[1.1] nsenter: cloned_binary: remove bindfd logic entirely

(This is a cherry-pick of b999376.)

While the ro-bind-mount trick did eliminate the memory overhead of
copying the runc binary for each "runc init" invocation, on machines
with very significant container churn, creating a temporary mount
namespace on every container invocation can trigger severe lock
contention on namespace_sem that makes containers fail to spawn.

The only reason we added bindfd in commit 16612d7 ("nsenter:
cloned_binary: try to ro-bind /proc/self/exe before copying") was due to
a Kubernetes e2e test failure where they had a ridiculously small memory
limit. It seems incredibly unlikely that real workloads are running
without 10MB to spare for the very short time that runc is interacting
with the container.

In addition, since the original cloned_binary implementation, cgroupv2
is now almost universally used on modern systems. Unlike cgroupv1, the
cgroupv2 memcg implementation does not migrate memory usage when
processes change cgroups (even cgroupv1 only did this if you had
memory.move_charge_at_immigrate enabled). In addition, because we do the
/proc/self/exe clone before synchronising the bootstrap data read, we
are guaranteed to do the clone before "runc init" is moved into the
container cgroup -- meaning that the memory used by the /proc/self/exe
clone is charged against the root cgroup, and thus container workloads
should not be affected at all with memfd cloning.

The long-term fix for this problem is to block the /proc/self/exe
re-opening attack entirely in-kernel, which is something I'm working
on[1]. Though it should also be noted that because the memfd is
completely separate to the host binary, even attacks like Dirty COW
against the runc binary can be defended against with the memfd approach.
Of course, once we have in-kernel protection against the /proc/self/exe
re-opening attack, we won't have that protection anymore...

[1]: https://lwn.net/Articles/934460/

Signed-off-by: Aleksa Sarai <[email protected]>

libcontainer/nsenter/cloned_binary.c

@@ -396,61 +396,6 @@ static int seal_execfd(int *fd, int fdtype)
 	return -1;
 }
 
-static int try_bindfd(void)
-{
-	int fd, ret = -1;
-	char template[PATH_MAX] = { 0 };
-	char *prefix = getenv("_LIBCONTAINER_STATEDIR");
-
-	if (!prefix || *prefix != '/')
-		prefix = "/tmp";
-	if (snprintf(template, sizeof(template), "%s/runc.XXXXXX", prefix) < 0)
-		return ret;
-
-	/*
-	 * We need somewhere to mount it, mounting anything over /proc/self is a
-	 * BAD idea on the host -- even if we do it temporarily.
-	 */
-	fd = mkstemp(template);
-	if (fd < 0)
-		return ret;
-	close(fd);
-
-	/*
-	 * For obvious reasons this won't work in rootless mode because we haven't
-	 * created a userns+mntns -- but getting that to work will be a bit
-	 * complicated and it's only worth doing if someone actually needs it.
-	 */
-	ret = -EPERM;
-	if (mount("/proc/self/exe", template, "", MS_BIND, "") < 0)
-		goto out;
-	if (mount("", template, "", MS_REMOUNT | MS_BIND | MS_RDONLY, "") < 0)
-		goto out_umount;
-
-	/* Get read-only handle that we're sure can't be made read-write. */
-	ret = open(template, O_PATH | O_CLOEXEC);
-
-out_umount:
-	/*
-	 * Make sure the MNT_DETACH works, otherwise we could get remounted
-	 * read-write and that would be quite bad (the fd would be made read-write
-	 * too, invalidating the protection).
-	 */
-	if (umount2(template, MNT_DETACH) < 0) {
-		if (ret >= 0)
-			close(ret);
-		ret = -ENOTRECOVERABLE;
-	}
-
-out:
-	/*
-	 * We don't care about unlink errors, the worst that happens is that
-	 * there's an empty file left around in STATEDIR.
-	 */
-	unlink(template);
-	return ret;
-}
-
 static ssize_t fd_to_fd(int outfd, int infd)
 {
 	ssize_t total = 0;
@@ -485,18 +430,6 @@ static int clone_binary(void)
 	size_t sent = 0;
 	int fdtype = EFD_NONE;
 
-	/*
-	 * Before we resort to copying, let's try creating an ro-binfd in one shot
-	 * by getting a handle for a read-only bind-mount of the execfd.
-	 */
-	execfd = try_bindfd();
-	if (execfd >= 0)
-		return execfd;
-
-	/*
-	 * Dammit, that didn't work -- time to copy the binary to a safe place we
-	 * can seal the contents.
-	 */
 	execfd = make_execfd(&fdtype);
 	if (execfd < 0 || fdtype == EFD_NONE)
 		return -ENOTRECOVERABLE;