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Zulip attempts to validate that the regular expressions that admins enter for linkifiers are well-formatted, and only contain a specific subset of regex grammar. The process of checking these properties (via a regex!) can cause denial-of-service via backtracking. Furthermore, this validation itself does not prevent the creation of linkifiers which themselves cause denial-of-service when they are executed. As the validator accepts literally anything inside of a `(?P<word>...)` block, any quadratic backtracking expression can be hidden therein. Switch user-provided linkifier patterns to be matched in the Markdown processor by the `re2` library, which is guaranteed constant-time. This somewhat limits the possible features of the regular expression (notably, look-head and -behind, and back-references); however, these features had never been advertised as working in the context of linkifiers. A migration removes any existing linkifiers which would not function under re2, after printing them for posterity during the upgrade; they are unlikely to be common, and are impossible to fix automatically. The denial-of-service in the linkifier validator was discovered by @erik-krogh and @yoff, as GHSL-2021-118.
40 lines
1.2 KiB
Python
40 lines
1.2 KiB
Python
import re2
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from django.db import migrations
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from django.db.backends.postgresql.schema import DatabaseSchemaEditor
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from django.db.migrations.state import StateApps
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def delete_re2_invalid(apps: StateApps, schema_editor: DatabaseSchemaEditor) -> None:
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options = re2.Options()
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options.log_errors = False
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RealmFilter = apps.get_model("zerver", "RealmFilter")
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found_errors = False
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for linkifier in RealmFilter.objects.all():
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try:
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re2.compile(linkifier.pattern, options=options)
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except re2.error:
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if not found_errors:
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print()
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found_errors = True
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print(
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f"Deleting linkifier {linkifier.id} in realm {linkifier.realm.string_id} which is not compatible with new re2 engine:"
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)
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print(f" {linkifier.pattern} -> {linkifier.url_format_string}")
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linkifier.delete()
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class Migration(migrations.Migration):
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dependencies = [
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("zerver", "0325_alter_realmplayground_unique_together"),
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]
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operations = [
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migrations.RunPython(
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delete_re2_invalid,
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reverse_code=migrations.RunPython.noop,
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elidable=True,
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)
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]
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