gecko-dev/caps/ContentPrincipal.cpp

/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
/* vim: set ts=2 sw=2 et tw=80: */
/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#include "ContentPrincipal.h"

#include "mozIThirdPartyUtil.h"
#include "nsContentUtils.h"
#include "nscore.h"
#include "nsScriptSecurityManager.h"
#include "nsString.h"
#include "nsReadableUtils.h"
#include "pratom.h"
#include "nsIURI.h"
#include "nsIURL.h"
#include "nsIStandardURL.h"
#include "nsIURIWithSpecialOrigin.h"
#include "nsIURIMutator.h"
#include "nsJSPrincipals.h"
#include "nsIEffectiveTLDService.h"
#include "nsIClassInfoImpl.h"
#include "nsIObjectInputStream.h"
#include "nsIObjectOutputStream.h"
#include "nsIProtocolHandler.h"
#include "nsError.h"
#include "nsIContentSecurityPolicy.h"
#include "nsNetCID.h"
#include "js/RealmIterators.h"
#include "js/Wrapper.h"

#include "mozilla/dom/BlobURLProtocolHandler.h"
#include "mozilla/dom/ScriptSettings.h"
#include "mozilla/ClearOnShutdown.h"
#include "mozilla/ExtensionPolicyService.h"
#include "mozilla/Preferences.h"
#include "mozilla/HashFunctions.h"

#include "nsSerializationHelper.h"
#include "json/json.h"

using namespace mozilla;

static inline ExtensionPolicyService& EPS() {
  return ExtensionPolicyService::GetSingleton();
}

NS_IMPL_CLASSINFO(ContentPrincipal, nullptr, nsIClassInfo::MAIN_THREAD_ONLY,
                  NS_PRINCIPAL_CID)
NS_IMPL_QUERY_INTERFACE_CI(ContentPrincipal, nsIPrincipal)
NS_IMPL_CI_INTERFACE_GETTER(ContentPrincipal, nsIPrincipal)

ContentPrincipal::ContentPrincipal(nsIURI* aURI,
                                   const OriginAttributes& aOriginAttributes,
                                   const nsACString& aOriginNoSuffix)
    : BasePrincipal(eContentPrincipal, aOriginNoSuffix, aOriginAttributes),
      mURI(aURI) {
#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
  // Assert that the URI we get here isn't any of the schemes that we know we
  // should not get here.  These schemes always either inherit their principal
  // or fall back to a null principal.  These are schemes which return
  // URI_INHERITS_SECURITY_CONTEXT from their protocol handler's
  // GetProtocolFlags function.
  bool hasFlag = false;
  MOZ_DIAGNOSTIC_ASSERT(
      NS_SUCCEEDED(NS_URIChainHasFlags(
          aURI, nsIProtocolHandler::URI_INHERITS_SECURITY_CONTEXT, &hasFlag)) &&
      !hasFlag);
#endif
}

ContentPrincipal::ContentPrincipal(ContentPrincipal* aOther,
                                   const OriginAttributes& aOriginAttributes)
    : BasePrincipal(aOther, aOriginAttributes),
      mURI(aOther->mURI),
      mDomain(aOther->mDomain),
      mAddon(aOther->mAddon) {}

ContentPrincipal::~ContentPrincipal() = default;

nsresult ContentPrincipal::GetScriptLocation(nsACString& aStr) {
  return mURI->GetSpec(aStr);
}

/* static */
nsresult ContentPrincipal::GenerateOriginNoSuffixFromURI(
    nsIURI* aURI, nsACString& aOriginNoSuffix) {
  if (!aURI) {
    return NS_ERROR_FAILURE;
  }

  nsCOMPtr<nsIURI> origin = NS_GetInnermostURI(aURI);
  if (!origin) {
    return NS_ERROR_FAILURE;
  }

  MOZ_ASSERT(!NS_IsAboutBlank(origin),
             "The inner URI for about:blank must be moz-safe-about:blank");

