[PATCH v2 1/3] landlock: Add LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS

[Justin Suess <utilityemal77@gmail.com>]

Add a landlock_restrict_self(2) flag to set the no_new_privs attribute
of the calling thread atomically with the enforcement of the ruleset:
no_new_privs is set if and only if the call succeeds.  This removes the
need for a prior prctl(2) PR_SET_NO_NEW_PRIVS call and guarantees that a
failed enforcement leaves the attribute unchanged.

Because no_new_privs is set by the call itself, the no_new_privs /
CAP_SYS_ADMIN requirement of landlock_restrict_self(2) is fulfilled by
construction, and the related EPERM check is skipped.  As a consequence,
an unprivileged caller passing unknown flags along with this flag gets
EINVAL instead of EPERM.

The attribute is only set past the last point of failure, just before
committing the new credentials.  When combined with
LANDLOCK_RESTRICT_SELF_TSYNC, no_new_privs is set on the sibling threads
as well, in their commit phase, with the same atomicity.

Bump the Landlock ABI version to 11.

Cc: Mickaël Salaün <mic@digikod.net>
Signed-off-by: Justin Suess <utilityemal77@gmail.com>
---
 include/uapi/linux/landlock.h | 13 +++++++++++++
 security/landlock/limits.h    |  2 +-
 security/landlock/syscalls.c  | 28 +++++++++++++++++++++-------
 security/landlock/tsync.c     |  8 ++++++--
 security/landlock/tsync.h     |  4 +++-
 5 files changed, 44 insertions(+), 11 deletions(-)

diff --git a/include/uapi/linux/landlock.h b/include/uapi/linux/landlock.h
index 272f047df438..77820e430ab8 100644
--- a/include/uapi/linux/landlock.h
+++ b/include/uapi/linux/landlock.h
@@ -191,12 +191,25 @@ struct landlock_ruleset_attr {
  *
  *     If the calling thread is running with no_new_privs, this operation
  *     enables no_new_privs on the sibling threads as well.
+ *
+ * The following flag ties the no_new_privs attribute to the ruleset
+ * enforcement:
+ *
+ * %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
+ *     Sets the no_new_privs attribute of the calling thread atomically with
+ *     the enforcement of the ruleset: no_new_privs is set if and only if
+ *     sys_landlock_restrict_self() succeeds.  This removes the need for a
+ *     prior :manpage:`prctl(2)` ``PR_SET_NO_NEW_PRIVS`` call, and with it the
+ *     %CAP_SYS_ADMIN requirement.  This flag requires a ruleset.  When
+ *     combined with %LANDLOCK_RESTRICT_SELF_TSYNC, no_new_privs is set on the
+ *     sibling threads as well.
  */
 /* clang-format off */
 #define LANDLOCK_RESTRICT_SELF_LOG_SAME_EXEC_OFF		(1U << 0)
 #define LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON			(1U << 1)
 #define LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF		(1U << 2)
 #define LANDLOCK_RESTRICT_SELF_TSYNC				(1U << 3)
+#define LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS			(1U << 4)
 /* clang-format on */
 
 /**
diff --git a/security/landlock/limits.h b/security/landlock/limits.h
index 08d5f2f6d321..1a7c5fb8f6fd 100644
--- a/security/landlock/limits.h
+++ b/security/landlock/limits.h
@@ -34,7 +34,7 @@
 #define LANDLOCK_NUM_ACCESS_MAX \
 	MAX(MAX(LANDLOCK_NUM_ACCESS_FS, LANDLOCK_NUM_ACCESS_NET), LANDLOCK_NUM_SCOPE)
 
-#define LANDLOCK_LAST_RESTRICT_SELF	LANDLOCK_RESTRICT_SELF_TSYNC
+#define LANDLOCK_LAST_RESTRICT_SELF	LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
 #define LANDLOCK_MASK_RESTRICT_SELF	((LANDLOCK_LAST_RESTRICT_SELF << 1) - 1)
 
 /* clang-format on */
diff --git a/security/landlock/syscalls.c b/security/landlock/syscalls.c
index 36b02892c62f..36b8a3fb506f 100644
--- a/security/landlock/syscalls.c
+++ b/security/landlock/syscalls.c
@@ -169,7 +169,7 @@ static const struct file_operations ruleset_fops = {
  * If the change involves a fix that requires userspace awareness, also update
  * the errata documentation in Documentation/userspace-api/landlock.rst .
  */
-const int landlock_abi_version = 10;
+const int landlock_abi_version = 11;
 
