userns: don't drop the exec securebits

SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE tell script
interpreters to only run what AT_EXECVE_CHECK allows and to refuse
interactive input. A service manager sets them together with their lock
bits so that nothing below it can turn them off again.

But set_cred_user_ns() resets all securebits to SECUREBITS_DEFAULT. So
any task that can create a user namespace sheds the locked exec bits
with a single unshare(CLONE_NEWUSER) and joining one with setns() does
the same. The lock is pointless if anyone who may create a user
namespace can drop it, and that is pretty much everyone.

Resetting the other bits is fine. They are about how capabilities are
computed and a new user namespace starts out with a full set of
capabilities anyway. The exec bits grant nothing though. They are
restrictions a task placed on itself, just like no_new_privs which a
user namespace doesn't touch either. So keep them and their locks.

Reproducer:

  // SPDX-License-Identifier: GPL-2.0
  /* Lock the exec securebits, then create a user namespace and look at them again. */
  #define _GNU_SOURCE
  #include <sched.h>
  #include <stdio.h>
  #include <sys/prctl.h>

  #define SECBIT_EXEC_RESTRICT_FILE		(1 << 8)
  #define SECBIT_EXEC_RESTRICT_FILE_LOCKED	(1 << 9)
  #define SECBIT_EXEC_DENY_INTERACTIVE		(1 << 10)
  #define SECBIT_EXEC_DENY_INTERACTIVE_LOCKED	(1 << 11)

  int main(void)
  {
  	int bits = SECBIT_EXEC_RESTRICT_FILE | SECBIT_EXEC_RESTRICT_FILE_LOCKED |
  		   SECBIT_EXEC_DENY_INTERACTIVE | SECBIT_EXEC_DENY_INTERACTIVE_LOCKED;

  	if (prctl(PR_SET_SECUREBITS, bits, 0, 0, 0)) {
  		perror("PR_SET_SECUREBITS");
  		return 1;
  	}
  	printf("before: 0x%x\n", prctl(PR_GET_SECUREBITS, 0, 0, 0, 0));
  	printf("clear locked bits: %s\n", prctl(PR_SET_SECUREBITS, 0, 0, 0, 0) ? "refused" : "ALLOWED");

  	if (unshare(CLONE_NEWUSER)) {
  		perror("unshare(CLONE_NEWUSER)");
  		return 1;
  	}
  	printf("after unshare(CLONE_NEWUSER): 0x%x\n", prctl(PR_GET_SECUREBITS, 0, 0, 0, 0));
  	return 0;
  }

On v7.1 this prints "before: 0xf00", "clear locked bits: refused" and
"after unshare(CLONE_NEWUSER): 0x0". With this patch the last line
reads 0xf00 as well.

Fixes: a0623b2a1d ("security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits")
Cc: stable@vger.kernel.org # v6.14+
Link: https://patch.msgid.link/20261026-work-securebits-userns-v1-1-b35473fa1a34@kernel.org
Reviewed-by: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

Documentation/userspace-api/check_exec.rst

@@ -99,6 +99,10 @@ This secure bit may be set by user session managers, service managers,
 container runtimes, sandboxer tools...  Except for test environments, the
 related ``SECBIT_EXEC_DENY_INTERACTIVE_LOCKED`` bit should also be set.
 
+Both secure bits and their locks are inherited across :manpage:`fork(2)` and
+:manpage:`execve(2)`, and they are kept when a task creates or joins a user
+namespace.
+
 Here is the expected behavior for a script interpreter according to combination
 of any exec securebits:
 

kernel/user_namespace.c

@@ -46,8 +46,11 @@ static void set_cred_user_ns(struct cred *cred, struct user_namespace *user_ns)
 {
 	/* Start with the same capabilities as init but useless for doing
 	 * anything as the capabilities are bound to the new user namespace.
+	 * The exec securebits grant nothing, they are restrictions the task
+	 * placed on itself. Keep them and their locks.
 	 */
-	cred->securebits = SECUREBITS_DEFAULT;
+	cred->securebits = SECUREBITS_DEFAULT |
+		(cred->securebits & (SECURE_ALL_UNPRIVILEGED | SECURE_ALL_UNPRIVILEGED << 1));
 	cred->cap_inheritable = CAP_EMPTY_SET;
 	cred->cap_permitted = CAP_FULL_SET;
 	cred->cap_effective = CAP_FULL_SET;