  // Handle non-strict file:// uris.
  if (!nsScriptSecurityManager::GetStrictFileOriginPolicy() &&
      NS_URIIsLocalFile(origin)) {
    // If strict file origin policy is not in effect, all local files are
    // considered to be same-origin, so return a known dummy origin here.
    aOriginNoSuffix.AssignLiteral("file://UNIVERSAL_FILE_URI_ORIGIN");
    return NS_OK;
  }

  nsresult rv;
// NB: This is only compiled for Thunderbird/Suite.
#if IS_ORIGIN_IS_FULL_SPEC_DEFINED
  bool fullSpec = false;
  rv = NS_URIChainHasFlags(origin, nsIProtocolHandler::ORIGIN_IS_FULL_SPEC,
                           &fullSpec);
  NS_ENSURE_SUCCESS(rv, rv);
  if (fullSpec) {
    return origin->GetAsciiSpec(aOriginNoSuffix);
  }
#endif

  // We want the invariant that prinA.origin == prinB.origin i.f.f.
  // prinA.equals(prinB). However, this requires that we impose certain
  // constraints on the behavior and origin semantics of principals, and in
  // particular, forbid creating origin strings for principals whose equality
  // constraints are not expressible as strings (i.e. object equality).
  // Moreover, we want to forbid URIs containing the magic "^" we use as a
  // separating character for origin attributes.
  //
  // These constraints can generally be achieved by restricting .origin to
  // nsIStandardURL-based URIs, but there are a few other URI schemes that we
  // need to handle.
  if (origin->SchemeIs("about") ||
      (origin->SchemeIs("moz-safe-about") &&
       // We generally consider two about:foo origins to be same-origin, but
       // about:blank is special since it can be generated from different
       // sources. We check for moz-safe-about:blank since origin is an
       // innermost URI.
       !StringBeginsWith(origin->GetSpecOrDefault(),
                         "moz-safe-about:blank"_ns))) {
    rv = origin->GetAsciiSpec(aOriginNoSuffix);
    NS_ENSURE_SUCCESS(rv, rv);

    int32_t pos = aOriginNoSuffix.FindChar('?');
    int32_t hashPos = aOriginNoSuffix.FindChar('#');

    if (hashPos != kNotFound && (pos == kNotFound || hashPos < pos)) {
      pos = hashPos;
    }

    if (pos != kNotFound) {
      aOriginNoSuffix.Truncate(pos);
    }

    // These URIs could technically contain a '^', but they never should.
    if (NS_WARN_IF(aOriginNoSuffix.FindChar('^', 0) != -1)) {
      aOriginNoSuffix.Truncate();
      return NS_ERROR_FAILURE;
    }
    return NS_OK;
  }

  // This URL can be a blobURL. In this case, we should use the 'parent'
  // principal instead.
  nsCOMPtr<nsIPrincipal> blobPrincipal;
  if (dom::BlobURLProtocolHandler::GetBlobURLPrincipal(
          origin, getter_AddRefs(blobPrincipal))) {
    MOZ_ASSERT(blobPrincipal);
    return blobPrincipal->GetOriginNoSuffix(aOriginNoSuffix);
  }

  // If we reached this branch, we can only create an origin if we have a
  // nsIStandardURL.  So, we query to a nsIStandardURL, and fail if we aren't
  // an instance of an nsIStandardURL nsIStandardURLs have the good property
  // of escaping the '^' character in their specs, which means that we can be
  // sure that the caret character (which is reserved for delimiting the end
  // of the spec, and the beginning of the origin attributes) is not present
  // in the origin string
  nsCOMPtr<nsIStandardURL> standardURL = do_QueryInterface(origin);
  if (!standardURL) {
    return NS_ERROR_FAILURE;
  }

