seccomp: allow turning off of seccomp filtering via env var

Fixes: #17504

(While we are it, also move $SYSTEMD_SECCOMP_LOG= env var description
into the right document section)

Also suggested in: https://github.com/systemd/systemd/issues/17245#issuecomment-704773603

(cherry picked from commit ce8f6d478e3f6c6a313fb19615aa5029bb18f86d)

Resolves: #1916835

3 files changed, 19 insertions, 5 deletions

diff --git a/doc/ENVIRONMENT.md b/doc/ENVIRONMENT.md
index 0e763b6302..36b649afe1 100644
--- a/doc/ENVIRONMENT.md
+++ b/doc/ENVIRONMENT.md
@@ -117,3 +117,6 @@ systemd-sulogin-shell:
 * `$SYSTEMD_SULOGIN_FORCE=1` — This skips asking for the root password if the
   root password is not available (such as when the root account is locked).
   See `sulogin(8)` for more details.
+
+* `$SYSTEMD_SECCOMP=0` – if set, seccomp filters will not be enforced, even if
+  support for it is compiled in and available in the kernel.
diff --git a/src/nspawn/nspawn-seccomp.c b/src/nspawn/nspawn-seccomp.c
index b56c5b04a8..fba22644da 100644
--- a/src/nspawn/nspawn-seccomp.c
+++ b/src/nspawn/nspawn-seccomp.c
@@ -172,7 +172,7 @@ int setup_seccomp(uint64_t cap_list_retain, char **syscall_whitelist, char **sys
         int r;
 
         if (!is_seccomp_available()) {
-                log_debug("SECCOMP features not detected in the kernel, disabling SECCOMP filterering");
+                log_debug("SECCOMP features not detected in the kernel or disabled at runtime, disabling SECCOMP filtering");
                 return 0;
         }
 
diff --git a/src/shared/seccomp-util.c b/src/shared/seccomp-util.c
index d91fb4e269..e903512d45 100644
--- a/src/shared/seccomp-util.c
+++ b/src/shared/seccomp-util.c
@@ -12,6 +12,7 @@
 
 #include "af-list.h"
 #include "alloc-util.h"
+#include "env-util.h"
 #include "macro.h"
 #include "nsflags.h"
 #include "process-util.h"
@@ -244,10 +245,20 @@ static bool is_seccomp_filter_available(void) {
 bool is_seccomp_available(void) {
         static int cached_enabled = -1;
 
-        if (cached_enabled < 0)
-                cached_enabled =
-                        is_basic_seccomp_available() &&
-                        is_seccomp_filter_available();
+        if (cached_enabled < 0) {
+                int b;
+
+                b = getenv_bool("SYSTEMD_SECCOMP");
+                if (b != 0) {
+                        if (b < 0 && b != -ENXIO) /* ENXIO: env var unset */
+                                log_debug_errno(b, "Failed to parse $SYSTEMD_SECCOMP value, ignoring.");
+
+                        cached_enabled =
+                                is_basic_seccomp_available() &&
+                                is_seccomp_filter_available();
+                } else
+                        cached_enabled = false;
+        }
 
         return cached_enabled;
 }