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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     David Howells <dhowells@redhat.com>
Cc:     Alan Jenkins <alan.christopher.jenkins@gmail.com>,
        viro@zeniv.linux.org.uk, torvalds@linux-foundation.org,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mszeredi@redhat.com
References: <5c6f3d62-4cec-2aea-4693-62928611c526@gmail.com>
        <153754740781.17872.7869536526927736855.stgit@warthog.procyon.org.uk>
        <153754743491.17872.12115848333103740766.stgit@warthog.procyon.org.uk>
        <de050902-f3d0-b2db-2627-983a36c87b3c@gmail.com>
        <862e36a2-2a6f-4e26-3228-8cab4b4cf230@gmail.com>
        <17405.1539272035@warthog.procyon.org.uk>
Date:   Thu, 11 Oct 2018 13:38:31 -0500
In-Reply-To: <17405.1539272035@warthog.procyon.org.uk> (David Howells's
        message of "Thu, 11 Oct 2018 16:33:55 +0100")
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Subject: Re: [PATCH 03/34] teach move_mount(2) to work with OPEN_TREE_CLONE [ver #12]
Sender: linux-kernel-owner@vger.kernel.org
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David Howells <dhowells@redhat.com> writes:

> Okay, this appears to fix the cycle-creation problem.
>
> It could probably be improved by comparing sequence numbers as Alan suggests,
> but I need to work out how to get at that.

It should just be a matter of replacing the test
"if (p->mnt.mnt_sb->s_type == &nsfs)" with "if mnt_ns_loop(p->mnt.mnt_root)"

That would allow reusing 100% of the existing logic, and remove the need
to export file_system_type nsfs;

As your test exists below it will reject a lot more than mount namespace
file descriptors.  It will reject file descriptors for every other
namespace as well.

Eric

> ---
> commit 069c3376f7849044117c866aeafbb1a525f84926
> Author: David Howells <dhowells@redhat.com>
> Date:   Thu Oct 4 23:18:59 2018 +0100
>
>     fixes
>
> diff --git a/fs/internal.h b/fs/internal.h
> index 17029b30e196..47a6c80c3c51 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -172,6 +172,7 @@ extern void mnt_pin_kill(struct mount *m);
>   * fs/nsfs.c
>   */
>  extern const struct dentry_operations ns_dentry_operations;
> +extern struct file_system_type nsfs;
>  
>  /*
>   * fs/ioctl.c
> diff --git a/fs/namespace.c b/fs/namespace.c
> index e969ded7d54b..25ecd8b3c76b 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -2388,6 +2388,27 @@ static inline int tree_contains_unbindable(struct mount *mnt)
>  	return 0;
>  }
>  
> +/*
> + * Object if there are any nsfs mounts in the specified subtree.  These can act
> + * as pins for mount namespaces that aren't checked by the mount-cycle checking
> + * code, thereby allowing cycles to be made.
> + */
> +static bool check_for_nsfs_mounts(struct mount *subtree)
> +{
> +	struct mount *p;
> +	bool ret = false;
> +
> +	lock_mount_hash();
> +	for (p = subtree; p; p = next_mnt(p, subtree))
> +		if (p->mnt.mnt_sb->s_type == &nsfs)
> +			goto out;
> +
> +	ret = true;
> +out:
> +	unlock_mount_hash();
> +	return ret;
> +}
> +
>  static int do_move_mount(struct path *old_path, struct path *new_path)
>  {
>  	struct path parent_path = {.mnt = NULL, .dentry = NULL};
> @@ -2442,6 +2463,8 @@ static int do_move_mount(struct path *old_path, struct path *new_path)
>  	if (IS_MNT_SHARED(p) && tree_contains_unbindable(old))
>  		goto out1;
>  	err = -ELOOP;
> +	if (!check_for_nsfs_mounts(old))
> +		goto out1;
>  	for (; mnt_has_parent(p); p = p->mnt_parent)
>  		if (p == old)
>  			goto out1;
> diff --git a/fs/nsfs.c b/fs/nsfs.c
> index f069eb6495b0..d3abcd5c2a23 100644
> --- a/fs/nsfs.c
> +++ b/fs/nsfs.c
> @@ -269,7 +269,7 @@ static struct dentry *nsfs_mount(struct file_system_type *fs_type,
>  	return mount_pseudo(fs_type, "nsfs:", &nsfs_ops,
>  			&ns_dentry_operations, NSFS_MAGIC);
>  }
> -static struct file_system_type nsfs = {
> +struct file_system_type nsfs = {
>  	.name = "nsfs",
>  	.mount = nsfs_mount,
>  	.kill_sb = kill_anon_super,