 3.1.11 release notes
+===========================
+
+*May 13, 2021*
+
+Django 3.1.11 fixes a regression in 3.1.9.
+
+Bugfixes
+========
+
+* Fixed a regression in Django 3.1.9 where saving ``FileField`` would raise a
+  ``SuspiciousFileOperation`` even when a custom
+  :attr:`~django.db.models.FileField.upload_to` returns a valid file path
+  (:ticket:`32718`).
diff --git a/docs/releases/index.txt b/docs/releases/index.txt
index 93bc8248b7..7d75e28b5e 100644
--- a/docs/releases/index.txt
+++ b/docs/releases/index.txt
@@ -25,6 +25,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   3.1.11
    3.1.10
    3.1.9
    3.1.8
@@ -63,6 +64,7 @@ versions of the documentation contain the release notes for any later releases.
 .. toctree::
    :maxdepth: 1
 
+   2.2.23
    2.2.22
    2.2.21
    2.2.20
diff --git a/tests/file_storage/test_generate_filename.py b/tests/file_storage/test_generate_filename.py
index 4746a53f69..66551c495b 100644
--- a/tests/file_storage/test_generate_filename.py
+++ b/tests/file_storage/test_generate_filename.py
@@ -1,6 +1,4 @@
 import os
-import sys
-from unittest import skipIf
 
 from django.core.exceptions import SuspiciousFileOperation
 from django.core.files.base import ContentFile
@@ -64,19 +62,37 @@ class GenerateFilenameStorageTests(SimpleTestCase):
             s.generate_filename(file_name)
 
     def test_filefield_dangerous_filename(self):
-        candidates = ['..', '.', '', '???', '$.$.$']
+        candidates = [
+            ('..', 'some/folder/..'),
+            ('.', 'some/folder/.'),
+            ('', 'some/folder/'),
+            ('???', '???'),
+            ('$.$.$', '$.$.$'),
+        ]
         f = FileField(upload_to='some/folder/')
-        msg = "Could not derive file name from '%s'"
-        for file_name in candidates:
+        for file_name, msg_file_name in candidates:
+            msg = f"Could not derive file name from '{msg_file_name}'"
             with self.subTest(file_name=file_name):
-                with self.assertRaisesMessage(SuspiciousFileOperation, msg % file_name):
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg):
                     f.generate_filename(None, file_name)
 
-    def test_filefield_dangerous_filename_dir(self):
+    def test_filefield_dangerous_filename_dot_segments(self):
         f = FileField(upload_to='some/folder/')
-        msg = "File name '/tmp/path' includes path elements"
+        msg = "Detected path traversal attempt in 'some/folder/../path'"
         with self.assertRaisesMessage(SuspiciousFileOperation, msg):
-            f.generate_filename(None, '/tmp/path')
+            f.generate_filename(None, '../path')
+
+    def test_filefield_generate_filename_absolute_path(self):
+        f = FileField(upload_to='some/folder/')
+        candidates = [
+            '/tmp/path',
+            '/tmp/../path',
+        ]
+        for file_name in candidates:
+            msg = f"Detected path traversal attempt in '{file_name}'"
+            with self.subTest(file_name=file_name):
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+                    f.generate_filename(None, file_name)
 
     def test_filefield_generate_filename(self):
         f = FileField(upload_to='some/folder/')
@@ -95,7 +111,57 @@ class GenerateFilenameStorageTests(SimpleTestCase):
             os.path.normpath('some/folder/test_with_space.txt')
         )
 
-    @skipIf(sys.platform == 'win32', 'Path components in filename are not supported after 0b79eb3.')
+    def test_filefield_generate_filename_upload_to_overrides_dangerous_filename(self):
+        def upload_to(instance, filename):
+            return 'test.txt'
+
+        f = FileField(upload_to=upload_to)
+        candidates = [
+            '/tmp/.',
+            '/tmp/..',
+            '/tmp/../path',
+            '/tmp/path',
+            'some/folder/',
+            'some/folder/.',
+            'some/folder/..',
+            'some/folder/???',
+            'some/folder/$.$.$',
+            'some/../test.txt',
+            '',
+        ]
+        for file_name in candidates:
+            with self.subTest(file_name=file_name):
+                self.assertEqual(f.generate_filename(None, file_name), 'test.txt')
+
+    def test_filefield_generate_filename_upload_to_absolute_path(self):
+        def upload_to(instance, filename):
+            return '/tmp/' + filename
+
+        f = FileField(upload_to=upload_to)
+        candidates = [
+            'path',
+            '../path',
+            '???',
+            '$.$.$',
+        ]
+        for file_name in candidates:
+            msg = f"Detected path traversal attempt in '/tmp/{file_name}'"
+            with self.subTest(file_name=file_name):
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+                    f.generate_filename(None, file_name)
+
+    def test_filefield_generate_filename_upload_to_dangerous_filename(self):
+        def upload_to(instance, filename):
+            return '/tmp/' + filename
+
+        f = FileField(upload_to=upload_to)
+        candidates = ['..', '.', '']
+        for file_name in candidates:
+            msg = f"Could not derive file name from '/tmp/{file_name}'"
+            with self.subTest(file_name=file_name):
+                with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+                    f.generate_filename(None, file_name)
+
     def test_filefield_awss3_storage(self):
         """
         Simulate a FileField with an S3 storage which uses keys rather than
diff --git a/tests/model_fields/test_filefield.py b/tests/model_fields/test_filefield.py
index d4e70d6041..c269b7c035 100644
--- a/tests/model_fields/test_filefield.py
+++ b/tests/model_fields/test_filefield.py
@@ -5,6 +5,7 @@ import tempfile
 import unittest
 from pathlib import Path
 
+from django.core.exceptions import SuspiciousFileOperation
 from django.core.files import File, temp
 from django.core.files.base import ContentFile
 from django.core.files.uploadedfile import TemporaryUploadedFile
@@ -62,6 +63,15 @@ class FileFieldTests(TestCase):
         d.refresh_from_db()
         self.assertIs(d.myfile.instance, d)
 
+    @unittest.skipIf(sys.platform == 'win32', "Crashes with OSError on Windows.")
+    def test_save_without_name(self):
+        with tempfile.NamedTemporaryFile(suffix='.txt') as tmp:
+            document = Document.objects.create(myfile='something.txt')
+            document.myfile = File(tmp)
+            msg = f"Detected path traversal attempt in '{tmp.name}'"
+            with self.assertRaisesMessage(SuspiciousFileOperation, msg):
+                document.save()
+
     def test_defer(self):
         Document.objects.create(myfile='something.txt')
         self.assertEqual(Document.objects.defer('myfile')[0].myfile, 'something.txt')