proc: allow to mount many instances of proc in one pid namespace

This patch allows to have multiple procfs instances inside the
same pid namespace. The aim here is lightweight sandboxes, and to allow
that we have to modernize procfs internals.

1) The main aim of this work is to have on embedded systems one
supervisor for apps. Right now we have some lightweight sandbox support,
however if we create pid namespacess we have to manages all the
processes inside too, where our goal is to be able to run a bunch of
apps each one inside its own mount namespace without being able to
notice each other. We only want to use mount namespaces, and we want
procfs to behave more like a real mount point.

2) Linux Security Modules have multiple ptrace paths inside some
subsystems, however inside procfs, the implementation does not guarantee
that the ptrace() check which triggers the security_ptrace_check() hook
will always run. We have the 'hidepid' mount option that can be used to
force the ptrace_may_access() check inside has_pid_permissions() to run.
The problem is that 'hidepid' is per pid namespace and not attached to
the mount point, any remount or modification of 'hidepid' will propagate
to all other procfs mounts.

This also does not allow to support Yama LSM easily in desktop and user
sessions. Yama ptrace scope which restricts ptrace and some other
syscalls to be allowed only on inferiors, can be updated to have a
per-task context, where the context will be inherited during fork(),
clone() and preserved across execve(). If we support multiple private
procfs instances, then we may force the ptrace_may_access() on
/proc/<pids>/ to always run inside that new procfs instances. This will
allow to specifiy on user sessions if we should populate procfs with
pids that the user can ptrace or not.

By using Yama ptrace scope, some restricted users will only be able to see
inferiors inside /proc, they won't even be able to see their other
processes. Some software like Chromium, Firefox's crash handler, Wine
and others are already using Yama to restrict which processes can be
ptracable. With this change this will give the possibility to restrict
/proc/<pids>/ but more importantly this will give desktop users a
generic and usuable way to specifiy which users should see all processes
and which users can not.

Side notes:
* This covers the lack of seccomp where it is not able to parse
arguments, it is easy to install a seccomp filter on direct syscalls
that operate on pids, however /proc/<pid>/ is a Linux ABI using
filesystem syscalls. With this change LSMs should be able to analyze
open/read/write/close...

In the new patch set version I removed the 'newinstance' option
as suggested by Eric W. Biederman.

Selftest has been added to verify new behavior.

Signed-off-by: Alexey Gladkov <gladkov.alexey@gmail.com>
Reviewed-by: Alexey Dobriyan <adobriyan@gmail.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>

3 files changed, 50 insertions, 0 deletions

diff --git a/tools/testing/selftests/proc/.gitignore b/tools/testing/selftests/proc/.gitignore
index 4bca5a9327a4..126901c5d6f4 100644
--- a/tools/testing/selftests/proc/.gitignore
+++ b/tools/testing/selftests/proc/.gitignore
@@ -3,6 +3,7 @@
 /fd-002-posix-eq
 /fd-003-kthread
 /proc-loadavg-001
+/proc-multiple-procfs
 /proc-pid-vm
 /proc-self-map-files-001
 /proc-self-map-files-002
diff --git a/tools/testing/selftests/proc/Makefile b/tools/testing/selftests/proc/Makefile
index a8ed0f684829..bf22457256be 100644
--- a/tools/testing/selftests/proc/Makefile
+++ b/tools/testing/selftests/proc/Makefile
@@ -19,5 +19,6 @@ TEST_GEN_PROGS += self
 TEST_GEN_PROGS += setns-dcache
 TEST_GEN_PROGS += setns-sysvipc
 TEST_GEN_PROGS += thread-self
+TEST_GEN_PROGS += proc-multiple-procfs
 
 include ../lib.mk
diff --git a/tools/testing/selftests/proc/proc-multiple-procfs.c b/tools/testing/selftests/proc/proc-multiple-procfs.c
new file mode 100644
index 000000000000..ab912ad95dab
--- /dev/null
+++ b/tools/testing/selftests/proc/proc-multiple-procfs.c
@@ -0,0 +1,48 @@
+/*
+ * Copyright © 2020 Alexey Gladkov <gladkov.alexey@gmail.com>
+ *
+ * Permission to use, copy, modify, and distribute this software for any
+ * purpose with or without fee is hereby granted, provided that the above
+ * copyright notice and this permission notice appear in all copies.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
+ * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
+ * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
+ * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
+ * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
+ * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
+ * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
+ */
+#include <assert.h>
+#include <stdlib.h>
+#include <stdio.h>
+#include <sys/mount.h>
+#include <sys/types.h>
+#include <sys/stat.h>
+
+int main(void)
+{
+	struct stat proc_st1, proc_st2;
+	char procbuff[] = "/tmp/proc.XXXXXX/meminfo";
+	char procdir1[] = "/tmp/proc.XXXXXX";
+	char procdir2[] = "/tmp/proc.XXXXXX";
+
+	assert(mkdtemp(procdir1) != NULL);
+	assert(mkdtemp(procdir2) != NULL);
+
+	assert(!mount("proc", procdir1, "proc", 0, "hidepid=1"));
+	assert(!mount("proc", procdir2, "proc", 0, "hidepid=2"));
+
+	snprintf(procbuff, sizeof(procbuff), "%s/meminfo", procdir1);
+	assert(!stat(procbuff, &proc_st1));
+
+	snprintf(procbuff, sizeof(procbuff), "%s/meminfo", procdir2);
+	assert(!stat(procbuff, &proc_st2));
+
+	umount(procdir1);
+	umount(procdir2);
+
+	assert(proc_st1.st_dev != proc_st2.st_dev);
+
+	return 0;
+}