passt, pasta: Namespace-based sandboxing, defer seccomp policy application

To reach (at least) a conceptually equivalent security level as
implemented by --enable-sandbox in slirp4netns, we need to create a
new mount namespace and pivot_root() into a new (empty) mountpoint, so
that passt and pasta can't access any filesystem resource after
initialisation.

While at it, also detach IPC, PID (only for passt, to prevent
vulnerabilities based on the knowledge of a target PID), and UTS
namespaces.

With this approach, if we apply the seccomp filters right after the
configuration step, the number of allowed syscalls grows further. To
prevent this, defer the application of seccomp policies after the
initialisation phase, before the main loop, that's where we expect bad
things to happen, potentially. This way, we get back to 22 allowed
syscalls for passt and 34 for pasta, on x86_64.

While at it, move #syscalls notes to specific code paths wherever it
conceptually makes sense.

We have to open all the file handles we'll ever need before
sandboxing:

- the packet capture file can only be opened once, drop instance
  numbers from the default path and use the (pre-sandbox) PID instead

- /proc/net/tcp{,v6} and /proc/net/udp{,v6}, for automatic detection
  of bound ports in pasta mode, are now opened only once, before
  sandboxing, and their handles are stored in the execution context

- the UNIX domain socket for passt is also bound only once, before
  sandboxing: to reject clients after the first one, instead of
  closing the listening socket, keep it open, accept and immediately
  discard new connection if we already have a valid one

Clarify the (unchanged) behaviour for --netns-only in the man page.

To actually make passt and pasta processes run in a separate PID
namespace, we need to unshare(CLONE_NEWPID) before forking to
background (if configured to do so). Introduce a small daemon()
implementation, __daemon(), that additionally saves the PID file
before forking. While running in foreground, the process itself can't
move to a new PID namespace (a process can't change the notion of its
own PID): mention that in the man page.

For some reason, fork() in a detached PID namespace causes SIGTERM
and SIGQUIT to be ignored, even if the handler is still reported as
SIG_DFL: add a signal handler that just exits.

We can now drop most of the pasta_child_handler() implementation,
that took care of terminating all processes running in the same
namespace, if pasta started a shell: the shell itself is now the
init process in that namespace, and all children will terminate
once the init process exits.

Issuing 'echo $$' in a detached PID namespace won't return the
actual namespace PID as seen from the init namespace: adapt
demo and test setup scripts to reflect that.

Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

2 files changed, 5 insertions, 3 deletions

diff --git a/test/demo/passt b/test/demo/passt
index b5762aa..76aac86 100644
--- a/test/demo/passt
+++ b/test/demo/passt
@@ -84,7 +84,8 @@ say	Now let's run 'passt' in the new namespace, and
 nl
 say	  enter this namespace from the guest terminal too.
 sleep	3
-pout	TARGET_PID echo $$
+guest	pstree -p | grep pasta
+gout	TARGET_PID pstree -p | grep pasta | sed -n 's/.*(\([0-9].*\))$/\1/p'
 sleep	1
 
 passtb	./passt -f -t 5201,5203
diff --git a/test/demo/pasta b/test/demo/pasta
index f8f0cd0..c136965 100644
--- a/test/demo/pasta
+++ b/test/demo/pasta
@@ -58,7 +58,8 @@ say	For convenience, let's enter this namespace
 nl
 say	  from another terminal.
 sleep	3
-pout	TARGET_PID echo $$
+ns	pstree -p | grep pasta
+nsout	TARGET_PID pstree -p | grep pasta | sed -n 's/.*(\([0-9].*\))$/\1/p'
 sleep	1
 
 ns	nsenter -t __TARGET_PID__ -U -n --preserve-credentials
@@ -172,7 +173,7 @@ sleep	2
 passtb	perf record -g ./pasta
 sleep	2
 
-pout	TARGET_PID echo $$
+nsout	TARGET_PID pstree -p | grep pasta | sed -n 's/.*(\([0-9].*\))$/\1/p'
 sleep	1
 ns	nsenter -t __TARGET_PID__ -U -n --preserve-credentials
 sleep	5