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

[Christian Brauner]

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