linux-kernel - [PATCH 5.4 67/71] seccomp: Invalidate seccomp mode to catch death failures

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Message-Id: <20220214092454.290611893@linuxfoundation.org>
Date:   Mon, 14 Feb 2022 10:26:35 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Andy Lutomirski <luto@...capital.net>,
        Will Drewry <wad@...omium.org>,
        Kees Cook <keescook@...omium.org>
Subject: [PATCH 5.4 67/71] seccomp: Invalidate seccomp mode to catch death failures

From: Kees Cook <keescook@...omium.org>

commit 495ac3069a6235bfdf516812a2a9b256671bbdf9 upstream.

If seccomp tries to kill a process, it should never see that process
again. To enforce this proactively, switch the mode to something
impossible. If encountered: WARN, reject all syscalls, and attempt to
kill the process again even harder.

Cc: Andy Lutomirski <luto@...capital.net>
Cc: Will Drewry <wad@...omium.org>
Fixes: 8112c4f140fa ("seccomp: remove 2-phase API")
Cc: stable@...r.kernel.org
Signed-off-by: Kees Cook <keescook@...omium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/seccomp.c |   10 ++++++++++
 1 file changed, 10 insertions(+)

--- a/kernel/seccomp.c
+++ b/kernel/seccomp.c
@@ -28,6 +28,9 @@
 #include <linux/syscalls.h>
 #include <linux/sysctl.h>
 
+/* Not exposed in headers: strictly internal use only. */
+#define SECCOMP_MODE_DEAD	(SECCOMP_MODE_FILTER + 1)
+
 #ifdef CONFIG_HAVE_ARCH_SECCOMP_FILTER
 #include <asm/syscall.h>
 #endif
@@ -706,6 +709,7 @@ static void __secure_computing_strict(in
 #ifdef SECCOMP_DEBUG
 	dump_stack();
 #endif
+	current->seccomp.mode = SECCOMP_MODE_DEAD;
 	seccomp_log(this_syscall, SIGKILL, SECCOMP_RET_KILL_THREAD, true);
 	do_exit(SIGKILL);
 }
@@ -892,6 +896,7 @@ static int __seccomp_filter(int this_sys
 	case SECCOMP_RET_KILL_THREAD:
 	case SECCOMP_RET_KILL_PROCESS:
 	default:
+		current->seccomp.mode = SECCOMP_MODE_DEAD;
 		seccomp_log(this_syscall, SIGSYS, action, true);
 		/* Dump core only if this is the last remaining thread. */
 		if (action == SECCOMP_RET_KILL_PROCESS ||
@@ -944,6 +949,11 @@ int __secure_computing(const struct secc
 		return 0;
 	case SECCOMP_MODE_FILTER:
 		return __seccomp_filter(this_syscall, sd, false);
+	/* Surviving SECCOMP_RET_KILL_* must be proactively impossible. */
+	case SECCOMP_MODE_DEAD:
+		WARN_ON_ONCE(1);
+		do_exit(SIGKILL);
+		return -1;
 	default:
 		BUG();
 	}