passt, util: Close any open file that the parent might have leaked

If a parent accidentally or due to implementation reasons leaks any
open file, we don't want to have access to them, except for the file
passed via --fd, if any.

This is the case for Podman when Podman's parent leaks files into
Podman: it's not practical for Podman to close unrelated files before
starting pasta, as reported by Paul.

Use close_range(2) to close all open files except for standard streams
and the one from --fd.

Given that parts of conf() depend on other files to be already opened,
such as the epoll file descriptor, we can't easily defer this to a
more convenient point, where --fd was already parsed. Introduce a
minimal, duplicate version of --fd parsing to keep this simple.

As we need to check that the passed --fd option doesn't exceed
INT_MAX, because we'll parse it with strtol() but file descriptor
indices are signed ints (regardless of the arguments close_range()
take), extend the existing check in the actual --fd parsing in conf(),
also rejecting file descriptors numbers that match standard streams,
while at it.

Suggested-by: Paul Holzinger <pholzing@redhat.com>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Paul Holzinger <pholzing@redhat.com>

1 files changed, 9 insertions, 3 deletions

diff --git a/isolation.c b/isolation.c
index 4956d7e..45fba1e 100644
--- a/isolation.c
+++ b/isolation.c
@@ -29,7 +29,8 @@
  *
  * Executed immediately after startup, drops capabilities we don't
  * need at any point during execution (or which we gain back when we
- * need by joining other namespaces).
+ * need by joining other namespaces), and closes any leaked file we
+ * might have inherited from the parent process.
  *
  * 2. isolate_user()
  * =================
@@ -166,14 +167,17 @@ static void clamp_caps(void)
 }
 
 /**
- * isolate_initial() - Early, config independent self isolation
+ * isolate_initial() - Early, mostly config independent self isolation
+ * @argc:	Argument count
+ * @argv:	Command line options: only --fd (if present) is relevant here
  *
  * Should:
  *  - drop unneeded capabilities
+ *  - close all open files except for standard streams and the one from --fd
  * Musn't:
  *  - remove filesytem access (we need to access files during setup)
  */
-void isolate_initial(void)
+void isolate_initial(int argc, char **argv)
 {
 	uint64_t keep;
 
@@ -207,6 +211,8 @@ void isolate_initial(void)
 		keep |= BIT(CAP_SETFCAP) | BIT(CAP_SYS_PTRACE);
 
 	drop_caps_ep_except(keep);
+
+	close_open_files(argc, argv);
 }
 
 /**