 /**
  * sys_landlock_create_ruleset - Create a new ruleset
@@ -502,21 +502,28 @@ SYSCALL_DEFINE4(landlock_add_rule, const int, ruleset_fd,
  *         - %LANDLOCK_RESTRICT_SELF_LOG_NEW_EXEC_ON
  *         - %LANDLOCK_RESTRICT_SELF_LOG_SUBDOMAINS_OFF
  *         - %LANDLOCK_RESTRICT_SELF_TSYNC
+ *         - %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS
  *
  * This system call enforces a Landlock ruleset on the current thread.
  * Enforcing a ruleset requires that the task has %CAP_SYS_ADMIN in its
  * namespace or is running with no_new_privs.  This avoids scenarios where
  * unprivileged tasks can affect the behavior of privileged children.
  *
+ * With %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS, the no_new_privs attribute of the
+ * calling thread is set atomically with the enforcement of the ruleset, which
+ * fulfills the above requirement: no_new_privs is set if and only if the call
+ * succeeds.
+ *
  * Return: 0 on success, or -errno on failure.  Possible returned errors are:
  *
  * - %EOPNOTSUPP: Landlock is supported by the kernel but disabled at boot time;
  * - %EINVAL: @flags contains an unknown bit.
  * - %EBADF: @ruleset_fd is not a file descriptor for the current thread;
  * - %EBADFD: @ruleset_fd is not a ruleset file descriptor;
- * - %EPERM: @ruleset_fd has no read access to the underlying ruleset, or the
- *   current thread is not running with no_new_privs, or it doesn't have
- *   %CAP_SYS_ADMIN in its namespace.
+ * - %EPERM: @ruleset_fd has no read access to the underlying ruleset, or
+ *   %LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS is not set while the current thread
+ *   is not running with no_new_privs and doesn't have %CAP_SYS_ADMIN in its
+ *   namespace.
  * - %E2BIG: The maximum number of stacked rulesets is reached for the current
  *   thread.
  *
@@ -529,6 +536,8 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 	struct landlock_ruleset *ruleset __free(landlock_put_ruleset) = NULL;
 	struct cred *new_cred;
 	struct landlock_cred_security *new_llcred;
+	const bool set_no_new_privs =
+		!!(flags & LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS);
 	bool __maybe_unused log_same_exec, log_new_exec, log_subdomains,
 		prev_log_subdomains;
 
@@ -537,9 +546,10 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 
 	/*
 	 * Similar checks as for seccomp(2), except that an -EPERM may be
-	 * returned.
+	 * returned.  LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS fulfills this
+	 * requirement.
 	 */
-	if (!task_no_new_privs(current) &&
+	if (!set_no_new_privs && !task_no_new_privs(current) &&
 	    !ns_capable_noaudit(current_user_ns(), CAP_SYS_ADMIN))
 		return -EPERM;
 
@@ -620,12 +630,16 @@ SYSCALL_DEFINE2(landlock_restrict_self, const int, ruleset_fd, const __u32,
 
 	if (flags & LANDLOCK_RESTRICT_SELF_TSYNC) {
 		const int err = landlock_restrict_sibling_threads(
-			current_cred(), new_cred);
+			current_cred(), new_cred, flags);
 		if (err) {
 			abort_creds(new_cred);
 			return err;
 		}
 	}
 
+	/* Sets no_new_privs past the last point of failure. */
+	if (set_no_new_privs)
+		task_set_no_new_privs(current);
+
 	return commit_creds(new_cred);
 }
diff --git a/security/landlock/tsync.c b/security/landlock/tsync.c
index c5730bbd9ed3..0b71e158c3f5 100644
--- a/security/landlock/tsync.c
+++ b/security/landlock/tsync.c
@@ -17,6 +17,7 @@
 #include <linux/sched/task.h>
 #include <linux/slab.h>
 #include <linux/task_work.h>
+#include <uapi/linux/landlock.h>
 
 #include "cred.h"
 #include "tsync.h"
@@ -466,7 +467,8 @@ static void cancel_tsync_works(const struct tsync_works *works,
  * restrict_sibling_threads - enables a Landlock policy for all sibling threads
  */
 int landlock_restrict_sibling_threads(const struct cred *old_cred,
-				      const struct cred *new_cred)
+				      const struct cred *new_cred,
+				      const u32 restrict_flags)
 {
 	int err;
 	struct tsync_shared_context shared_ctx;
@@ -481,7 +483,9 @@ int landlock_restrict_sibling_threads(const struct cred *old_cred,
 	init_completion(&shared_ctx.all_finished);
 	shared_ctx.old_cred = old_cred;
 	shared_ctx.new_cred = new_cred;
-	shared_ctx.set_no_new_privs = task_no_new_privs(current);
+	shared_ctx.set_no_new_privs =
+		(restrict_flags & LANDLOCK_RESTRICT_SELF_NO_NEW_PRIVS) ||
+		task_no_new_privs(current);
 
 	/*
 	 * Serialize concurrent TSYNC operations to prevent deadlocks when
diff --git a/security/landlock/tsync.h b/security/landlock/tsync.h
index ef86bb61c2f6..2ae4f938ca00 100644
--- a/security/landlock/tsync.h
+++ b/security/landlock/tsync.h
@@ -9,8 +9,10 @@
 #define _SECURITY_LANDLOCK_TSYNC_H
 
 #include <linux/cred.h>
+#include <linux/types.h>
 
 int landlock_restrict_sibling_threads(const struct cred *old_cred,
-				      const struct cred *new_cred);
+				      const struct cred *new_cred,
+				      u32 restrict_flags);
 
 #endif /* _SECURITY_LANDLOCK_TSYNC_H */
-- 
2.54.0