nspawn: enable FUSE in containers

Linux kernel v4.18 (2018-08-12) added user-namespace support to FUSE, and
bumped the FUSE version to 7.27 (see: da315f6e0398 (Merge tag
'fuse-update-4.18' of
git://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse, Linus Torvalds,
2018-06-07).  This means that on such kernels it is safe to enable FUSE in
nspawn containers.

In outer_child(), before calling copy_devnodes(), check the FUSE version to
decide whether enable (>=7.27) or disable (<7.27) FUSE in the container.  We
look at the FUSE version instead of the kernel version in order to enable FUSE
support on older-versioned kernels that may have the mentioned patchset
backported ([as requested by @poettering][1]).  However, I am not sure that
this is safe; user-namespace support is not a documented part of the FUSE
protocol, which is what FUSE_KERNEL_VERSION/FUSE_KERNEL_MINOR_VERSION are meant
to capture.  While the same patchset
 - added FUSE_ABORT_ERROR (which is all that the 7.27 version bump
   is documented as including),
 - bumped FUSE_KERNEL_MINOR_VERSION from 26 to 27, and
 - added user-namespace support
these 3 things are not inseparable; it is conceivable to me that a backport
could include the first 2 of those things and exclude the 3rd; perhaps it would
be safer to check the kernel version.

Do note that our get_fuse_version() function uses the fsopen() family of
syscalls, which were not added until Linux kernel v5.2 (2019-07-07); so if
nothing has been backported, then the minimum kernel version for FUSE-in-nspawn
is actually v5.2, not v4.18.

Pass whether or not to enable FUSE to copy_devnodes(); have copy_devnodes()
copy in /dev/fuse if enabled.

Pass whether or not to enable FUSE back over fd_outer_socket to run_container()
so that it can pass that to append_machine_properties() (via either
register_machine() or allocate_scope()); have append_machine_properties()
append "DeviceAllow=/dev/fuse rw" if enabled.

For testing, simply check that /dev/fuse can be opened for reading and writing,
but that actually reading from it fails with EPERM.  The test assumes that if
FUSE is supported (/dev/fuse exists), then the testsuite is running on a kernel
with FUSE >= 7.27; I am unsure how to go about writing a test that validates
that the version check disables FUSE on old kernels.

[1]: https://github.com/systemd/systemd/issues/17607#issuecomment-745418835

Closes #17607

1 files changed, 9 insertions, 0 deletions

diff --git a/src/nspawn/nspawn-register.c b/src/nspawn/nspawn-register.c
index 855172c09c..52f7384468 100644
--- a/src/nspawn/nspawn-register.c
+++ b/src/nspawn/nspawn-register.c
@@ -15,6 +15,7 @@
 
 static int append_machine_properties(
                 sd_bus_message *m,
+                bool enable_fuse,
                 CustomMount *mounts,
                 unsigned n_mounts,
                 int kill_signal,
@@ -40,6 +41,12 @@ static int append_machine_properties(
                                   "char-pts", "rw");
         if (r < 0)
                 return bus_log_create_error(r);
+        if (enable_fuse) {
+                r = sd_bus_message_append(m, "(sv)", "DeviceAllow", "a(ss)", 1,
+                                          "/dev/fuse", "rw");
+                if (r < 0)
+                        return bus_log_create_error(r);
+        }
 
         for (j = 0; j < n_mounts; j++) {
                 CustomMount *cm = mounts + j;
@@ -200,6 +207,7 @@ int register_machine(
 
                 r = append_machine_properties(
                                 m,
+                                FLAGS_SET(flags, REGISTER_MACHINE_ENABLE_FUSE),
                                 mounts,
                                 n_mounts,
                                 kill_signal,
@@ -320,6 +328,7 @@ int allocate_scope(
 
         r = append_machine_properties(
                         m,
+                        FLAGS_SET(flags, ALLOCATE_SCOPE_ENABLE_FUSE),
                         mounts,
                         n_mounts,
                         kill_signal,