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From:   Olga Kornievskaia <aglo@umich.edu>
Date:   Wed, 19 May 2021 13:07:48 -0400
Message-ID: <CAN-5tyF8J2+kpVtHHmwc9rASmn=EJmei8RB47cQAgYC6P1=GSw@mail.gmail.com>
Subject: Re: [PATCH v2 1/2] vfs,LSM: introduce the FS_HANDLES_LSM_OPTS flag
To:     Ondrej Mosnacek <omosnace@redhat.com>
Cc:     Linux Security Module list 
        <linux-security-module@vger.kernel.org>,
        SElinux list <selinux@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        linux-nfs <linux-nfs@vger.kernel.org>,
        linux-btrfs@vger.kernel.org, Paul Moore <paul@paul-moore.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        David Howells <dhowells@redhat.com>,
        Stephen Smalley <stephen.smalley.work@gmail.com>,
        Richard Haines <richard_c_haines@btinternet.com>
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Precedence: bulk

On Mon, May 17, 2021 at 9:42 AM Ondrej Mosnacek <omosnace@redhat.com> wrote:
>
> Add a new FS_HANDLES_LSM_OPTS filesystem flag to singal to VFS that the
> filesystem does LSM option setting for the given mount on its own, so
> the security_sb_set_mnt_opts() call in vfs_get_tree() can be skipped.
>
> This allows the following simplifications:
> 1. Removal of explicit LSM option handling from BTRFS.
>
>    This exists only because of the double-layer mount that BTRFS is
>    doing for its subvolume support. Setting FS_HANDLES_LSM_OPTS on the
>    inner layer (btrfs_root_fs_type) and unsetting FS_BINARY_MOUNTDATA
>    from both layers allows us to leave the LSM option handling entirely
>    on VFS as part of the outer vfs_get_tree() call.
>
> 2. Removal of FS_BINARY_MOUNTDATA flags from BTRFS's fs_types.
>
>    After applying (1.), we can let VFS eat away LSM opts at the outer
>    mount layer and then do selinux_set_mnt_opts() with these opts, so
>    setting the flag is no longer needed neither for preserving the LSM
>    opts, nor for the SELinux double-set_mnt_opts exception.
>
> 3. Removal of the ugly FS_BINARY_MOUNTDATA special case from
>    selinux_set_mnt_opts().
>
>    Applying (1.) and also setting FS_HANDLES_LSM_OPTS on NFS fs_types
>    (which needs to unavoidably do the LSM options handling on its own
>    due to the SECURITY_LSM_NATIVE_LABELS flag usage) gets us to the
>    state where there is an exactly one security_sb_set_mnt_opts() or
>    security_sb_clone_mnt_opts() call for each superblock, so the rather
>    hacky FS_BINARY_MOUNTDATA special case can be finally removed from
>    security_sb_set_mnt_opts().
>
> The only other filesystem that sets FS_BINARY_MOUNTDATA is coda, which
> is also the only one that has binary mount data && doesn't do its own
> LSM options handling. So for coda we leave FS_HANDLES_LSM_OPTS unset and
> the behavior remains unchanged - with fsconfig(2) it (probably) won't
> even mount and with mount(2) it still won't support LSM options (and the
> security_sb_set_mnt_opts() will be always performed with empty LSM
> options as before).
>
> AFAICT, this shouldn't negatively affect the other LSMs. In fact, I
> think AppArmor will now gain the ability to do its DFA matching on BTRFS
> mount options, which was prevented before due to FS_BINARY_MOUNTDATA
> being set on both its fs_types.

Tested-by: Olga Kornievskaia <kolga@netapp.com> (both patches).

