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

3 files changed, 22 insertions, 9 deletions

diff --git a/docs/ENVIRONMENT.md b/docs/ENVIRONMENT.md
index 38752c9169..74a71bba93 100644
--- a/docs/ENVIRONMENT.md
+++ b/docs/ENVIRONMENT.md
@@ -83,6 +83,13 @@ All tools:
 * `$SYSTEMD_RDRAND=0` — if set, the RDRAND instruction will never be used,
   even if the CPU supports it.
 
+* `$SYSTEMD_SECCOMP=0` – if set, seccomp filters will not be enforced, even if
+  support for it is compiled in and available in the kernel.
+
+* `$SYSTEMD_LOG_SECCOMP=1` — if set, system calls blocked by seccomp filtering,
+  for example in systemd-nspawn, will be logged to the audit log, if the current
+  kernel version supports this.
+
 systemctl:
 
 * `$SYSTEMCTL_FORCE_BUS=1` — if set, do not connect to PID1's private D-Bus
@@ -93,10 +100,6 @@ systemctl:
 
 * `$SYSTEMCTL_SKIP_SYSV=1` — if set, do not call out to SysV compatibility hooks.
 
-* `$SYSTEMD_LOG_SECCOMP=1` — if set, system calls blocked by seccomp filtering,
-  for example in systemd-nspawn, will be logged to the audit log, if the current
-  kernel version supports this.
-
 systemd-nspawn:
 
 * `$SYSTEMD_NSPAWN_UNIFIED_HIERARCHY=1` — if set, force nspawn into unified
diff --git a/src/nspawn/nspawn-seccomp.c b/src/nspawn/nspawn-seccomp.c
index 1ab50553a9..76f2bfe77e 100644
--- a/src/nspawn/nspawn-seccomp.c
+++ b/src/nspawn/nspawn-seccomp.c
@@ -186,7 +186,7 @@ int setup_seccomp(uint64_t cap_list_retain, char **syscall_allow_list, char **sy
         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 8475924297..f6a8e4963d 100644
--- a/src/shared/seccomp-util.c
+++ b/src/shared/seccomp-util.c
@@ -259,10 +259,20 @@ 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_secure("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;
 }