  // See whether we have a useful hostPort. If we do, use that.
  nsAutoCString hostPort;
  if (!origin->SchemeIs("chrome")) {
    rv = origin->GetAsciiHostPort(hostPort);
    NS_ENSURE_SUCCESS(rv, rv);
  }
  if (!hostPort.IsEmpty()) {
    rv = origin->GetScheme(aOriginNoSuffix);
    NS_ENSURE_SUCCESS(rv, rv);
    aOriginNoSuffix.AppendLiteral("://");
    aOriginNoSuffix.Append(hostPort);
    return NS_OK;
  }

  rv = aURI->GetAsciiSpec(aOriginNoSuffix);
  NS_ENSURE_SUCCESS(rv, rv);

  // The origin, when taken from the spec, should not contain the ref part of
  // the URL.

  int32_t pos = aOriginNoSuffix.FindChar('?');
  int32_t hashPos = aOriginNoSuffix.FindChar('#');

  if (hashPos != kNotFound && (pos == kNotFound || hashPos < pos)) {
    pos = hashPos;
  }

  if (pos != kNotFound) {
    aOriginNoSuffix.Truncate(pos);
  }

  return NS_OK;
}

bool ContentPrincipal::SubsumesInternal(
    nsIPrincipal* aOther,
    BasePrincipal::DocumentDomainConsideration aConsideration) {
  MOZ_ASSERT(aOther);

  // For ContentPrincipal, Subsumes is equivalent to Equals.
  if (aOther == this) {
    return true;
  }

  // If either the subject or the object has changed its principal by
  // explicitly setting document.domain then the other must also have
  // done so in order to be considered the same origin. This prevents
  // DNS spoofing based on document.domain (154930)
  nsresult rv;
  if (aConsideration == ConsiderDocumentDomain) {
    // Get .domain on each principal.
    nsCOMPtr<nsIURI> thisDomain, otherDomain;
    GetDomain(getter_AddRefs(thisDomain));
    aOther->GetDomain(getter_AddRefs(otherDomain));

    // If either has .domain set, we have equality i.f.f. the domains match.
    // Otherwise, we fall through to the non-document-domain-considering case.
    if (thisDomain || otherDomain) {
      bool isMatch =
          nsScriptSecurityManager::SecurityCompareURIs(thisDomain, otherDomain);
#ifdef DEBUG
      if (isMatch) {
        nsAutoCString thisSiteOrigin, otherSiteOrigin;
        MOZ_ALWAYS_SUCCEEDS(GetSiteOrigin(thisSiteOrigin));
        MOZ_ALWAYS_SUCCEEDS(aOther->GetSiteOrigin(otherSiteOrigin));
        MOZ_ASSERT(
            thisSiteOrigin == otherSiteOrigin,
            "SubsumesConsideringDomain passed with mismatched siteOrigin!");
      }
#endif
      return isMatch;
    }
  }

  // Compare uris.
  bool isSameOrigin = false;
  rv = aOther->IsSameOrigin(mURI, false, &isSameOrigin);
  NS_ENSURE_SUCCESS(rv, false);
  return isSameOrigin;
}

NS_IMETHODIMP
ContentPrincipal::GetURI(nsIURI** aURI) {
  *aURI = do_AddRef(mURI).take();
  return NS_OK;
}

bool ContentPrincipal::MayLoadInternal(nsIURI* aURI) {
  MOZ_ASSERT(aURI);

#if defined(MOZ_THUNDERBIRD) || defined(MOZ_SUITE)
  nsCOMPtr<nsIURIWithSpecialOrigin> uriWithSpecialOrigin =
      do_QueryInterface(aURI);
  if (uriWithSpecialOrigin) {
    nsCOMPtr<nsIURI> origin;
    nsresult rv = uriWithSpecialOrigin->GetOrigin(getter_AddRefs(origin));
    if (NS_WARN_IF(NS_FAILED(rv))) {
      return false;
    }
    MOZ_ASSERT(origin);
    OriginAttributes attrs;
    RefPtr<BasePrincipal> principal =
        BasePrincipal::CreateContentPrincipal(origin, attrs);
    return nsIPrincipal::Subsumes(principal);
  }
#endif

  nsCOMPtr<nsIPrincipal> blobPrincipal;
  if (dom::BlobURLProtocolHandler::GetBlobURLPrincipal(
          aURI, getter_AddRefs(blobPrincipal))) {
    MOZ_ASSERT(blobPrincipal);
    return nsIPrincipal::Subsumes(blobPrincipal);
  }

