[lxc-devel] [lxc/master] seccomp: handle arch inversion

[brauner on Github]

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From b5ed021bbc47efe77732b38b5946116be94367e1 Mon Sep 17 00:00:00 2001
From: Christian Brauner <christian.brauner at ubuntu.com>
Date: Fri, 13 Apr 2018 14:02:24 +0200
Subject: [PATCH] seccomp: handle arch inversion

This commit deals with different kernel and userspace layouts and nesting. Here
are three examples:
1. 64bit kernel and 64bit userspace running 32bit containers
2. 64bit kernel and 32bit userspace running 64bit containers
3. 64bit kernel and 64bit userspace running 32bit containers running 64bit containers
Two things to lookout for:
1. The compat arch that is detected might have already been present in the main
   context. So check that it actually hasn't been and only then add it.
2. The contexts don't need merging if the architectures are the same and also can't be.
With these changes I can run all crazy/weird combinations with proper seccomp
isolation.

Closes #654.

Link: https://bugs.chromium.org/p/chromium/issues/detail?id=832366
Reported-by: Chirantan Ekbote <chirantan at chromium.org>
Reported-by: Sonny Rao <sonnyrao at chromium.org>
Signed-off-by: Christian Brauner <christian.brauner at ubuntu.com>
---
 src/lxc/seccomp.c | 48 ++++++++++++++++++++++++++++++++----------------
 1 file changed, 32 insertions(+), 16 deletions(-)

diff --git a/src/lxc/seccomp.c b/src/lxc/seccomp.c
index 310a742fc..5da31a563 100644
--- a/src/lxc/seccomp.c
+++ b/src/lxc/seccomp.c
@@ -370,17 +370,21 @@ scmp_filter_ctx get_new_ctx(enum lxc_hostarch_t n_arch, uint32_t default_policy_
 		WARN("Failed to turn on seccomp nop-skip, continuing");
 	}
 #endif
-	ret = seccomp_arch_add(ctx, arch);
-	if (ret != 0) {
-		ERROR("Seccomp error %d (%s) adding arch: %d", ret,
-		      strerror(-ret), (int)n_arch);
-		seccomp_release(ctx);
-		return NULL;
-	}
-	if (seccomp_arch_remove(ctx, SCMP_ARCH_NATIVE) != 0) {
-		ERROR("Seccomp error removing native arch");
-		seccomp_release(ctx);
-		return NULL;
+
+	if (seccomp_arch_exist(ctx, arch) == -EEXIST) {
+		ret = seccomp_arch_add(ctx, arch);
+		if (ret != 0) {
+			ERROR("Seccomp error %d (%s) adding arch: %d", ret,
+					strerror(-ret), (int)n_arch);
+			seccomp_release(ctx);
+			return NULL;
+		}
+
+		if (seccomp_arch_remove(ctx, SCMP_ARCH_NATIVE) != 0) {
+			ERROR("Seccomp error removing native arch");
+			seccomp_release(ctx);
+			return NULL;
+		}
 	}
 
 	return ctx;
@@ -772,11 +776,23 @@ static int parse_config_v2(FILE *f, char *line, struct lxc_conf *conf)
 	}
 
 	if (compat_ctx[0]) {
-		INFO("Merging in the compat Seccomp ctx into the main one");
-		if (seccomp_merge(conf->seccomp_ctx, compat_ctx[0]) != 0 ||
-			(compat_ctx[1] != NULL && seccomp_merge(conf->seccomp_ctx, compat_ctx[1]) != 0)) {
-			ERROR("Error merging compat Seccomp contexts");
-			goto bad;
+		INFO("Merging compat seccomp contexts into main context");
+		if (compat_arch[0] != native_arch && compat_arch[0] != seccomp_arch_native()) {
+			ret = seccomp_merge(conf->seccomp_ctx, compat_ctx[0]);
+			if (ret < 0) {
+				ERROR("Failed to merge first compat seccomp context into main context");
+				goto bad;
+			}
+			TRACE("Merged first compat seccomp context into main context");
+		}
+
+		if (compat_arch[1] && compat_arch[1] != native_arch && compat_arch[1] != seccomp_arch_native()) {
+			ret = seccomp_merge(conf->seccomp_ctx, compat_ctx[1]);
+			if (ret < 0) {
+				ERROR("Failed to merge first compat seccomp context into main context");
+				goto bad;
+			}
+			TRACE("Merged second compat seccomp context into main context");
 		}
 	}