security: Add EXEC_RESTRICT_FILE and EXEC_DENY_INTERACTIVE securebits

The new SECBIT_EXEC_RESTRICT_FILE, SECBIT_EXEC_DENY_INTERACTIVE, and
their *_LOCKED counterparts are designed to be set by processes setting
up an execution environment, such as a user session, a container, or a
security sandbox.  Unlike other securebits, these ones can be set by
unprivileged processes.  Like seccomp filters or Landlock domains, the
securebits are inherited across processes.

When SECBIT_EXEC_RESTRICT_FILE is set, programs interpreting code should
control executable resources according to execveat(2) + AT_EXECVE_CHECK
(see previous commit).

When SECBIT_EXEC_DENY_INTERACTIVE is set, a process should deny
execution of user interactive commands (which excludes executable
regular files).

Being able to configure each of these securebits enables system
administrators or owner of image containers to gradually validate the
related changes and to identify potential issues (e.g. with interpreter
or audit logs).

It should be noted that unlike other security bits, the
SECBIT_EXEC_RESTRICT_FILE and SECBIT_EXEC_DENY_INTERACTIVE bits are
dedicated to user space willing to restrict itself.  Because of that,
they only make sense in the context of a trusted environment (e.g.
sandbox, container, user session, full system) where the process
changing its behavior (according to these bits) and all its parent
processes are trusted.  Otherwise, any parent process could just execute
its own malicious code (interpreting a script or not), or even enforce a
seccomp filter to mask these bits.

Such a secure environment can be achieved with an appropriate access
control (e.g. mount's noexec option, file access rights, LSM policy) and
an enlighten ld.so checking that libraries are allowed for execution
e.g., to protect against illegitimate use of LD_PRELOAD.

Ptrace restrictions according to these securebits would not make sense
because of the processes' trust assumption.

Scripts may need some changes to deal with untrusted data (e.g. stdin,
environment variables), but that is outside the scope of the kernel.

See chromeOS's documentation about script execution control and the
related threat model:
https://www.chromium.org/chromium-os/developer-library/guides/security/noexec-shell-scripts/

Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Paul Moore <paul@paul-moore.com>
Reviewed-by: Serge Hallyn <serge@hallyn.com>
Reviewed-by: Jeff Xu <jeffxu@chromium.org>
Tested-by: Jeff Xu <jeffxu@chromium.org>
Signed-off-by: Mickaël Salaün <mic@digikod.net>
Link: https://lore.kernel.org/r/20241212174223.389435-3-mic@digikod.net
Signed-off-by: Kees Cook <kees@kernel.org>

1 files changed, 23 insertions, 6 deletions

diff --git a/security/commoncap.c b/security/commoncap.c
index cefad323a0b1..52ea01acb453 100644
--- a/security/commoncap.c
+++ b/security/commoncap.c
@@ -1302,21 +1302,38 @@ int cap_task_prctl(int option, unsigned long arg2, unsigned long arg3,
 		     & (old->securebits ^ arg2))			/*[1]*/
 		    || ((old->securebits & SECURE_ALL_LOCKS & ~arg2))	/*[2]*/
 		    || (arg2 & ~(SECURE_ALL_LOCKS | SECURE_ALL_BITS))	/*[3]*/
-		    || (cap_capable(current_cred(),
-				    current_cred()->user_ns,
-				    CAP_SETPCAP,
-				    CAP_OPT_NONE) != 0)			/*[4]*/
 			/*
 			 * [1] no changing of bits that are locked
 			 * [2] no unlocking of locks
 			 * [3] no setting of unsupported bits
-			 * [4] doing anything requires privilege (go read about
-			 *     the "sendmail capabilities bug")
 			 */
 		    )
 			/* cannot change a locked bit */
 			return -EPERM;
 
+		/*
+		 * Doing anything requires privilege (go read about the
+		 * "sendmail capabilities bug"), except for unprivileged bits.
+		 * Indeed, the SECURE_ALL_UNPRIVILEGED bits are not
+		 * restrictions enforced by the kernel but by user space on
+		 * itself.
+		 */
+		if (cap_capable(current_cred(), current_cred()->user_ns,
+				CAP_SETPCAP, CAP_OPT_NONE) != 0) {
+			const unsigned long unpriv_and_locks =
+				SECURE_ALL_UNPRIVILEGED |
+				SECURE_ALL_UNPRIVILEGED << 1;
+			const unsigned long changed = old->securebits ^ arg2;
+
+			/* For legacy reason, denies non-change. */
+			if (!changed)
+				return -EPERM;
+
+			/* Denies privileged changes. */
+			if (changed & ~unpriv_and_locks)
+				return -EPERM;
+		}
+
 		new = prepare_creds();
 		if (!new)
 			return -ENOMEM;