  // If this principal is associated with an addon, check whether that addon
  // has been given permission to load from this domain.
  if (AddonAllowsLoad(aURI)) {
    return true;
  }

  if (nsScriptSecurityManager::SecurityCompareURIs(mURI, aURI)) {
    return true;
  }

  // If strict file origin policy is in effect, local files will always fail
  // SecurityCompareURIs unless they are identical. Explicitly check file origin
  // policy, in that case.
  if (nsScriptSecurityManager::GetStrictFileOriginPolicy() &&
      NS_URIIsLocalFile(aURI) && NS_RelaxStrictFileOriginPolicy(aURI, mURI)) {
    return true;
  }

  return false;
}

uint32_t ContentPrincipal::GetHashValue() {
  MOZ_ASSERT(mURI, "Need a principal URI");

  nsCOMPtr<nsIURI> uri;
  GetDomain(getter_AddRefs(uri));
  if (!uri) {
    GetURI(getter_AddRefs(uri));
  };
  return NS_SecurityHashURI(uri);
}

NS_IMETHODIMP
ContentPrincipal::GetDomain(nsIURI** aDomain) {
  if (!mDomain) {
    *aDomain = nullptr;
    return NS_OK;
  }

  NS_ADDREF(*aDomain = mDomain);
  return NS_OK;
}

NS_IMETHODIMP
ContentPrincipal::SetDomain(nsIURI* aDomain) {
  MOZ_ASSERT(aDomain);

  mDomain = aDomain;
  SetHasExplicitDomain();

  // Set the changed-document-domain flag on compartments containing realms
  // using this principal.
  auto cb = [](JSContext*, void*, JS::Realm* aRealm,
               const JS::AutoRequireNoGC& nogc) {
    JS::Compartment* comp = JS::GetCompartmentForRealm(aRealm);
    xpc::SetCompartmentChangedDocumentDomain(comp);
  };
  JSPrincipals* principals =
      nsJSPrincipals::get(static_cast<nsIPrincipal*>(this));

  dom::AutoJSAPI jsapi;
  jsapi.Init();
  JS::IterateRealmsWithPrincipals(jsapi.cx(), principals, nullptr, cb);

  return NS_OK;
}

static nsresult GetSpecialBaseDomain(const nsCOMPtr<nsIURI>& aURI,
                                     bool* aHandled, nsACString& aBaseDomain) {
  *aHandled = false;

  // Special handling for a file URI.
  if (NS_URIIsLocalFile(aURI)) {
    // If strict file origin policy is not in effect, all local files are
    // considered to be same-origin, so return a known dummy domain here.
    if (!nsScriptSecurityManager::GetStrictFileOriginPolicy()) {
      *aHandled = true;
      aBaseDomain.AssignLiteral("UNIVERSAL_FILE_URI_ORIGIN");
      return NS_OK;
    }

    // Otherwise, we return the file path.
    nsCOMPtr<nsIURL> url = do_QueryInterface(aURI);

    if (url) {
      *aHandled = true;
      return url->GetFilePath(aBaseDomain);
    }
  }

  bool hasNoRelativeFlag;
  nsresult rv = NS_URIChainHasFlags(aURI, nsIProtocolHandler::URI_NORELATIVE,
                                    &hasNoRelativeFlag);
  if (NS_WARN_IF(NS_FAILED(rv))) {
    return rv;
  }

