Fix clone(CLONE_NEWUSER).

- Use new user namespace for namespace creation checks.

- Ensure userns is never nil since it's used by other namespaces.

PiperOrigin-RevId: 234673175
Change-Id: I4b9d9d1e63ce4e24362089793961a996f7540cd9

pkg/sentry/kernel/task_clone.go

@@ -17,7 +17,6 @@ package kernel
 import (
 	"gvisor.googlesource.com/gvisor/pkg/abi/linux"
 	"gvisor.googlesource.com/gvisor/pkg/bpf"
-	"gvisor.googlesource.com/gvisor/pkg/sentry/kernel/auth"
 	"gvisor.googlesource.com/gvisor/pkg/sentry/usermem"
 	"gvisor.googlesource.com/gvisor/pkg/syserror"
 )
@@ -166,7 +165,7 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 	// privileges over the remaining namespaces created by the call." -
 	// user_namespaces(7)
 	creds := t.Credentials()
-	var userns *auth.UserNamespace
+	userns := creds.UserNamespace
 	if opts.NewUserNamespace {
 		var err error
 		// "EPERM (since Linux 3.9): CLONE_NEWUSER was specified in flags and
@@ -182,7 +181,7 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 			return 0, nil, err
 		}
 	}
-	if (opts.NewPIDNamespace || opts.NewNetworkNamespace || opts.NewUTSNamespace) && !creds.HasCapability(linux.CAP_SYS_ADMIN) {
+	if (opts.NewPIDNamespace || opts.NewNetworkNamespace || opts.NewUTSNamespace) && !creds.HasCapabilityIn(linux.CAP_SYS_ADMIN, userns) {
 		return 0, nil, syserror.EPERM
 	}
 
@@ -287,7 +286,7 @@ func (t *Task) Clone(opts *CloneOptions) (ThreadID, *SyscallControl, error) {
 		nt.SetSignalStack(t.SignalStack())
 	}
 
-	if userns != nil {
+	if userns != creds.UserNamespace {
 		if err := nt.SetUserNamespace(userns); err != nil {
 			// This shouldn't be possible: userns was created from nt.creds, so
 			// nt should have CAP_SYS_ADMIN in userns.

test/syscalls/linux/BUILD

@@ -732,7 +732,9 @@ cc_binary(
     srcs = ["fork.cc"],
     linkstatic = 1,
     deps = [
+        "//test/util:capability_util",
         "//test/util:logging",
+        "//test/util:memory_util",
         "//test/util:test_main",
         "//test/util:test_util",
         "//test/util:thread_util",

test/syscalls/linux/fork.cc

@@ -21,11 +21,14 @@
 #include <sys/types.h>
 #include <unistd.h>
 #include <atomic>
+#include <cstdlib>
 
 #include "gtest/gtest.h"
 #include "absl/time/clock.h"
 #include "absl/time/time.h"
+#include "test/util/capability_util.h"
 #include "test/util/logging.h"
+#include "test/util/memory_util.h"
 #include "test/util/test_util.h"
 #include "test/util/thread_util.h"
 
@@ -393,6 +396,34 @@ TEST_F(ForkTest, Affinity) {
   EXPECT_THAT(Wait(child), SyscallSucceedsWithValue(0));
 }
 
+TEST(CloneTest, NewUserNamespacePermitsAllOtherNamespaces) {
+  // "If CLONE_NEWUSER is specified along with other CLONE_NEW* flags in a
+  // single clone(2) or unshare(2) call, the user namespace is guaranteed to be
+  // created first, giving the child (clone(2)) or caller (unshare(2))
+  // privileges over the remaining namespaces created by the call. Thus, it is
+  // possible for an unprivileged caller to specify this combination of flags."
+  // - user_namespaces(7)
+  SKIP_IF(!ASSERT_NO_ERRNO_AND_VALUE(CanCreateUserNamespace()));
+  Mapping child_stack = ASSERT_NO_ERRNO_AND_VALUE(
+      MmapAnon(kPageSize, PROT_READ | PROT_WRITE, MAP_PRIVATE));
+  int child_pid;
+  // We only test with CLONE_NEWIPC, CLONE_NEWNET, and CLONE_NEWUTS since these
+  // namespaces were implemented in Linux before user namespaces.
+  ASSERT_THAT(
+      child_pid = clone(
+          +[](void*) { return 0; },
+          reinterpret_cast<void*>(child_stack.addr() + kPageSize),
+          CLONE_NEWUSER | CLONE_NEWIPC | CLONE_NEWNET | CLONE_NEWUTS | SIGCHLD,
+          /* arg = */ nullptr),
+      SyscallSucceeds());
+
+  int status;
+  ASSERT_THAT(waitpid(child_pid, &status, 0),
+              SyscallSucceedsWithValue(child_pid));
+  EXPECT_TRUE(WIFEXITED(status) && WEXITSTATUS(status) == 0)
+      << "status = " << status;
+}
+
 #ifdef __x86_64__
 // Clone with CLONE_SETTLS and a non-canonical TLS address is rejected.
 TEST(CloneTest, NonCanonicalTLS) {