Re: [PATCH v8 0/1] ns: introduce binfmt_misc namespace

Christian Brauner
2019-12-16T09:46:23Z

On Mon, Dec 16, 2019 at 10:12:19AM +0100, Laurent Vivier wrote:
> v8: s/file->f_path.dentry/file_dentry(file)/
>
> v7: Use the new mount API
>
> Replace
>
> static struct dentry *bm_mount(struct file_system_type *fs_type,
> int flags, const char *dev_name, void *data)
> {
> struct user_namespace *ns = current_user_ns();
>
> return mount_ns(fs_type, flags, data, ns, ns,
> bm_fill_super);
> }
>
> by
>
> static void bm_free(struct fs_context *fc)
> {
> if (fc->s_fs_info)
> put_user_ns(fc->s_fs_info);
> }
>
> static int bm_get_tree(struct fs_context *fc)
> {
> return get_tree_keyed(fc, bm_fill_super, get_user_ns(fc->user_ns));
> }
>
> static const struct fs_context_operations bm_context_ops = {
> .free = bm_free,
> .get_tree = bm_get_tree,
> };
>
> static int bm_init_fs_context(struct fs_context *fc)
> {
> fc->ops = &bm_context_ops;
> return 0;
> }
>
> v6: Return &init_binfmt_ns instead of NULL in binfmt_ns()
> This should never happen, but to stay safe return a
> value we can use.
> change subject from "RFC" to "PATCH"
>
> v5: Use READ_ONCE()/WRITE_ONCE()
> move mount pointer struct init to bm_fill_super() and add smp_wmb()
> remove useless NULL value init
> add WARN_ON_ONCE()
>
> v4: first user namespace is initialized with &init_binfmt_ns,
> all new user namespaces are initialized with a NULL and use
> the one of the first parent that is not NULL. The pointer
> is initialized to a valid value the first time the binfmt_misc
> fs is mounted in the current user namespace.
> This allows to not change the way it was working before:
> new ns inherits values from its parent, and if parent value is modified
> (or parent creates its own binfmt entry by mounting the fs) child
> inherits it (unless it has itself mounted the fs).
>
> v3: create a structure to store binfmt_misc data,
> add a pointer to this structure in the user_namespace structure,
> in init_user_ns structure this pointer points to an init_binfmt_ns
> structure. And all new user namespaces point to this init structure.
> A new binfmt namespace structure is allocated if the binfmt_misc
> filesystem is mounted in a user namespace that is not the initial
> one but its binfmt namespace pointer points to the initial one.
> add override_creds()/revert_creds() around open_exec() in
> bm_register_write()
>
> v2: no new namespace, binfmt_misc data are now part of
> the mount namespace
> I put this in mount namespace instead of user namespace
> because the mount namespace is already needed and
> I don't want to force to have the user namespace for that.
> As this is a filesystem, it seems logic to have it here.
>
> This allows to define a new interpreter for each new container.
>
> But the main goal is to be able to chroot to a directory
> using a binfmt_misc interpreter without being root.
>
> I have a modified version of unshare at:
>
> https://github.com/vivier/util-linux.git branch unshare-chroot
>
> with some new options to unshare binfmt_misc namespace and to chroot
> to a directory.
>
> If you have a directory /chroot/powerpc/jessie containing debian for powerpc
> binaries and a qemu-ppc interpreter, you can do for instance:
>
> $ uname -a
> Linux fedora28-wor-2 4.19.0-rc5+ #18 SMP Mon Oct 1 00:32:34 CEST 2018 x86_64 x86_64 x86_64 GNU/Linux
> $ ./unshare --map-root-user --fork --pid \
> --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/qemu-ppc:OC" \
> --root=/chroot/powerpc/jessie /bin/bash -l
> # uname -a
> Linux fedora28-wor-2 4.19.0-rc5+ #18 SMP Mon Oct 1 00:32:34 CEST 2018 ppc GNU/Linux
> # id
> uid=0(root) gid=0(root) groups=0(root),65534(nogroup)
> # ls -l
> total 5940
> drwxr-xr-x. 2 nobody nogroup 4096 Aug 12 00:58 bin
> drwxr-xr-x. 2 nobody nogroup 4096 Jun 17 20:26 boot
> drwxr-xr-x. 4 nobody nogroup 4096 Aug 12 00:08 dev
> drwxr-xr-x. 42 nobody nogroup 4096 Sep 28 07:25 etc
> drwxr-xr-x. 3 nobody nogroup 4096 Sep 28 07:25 home
> drwxr-xr-x. 9 nobody nogroup 4096 Aug 12 00:58 lib
> drwxr-xr-x. 2 nobody nogroup 4096 Aug 12 00:08 media
> drwxr-xr-x. 2 nobody nogroup 4096 Aug 12 00:08 mnt
> drwxr-xr-x. 3 nobody nogroup 4096 Aug 12 13:09 opt
> dr-xr-xr-x. 143 nobody nogroup 0 Sep 30 23:02 proc
> -rwxr-xr-x. 1 nobody nogroup 6009712 Sep 28 07:22 qemu-ppc
> drwx------. 3 nobody nogroup 4096 Aug 12 12:54 root
> drwxr-xr-x. 3 nobody nogroup 4096 Aug 12 00:08 run
> drwxr-xr-x. 2 nobody nogroup 4096 Aug 12 00:58 sbin
> drwxr-xr-x. 2 nobody nogroup 4096 Aug 12 00:08 srv
> drwxr-xr-x. 2 nobody nogroup 4096 Apr 6 2015 sys
> drwxrwxrwt. 2 nobody nogroup 4096 Sep 28 10:31 tmp
> drwxr-xr-x. 10 nobody nogroup 4096 Aug 12 00:08 usr
> drwxr-xr-x. 11 nobody nogroup 4096 Aug 12 00:08 var
>
> If you want to use the qemu binary provided by your distro, you can use
>
> --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff:/bin/qemu-ppc-static:OCF"
>
> With the 'F' flag, qemu-ppc-static will be then loaded from the main root
> filesystem before switching to the chroot.
>
> Another example is to use the 'P' flag in one chroot and not in another one (useful in a test
> environment to test different configurations of the same interpreter):
>
> ./unshare --fork --pid --mount-proc --map-root-user --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff://usr/bin/qemu-ppc-noargv0:OCF" --root=/chroot/powerpc/jessie /bin/bash -l
> root@localhost:/# sh -c 'echo $0'
> /bin/sh
>
> ./unshare --fork --pid --mount-proc --map-root-user --load-interp ":qemu-ppc:M::\x7fELF\x01\x02\x01\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x02\x00\x14:\xff\xff\xff\xff\xff\xff\xff\x00\xff\xff\xff\xff\xff\xff\xff\xff\xff\xfe\xff\xff://usr/bin/qemu-ppc-argv0:OCFP" --root=/chroot/powerpc/jessie /bin/bash -l
> root@localhost:/# sh -c 'echo $0'
> sh

Hey Laurent,

We have quite some time before the v5.6 merge window opens. So I would
really like for this new feature to come with proper testing!

I know you've been waiting on this patch quite some time but adding
tests will not endanger v5.6 inclusion - lest we find significant bugs. ;)

Christian