  // In case of FTP we want to get base domain via TLD service even if FTP
  // protocol handler is disabled and the scheme is handled by external protocol
  // handler which returns URI_NORELATIVE flag.
  if (hasNoRelativeFlag && !aURI->SchemeIs("ftp")) {
    *aHandled = true;
    return aURI->GetSpec(aBaseDomain);
  }

  if (aURI->SchemeIs("indexeddb")) {
    *aHandled = true;
    return aURI->GetSpec(aBaseDomain);
  }

  return NS_OK;
}

NS_IMETHODIMP
ContentPrincipal::GetBaseDomain(nsACString& aBaseDomain) {
  // Handle some special URIs first.
  bool handled;
  nsresult rv = GetSpecialBaseDomain(mURI, &handled, aBaseDomain);
  NS_ENSURE_SUCCESS(rv, rv);

  if (handled) {
    return NS_OK;
  }

  // For everything else, we ask the TLD service via the ThirdPartyUtil.
  nsCOMPtr<mozIThirdPartyUtil> thirdPartyUtil =
      do_GetService(THIRDPARTYUTIL_CONTRACTID);
  if (!thirdPartyUtil) {
    return NS_ERROR_FAILURE;
  }

  return thirdPartyUtil->GetBaseDomain(mURI, aBaseDomain);
}

NS_IMETHODIMP
ContentPrincipal::GetSiteOriginNoSuffix(nsACString& aSiteOrigin) {
  nsresult rv = GetOriginNoSuffix(aSiteOrigin);
  NS_ENSURE_SUCCESS(rv, rv);

  // It is possible for two principals with the same origin to have different
  // mURI values. In order to ensure that two principals with matching origins
  // also have matching siteOrigins, we derive the siteOrigin entirely from the
  // origin string and do not rely on mURI at all here.
  nsCOMPtr<nsIURI> origin;
  rv = NS_NewURI(getter_AddRefs(origin), aSiteOrigin);
  if (NS_FAILED(rv)) {
    // We got an error parsing the origin as a URI? siteOrigin == origin
    // aSiteOrigin was already filled with `OriginNoSuffix`
    return rv;
  }

  // Handle some special URIs first.
  nsAutoCString baseDomain;
  bool handled;
  rv = GetSpecialBaseDomain(origin, &handled, baseDomain);
  NS_ENSURE_SUCCESS(rv, rv);

  if (handled) {
    // This is a special URI ("file:", "about:", "view-source:", etc). Just
    // return the origin.
    return NS_OK;
  }

  // For everything else, we ask the TLD service. Note that, unlike in
  // GetBaseDomain, we don't use ThirdPartyUtil.getBaseDomain because if the
  // host is an IP address that returns the raw address and we can't use it with
  // SetHost below because SetHost expects '[' and ']' around IPv6 addresses.
  // See bug 1491728.
  nsCOMPtr<nsIEffectiveTLDService> tldService =
      do_GetService(NS_EFFECTIVETLDSERVICE_CONTRACTID);
  if (!tldService) {
    return NS_ERROR_FAILURE;
  }

  bool gotBaseDomain = false;
  rv = tldService->GetBaseDomain(origin, 0, baseDomain);
  if (NS_SUCCEEDED(rv)) {
    gotBaseDomain = true;
  } else {
    // If this is an IP address or something like "localhost", we just continue
    // with gotBaseDomain = false.
    if (rv != NS_ERROR_HOST_IS_IP_ADDRESS &&
        rv != NS_ERROR_INSUFFICIENT_DOMAIN_LEVELS) {
      return rv;
    }
  }

