migrations: Derive emoji content-type from the bytes.

tools/semgrep-py.yml

@@ -54,7 +54,7 @@ rules:
 
   - id: dont-import-models-in-migrations
     patterns:
-      - pattern-not: from zerver.lib.mime_types import guess_type
+      - pattern-not: from zerver.lib.mime_types import $X
       - pattern-not: from zerver.lib.redis_utils import get_redis_client
       - pattern-not: from zerver.lib.utils import generate_api_key
       - pattern-not: from zerver.models.linkifiers import filter_pattern_validator

zerver/migrations/0553_copy_emoji_images.py

@@ -7,6 +7,7 @@ from typing import Any
 
 import boto3
 import botocore
+import magic
 import pyvips
 from botocore.client import Config
 from django.conf import settings
@@ -14,7 +15,7 @@ from django.db import migrations
 from django.db.backends.base.schema import BaseDatabaseSchemaEditor
 from django.db.migrations.state import StateApps
 
-from zerver.lib.mime_types import guess_extension, guess_type
+from zerver.lib.mime_types import guess_extension
 
 # From zerver.lib.thumbnail
 DEFAULT_EMOJI_SIZE = 64
@@ -142,31 +143,50 @@ def thumbnail_local_emoji(apps: StateApps) -> None:
         if total_processed % 100 == 0:
             print(f"Processed {total_processed} custom emoji")
 
+        old_file_name = emoji.file_name
         try:
-            old_file_name = emoji.file_name
-            content_type = guess_type(old_file_name)[0]
+            base_path = os.path.join(
+                settings.LOCAL_AVATARS_DIR, str(emoji.realm_id), "emoji/images"
+            )
+            copy_from_path = f"{base_path}/{old_file_name}.original"
+            if not os.path.exists(copy_from_path) and os.path.exists(
+                f"{base_path}/{old_file_name}"
+            ):
+                # Imports currently don't write ".original" files, so check without that
+                copy_from_path = f"{base_path}/{old_file_name}"
+                if not os.path.exists(copy_from_path):
+                    raise SkipImageError("Failed to read .original file: Does not exist")
+
+            with open(copy_from_path, "rb") as fh:
+                original_bytes = fh.read()
+
+            # We used to accept any bytes which pillow could
+            # thumbnail, with any filename, and would use the
+            # guessed-from-filename content-type when serving the
+            # emoji.  Examine the bytes of the image to verify that it
+            # is an image of reasonable type, and then derive the real
+            # filename extension (which we will still use for deriving
+            # content-type at serving time) from that.  This ensures
+            # that the contents are a valid image, and that we put the
+            # right content-type on it when served -- the filename
+            # used for the initial upload becomes completely
+            # irrelevant.
+            content_type = magic.from_buffer(original_bytes[:1024], mime=True)
+
             if content_type not in VALID_EMOJI_CONTENT_TYPE:
                 raise SkipImageError(f"Invalid content-type: {content_type}")
 
             new_file_name = get_emoji_file_name(content_type, emoji.id)
             if old_file_name == new_file_name:
                 continue
-            base_path = os.path.join(
-                settings.LOCAL_AVATARS_DIR, str(emoji.realm_id), "emoji/images"
-            )
+
             print(f"{base_path}/{old_file_name} -> {base_path}/{new_file_name}")
             try:
                 if os.path.exists(f"{base_path}/{new_file_name}.original"):
                     os.unlink(f"{base_path}/{new_file_name}.original")
-                from_file = f"{base_path}/{old_file_name}.original"
-                if not os.path.exists(from_file) and os.path.exists(f"{base_path}/{old_file_name}"):
-                    # Imports currently don't write ".original" files, so check without that
-                    from_file = f"{base_path}/{old_file_name}"
-                os.link(from_file, f"{base_path}/{new_file_name}.original")
-                with open(f"{base_path}/{new_file_name}.original", "rb") as fh:
-                    original_bytes = fh.read()
+                os.link(copy_from_path, f"{base_path}/{new_file_name}.original")
             except OSError as e:
-                raise SkipImageError(f"Failed to read original file: {e}")
+                raise SkipImageError(f"Failed to update .original file: {e}")
 
             animated, still = resize_emoji(original_bytes, new_file_name)
             try:
@@ -230,7 +250,6 @@ def thumbnail_s3(apps: StateApps) -> None:
             try:
                 old_data = avatar_bucket.Object(copy_from_path).get()
                 original_bytes = old_data["Body"].read()
-                content_type = old_data["ContentType"]
             except botocore.exceptions.ClientError:
                 # Imports currently don't write ".original" files, so check without that
                 try:
@@ -239,10 +258,16 @@ def thumbnail_s3(apps: StateApps) -> None:
                 except botocore.exceptions.ClientError as e:
                     raise SkipImageError(f"Failed to read .original file: {e}")
                 original_bytes = old_data["Body"].read()
-                # They also may have uploaded as "application/octet-stream", so guess the
-                # content-type from the filename.  If we can't guess, then we'll hit the
-                # SkipImageError case right below this.
-                content_type = guess_type(old_file_name)[0] or "application/octet-stream"
+
+            # We used to accept any bytes which pillow could
+            # thumbnail, with any filename, and would store the
+            # guessed-from-filename content-type in S3, to be used
+            # when serving the emoji.  Examine the bytes of the image
+            # to verify that it is an image of reasonable type, and
+            # then both store that content-type in S3 (for later
+            # serving), as well as using it to derive the right
+            # filename extension (for clarity).
+            content_type = magic.from_buffer(original_bytes[:1024], mime=True)
 
             if content_type not in VALID_EMOJI_CONTENT_TYPE:
                 raise SkipImageError(f"Invalid content-type: {content_type}")