[PATCH 3/8] seccomp: Introduce SECCOMP_PIN_ARCHITECTURE

[Kees Cook <keescook@chromium.org>]

For systems that provide multiple syscall maps based on architectures
(e.g. AUDIT_ARCH_X86_64 and AUDIT_ARCH_I386 via CONFIG_COMPAT), allow
a fast way to pin the process to a specific syscall mapping, instead of
needing to generate all filters with an architecture check as the first
filter action.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Will Drewry <wad@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
---
 include/linux/seccomp.h      |  3 +++
 include/uapi/linux/seccomp.h |  1 +
 kernel/seccomp.c             | 37 ++++++++++++++++++++++++++++++++++--
 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/include/linux/seccomp.h b/include/linux/seccomp.h
index babcd6c02d09..6525ddec177a 100644
--- a/include/linux/seccomp.h
+++ b/include/linux/seccomp.h
@@ -30,6 +30,9 @@ struct seccomp_filter;
  */
 struct seccomp {
 	int mode;
+#ifdef CONFIG_COMPAT
+	u32 arch;
+#endif
 	atomic_t filter_count;
 	struct seccomp_filter *filter;
 };
diff --git a/include/uapi/linux/seccomp.h b/include/uapi/linux/seccomp.h
index c1735455bc53..84e89bb201ae 100644
--- a/include/uapi/linux/seccomp.h
+++ b/include/uapi/linux/seccomp.h
@@ -16,6 +16,7 @@
 #define SECCOMP_SET_MODE_FILTER		1
 #define SECCOMP_GET_ACTION_AVAIL	2
 #define SECCOMP_GET_NOTIF_SIZES		3
+#define SECCOMP_PIN_ARCHITECTURE	4
 
 /* Valid flags for SECCOMP_SET_MODE_FILTER */
 #define SECCOMP_FILTER_FLAG_TSYNC		(1UL << 0)
diff --git a/kernel/seccomp.c b/kernel/seccomp.c
index a319700c04c4..43edf53c2d84 100644
--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -268,9 +268,16 @@ static u32 seccomp_run_filters(const struct seccomp_data *sd,
 			       struct seccomp_filter **match)
 {
 	u32 ret = SECCOMP_RET_ALLOW;
+	struct seccomp_filter *f;
+
+#ifdef CONFIG_COMPAT
+	/* Block mismatched architectures. */
+	if (current->seccomp.arch && current->seccomp.arch != sd->arch)
+		return SECCOMP_RET_KILL_PROCESS;
+#endif
+
 	/* Make sure cross-thread synced filter points somewhere sane. */
-	struct seccomp_filter *f =
-			READ_ONCE(current->seccomp.filter);
+	f = READ_ONCE(current->seccomp.filter);
 
 	/* Ensure unexpected behavior doesn't result in failing open. */
 	if (WARN_ON(f == NULL))
@@ -478,6 +485,11 @@ static inline void seccomp_sync_threads(unsigned long flags)
 		if (task_no_new_privs(caller))
 			task_set_no_new_privs(thread);
 
+#ifdef CONFIG_COMPAT
+		/* Copy any pinned architecture. */
+		thread->seccomp.arch = caller->seccomp.arch;
+#endif
+
 		/*
 		 * Opt the other thread into seccomp if needed.
 		 * As threads are considered to be trust-realm
@@ -1456,6 +1468,20 @@ static long seccomp_get_notif_sizes(void __user *usizes)
 	return 0;
 }
 
+static long seccomp_pin_architecture(void)
+{
+#ifdef CONFIG_COMPAT
+	u32 arch = syscall_get_arch(current);
+
+	/* How did you even get here? */
+	if (current->seccomp.arch && current->seccomp.arch != arch)
+		return -EBUSY;
+
+	current->seccomp.arch = arch;
+#endif
+	return 0;
+}
+
 /* Common entry point for both prctl and syscall. */
 static long do_seccomp(unsigned int op, unsigned int flags,
 		       void __user *uargs)
@@ -1477,6 +1503,13 @@ static long do_seccomp(unsigned int op, unsigned int flags,
 			return -EINVAL;
 
 		return seccomp_get_notif_sizes(uargs);
+	case SECCOMP_PIN_ARCHITECTURE:
+		if (flags != 0)
+			return -EINVAL;
+		if (uargs != NULL)
+			return -EINVAL;
+
+		return seccomp_pin_architecture();
 	default:
 		return -EINVAL;
 	}
-- 
2.25.1