  // NOTE: Calling `SetHostPort` with a portless domain is insufficient to clear
  // the port, so an extra `SetPort` call has to be made.
  nsCOMPtr<nsIURI> siteUri;
  NS_MutateURI mutator(origin);
  mutator.SetUserPass(""_ns).SetPort(-1);
  if (gotBaseDomain) {
    mutator.SetHost(baseDomain);
  }
  rv = mutator.Finalize(siteUri);
  MOZ_ASSERT(NS_SUCCEEDED(rv), "failed to create siteUri");
  NS_ENSURE_SUCCESS(rv, rv);

  aSiteOrigin.Truncate();
  rv = GenerateOriginNoSuffixFromURI(siteUri, aSiteOrigin);
  MOZ_ASSERT(NS_SUCCEEDED(rv), "failed to create siteOriginNoSuffix");
  return rv;
}

nsresult ContentPrincipal::GetSiteIdentifier(SiteIdentifier& aSite) {
  nsCString siteOrigin;
  nsresult rv = GetSiteOrigin(siteOrigin);
  NS_ENSURE_SUCCESS(rv, rv);

  RefPtr<BasePrincipal> principal = CreateContentPrincipal(siteOrigin);
  if (!principal) {
    NS_WARNING("could not instantiate content principal");
    return NS_ERROR_FAILURE;
  }

  aSite.Init(principal);
  return NS_OK;
}

WebExtensionPolicy* ContentPrincipal::AddonPolicy() {
  if (!mAddon.isSome()) {
    NS_ENSURE_TRUE(mURI, nullptr);

    if (mURI->SchemeIs("moz-extension")) {
      mAddon.emplace(EPS().GetByURL(mURI.get()));
    } else {
      mAddon.emplace(nullptr);
    }
  }

  return mAddon.value();
}

NS_IMETHODIMP
ContentPrincipal::GetAddonId(nsAString& aAddonId) {
  auto* policy = AddonPolicy();
  if (policy) {
    policy->GetId(aAddonId);
  } else {
    aAddonId.Truncate();
  }
  return NS_OK;
}

NS_IMETHODIMP
ContentPrincipal::Deserializer::Read(nsIObjectInputStream* aStream) {
  MOZ_ASSERT(!mPrincipal);

  nsCOMPtr<nsISupports> supports;
  nsCOMPtr<nsIURI> principalURI;
  nsresult rv = NS_ReadOptionalObject(aStream, true, getter_AddRefs(supports));
  if (NS_FAILED(rv)) {
    return rv;
  }

  principalURI = do_QueryInterface(supports);
  // Enforce re-parsing about: URIs so that if they change, we continue to use
  // their new principals correctly.
  if (principalURI->SchemeIs("about")) {
    nsAutoCString spec;
    principalURI->GetSpec(spec);
    NS_ENSURE_SUCCESS(NS_NewURI(getter_AddRefs(principalURI), spec),
                      NS_ERROR_FAILURE);
  }

  nsCOMPtr<nsIURI> domain;
  rv = NS_ReadOptionalObject(aStream, true, getter_AddRefs(supports));
  if (NS_FAILED(rv)) {
    return rv;
  }

  domain = do_QueryInterface(supports);

  nsAutoCString suffix;
  rv = aStream->ReadCString(suffix);
  NS_ENSURE_SUCCESS(rv, rv);

  OriginAttributes attrs;
  bool ok = attrs.PopulateFromSuffix(suffix);
  NS_ENSURE_TRUE(ok, NS_ERROR_FAILURE);

  // Since Bug 965637 we do not serialize the CSP within the
  // Principal anymore. Nevertheless there might still be
  // serialized Principals that do have a serialized CSP.
  // For now, we just read the CSP here but do not actually
  // consume it. Please note that we deliberately ignore
  // the return value to avoid CSP deserialization problems.
  // After Bug 1508939 we will have a new serialization for
  // Principals which allows us to update the code here.
  // Additionally, the format for serialized CSPs changed
  // within Bug 965637 which also can cause failures within
  // the CSP deserialization code.
  Unused << NS_ReadOptionalObject(aStream, true, getter_AddRefs(supports));

  nsAutoCString originNoSuffix;
  rv = GenerateOriginNoSuffixFromURI(principalURI, originNoSuffix);
  NS_ENSURE_SUCCESS(rv, rv);

  RefPtr<ContentPrincipal> principal =
      new ContentPrincipal(principalURI, attrs, originNoSuffix);

  // Note: we don't call SetDomain here because we don't need the wrapper
  // recomputation code there (we just created this principal).
  if (domain) {
    principal->mDomain = domain;
    principal->SetHasExplicitDomain();
  }

  mPrincipal = principal.forget();
  return NS_OK;
}

nsresult ContentPrincipal::PopulateJSONObject(Json::Value& aObject) {
  nsAutoCString principalURI;
  nsresult rv = mURI->GetSpec(principalURI);
  NS_ENSURE_SUCCESS(rv, rv);

  // We turn each int enum field into a JSON string key of the object
  // aObject is the inner JSON object that has stringified enum keys
  // An example aObject might be:
  //
  // eURI                   eSuffix
  //    |                           |
  //  {"0": "https://mozilla.com", "2": "^privateBrowsingId=1"}
  //    |                |          |         |
  //    -----------------------------         |
  //         |           |                    |
  //        Key          ----------------------
  //                                |
  //                              Value
  aObject[std::to_string(eURI)] = principalURI.get();

  if (mDomain) {
    nsAutoCString domainStr;
    rv = mDomain->GetSpec(domainStr);
    NS_ENSURE_SUCCESS(rv, rv);
    aObject[std::to_string(eDomain)] = domainStr.get();
  }

  nsAutoCString suffix;
  OriginAttributesRef().CreateSuffix(suffix);
  if (suffix.Length() > 0) {
    aObject[std::to_string(eSuffix)] = suffix.get();
  }

  return NS_OK;
}

already_AddRefed<BasePrincipal> ContentPrincipal::FromProperties(
    nsTArray<ContentPrincipal::KeyVal>& aFields) {
  MOZ_ASSERT(aFields.Length() == eMax + 1, "Must have all the keys");
  nsresult rv;
  nsCOMPtr<nsIURI> principalURI;
  nsCOMPtr<nsIURI> domain;
  nsCOMPtr<nsIContentSecurityPolicy> csp;
  OriginAttributes attrs;

  // The odd structure here is to make the code to not compile
  // if all the switch enum cases haven't been codified
  for (const auto& field : aFields) {
    switch (field.key) {
      case ContentPrincipal::eURI:
        if (!field.valueWasSerialized) {
          MOZ_ASSERT(
              false,
              "Content principals require a principal URI in serialized JSON");
          return nullptr;
        }
        rv = NS_NewURI(getter_AddRefs(principalURI), field.value.get());
        NS_ENSURE_SUCCESS(rv, nullptr);

        {
          // Enforce re-parsing about: URIs so that if they change, we
          // continue to use their new principals correctly.
          if (principalURI->SchemeIs("about")) {
            nsAutoCString spec;
            principalURI->GetSpec(spec);
            if (NS_FAILED(NS_NewURI(getter_AddRefs(principalURI), spec))) {
              return nullptr;
            }
          }
        }
        break;
      case ContentPrincipal::eDomain:
        if (field.valueWasSerialized) {
          rv = NS_NewURI(getter_AddRefs(domain), field.value.get());
          NS_ENSURE_SUCCESS(rv, nullptr);
        }
        break;
      case ContentPrincipal::eSuffix:
        if (field.valueWasSerialized) {
          bool ok = attrs.PopulateFromSuffix(field.value);
          if (!ok) {
            return nullptr;
          }
        }
        break;
    }
  }
  nsAutoCString originNoSuffix;
  rv = ContentPrincipal::GenerateOriginNoSuffixFromURI(principalURI,
                                                       originNoSuffix);
  if (NS_FAILED(rv)) {
    return nullptr;
  }

  RefPtr<ContentPrincipal> principal =
      new ContentPrincipal(principalURI, attrs, originNoSuffix);

  principal->mDomain = domain;
  if (principal->mDomain) {
    principal->SetHasExplicitDomain();
  }

  return principal.forget();
}