>
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>  fs/btrfs/super.c         | 34 +++++-----------------------------
>  fs/nfs/fs_context.c      |  6 ++++--
>  fs/super.c               | 10 ++++++----
>  include/linux/fs.h       |  3 ++-
>  security/selinux/hooks.c | 15 ---------------
>  5 files changed, 17 insertions(+), 51 deletions(-)
>
> diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
> index 4a396c1147f1..80716ead1cde 100644
> --- a/fs/btrfs/super.c
> +++ b/fs/btrfs/super.c
> @@ -1666,19 +1666,12 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
>         struct btrfs_device *device = NULL;
>         struct btrfs_fs_devices *fs_devices = NULL;
>         struct btrfs_fs_info *fs_info = NULL;
> -       void *new_sec_opts = NULL;
>         fmode_t mode = FMODE_READ;
>         int error = 0;
>
>         if (!(flags & SB_RDONLY))
>                 mode |= FMODE_WRITE;
>
> -       if (data) {
> -               error = security_sb_eat_lsm_opts(data, &new_sec_opts);
> -               if (error)
> -                       return ERR_PTR(error);
> -       }
> -
>         /*
>          * Setup a dummy root and fs_info for test/set super.  This is because
>          * we don't actually fill this stuff out until open_ctree, but we need
> @@ -1688,10 +1681,9 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
>          * superblock with our given fs_devices later on at sget() time.
>          */
>         fs_info = kvzalloc(sizeof(struct btrfs_fs_info), GFP_KERNEL);
> -       if (!fs_info) {
> -               error = -ENOMEM;
> -               goto error_sec_opts;
> -       }
> +       if (!fs_info)
> +               return ERR_PTR(-ENOMEM);
> +
>         btrfs_init_fs_info(fs_info);
>
>         fs_info->super_copy = kzalloc(BTRFS_SUPER_INFO_SIZE, GFP_KERNEL);
> @@ -1748,9 +1740,6 @@ static struct dentry *btrfs_mount_root(struct file_system_type *fs_type,
>                         set_bit(BTRFS_FS_CSUM_IMPL_FAST, &fs_info->flags);
>                 error = btrfs_fill_super(s, fs_devices, data);
>         }
> -       if (!error)
> -               error = security_sb_set_mnt_opts(s, new_sec_opts, 0, NULL);
> -       security_free_mnt_opts(&new_sec_opts);
>         if (error) {
>                 deactivate_locked_super(s);
>                 return ERR_PTR(error);
> @@ -1762,8 +1751,6 @@ error_close_devices:
>         btrfs_close_devices(fs_devices);
>  error_fs_info:
>         btrfs_free_fs_info(fs_info);
> -error_sec_opts:
> -       security_free_mnt_opts(&new_sec_opts);
>         return ERR_PTR(error);
>  }
>
> @@ -1925,17 +1912,6 @@ static int btrfs_remount(struct super_block *sb, int *flags, char *data)
>         sync_filesystem(sb);
>         set_bit(BTRFS_FS_STATE_REMOUNTING, &fs_info->fs_state);
>
> -       if (data) {
> -               void *new_sec_opts = NULL;
> -
> -               ret = security_sb_eat_lsm_opts(data, &new_sec_opts);
> -               if (!ret)
> -                       ret = security_sb_remount(sb, new_sec_opts);
> -               security_free_mnt_opts(&new_sec_opts);
> -               if (ret)
> -                       goto restore;
> -       }
> -
>         ret = btrfs_parse_options(fs_info, data, *flags);
>         if (ret)
>                 goto restore;
> @@ -2385,7 +2361,7 @@ static struct file_system_type btrfs_fs_type = {
>         .name           = "btrfs",
>         .mount          = btrfs_mount,
>         .kill_sb        = btrfs_kill_super,
> -       .fs_flags       = FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA,
> +       .fs_flags       = FS_REQUIRES_DEV,
>  };
>
>  static struct file_system_type btrfs_root_fs_type = {
> @@ -2393,7 +2369,7 @@ static struct file_system_type btrfs_root_fs_type = {
>         .name           = "btrfs",
>         .mount          = btrfs_mount_root,
>         .kill_sb        = btrfs_kill_super,
> -       .fs_flags       = FS_REQUIRES_DEV | FS_BINARY_MOUNTDATA,
> +       .fs_flags       = FS_REQUIRES_DEV | FS_HANDLES_LSM_OPTS,
>  };
>
>  MODULE_ALIAS_FS("btrfs");
> diff --git a/fs/nfs/fs_context.c b/fs/nfs/fs_context.c
> index d95c9a39bc70..b5db4160e89b 100644
> --- a/fs/nfs/fs_context.c
> +++ b/fs/nfs/fs_context.c
> @@ -1557,7 +1557,8 @@ struct file_system_type nfs_fs_type = {
>         .init_fs_context        = nfs_init_fs_context,
>         .parameters             = nfs_fs_parameters,
>         .kill_sb                = nfs_kill_super,
> -       .fs_flags               = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
> +       .fs_flags               = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
> +                                 FS_HANDLES_LSM_OPTS,
>  };
>  MODULE_ALIAS_FS("nfs");
>  EXPORT_SYMBOL_GPL(nfs_fs_type);
> @@ -1569,7 +1570,8 @@ struct file_system_type nfs4_fs_type = {
>         .init_fs_context        = nfs_init_fs_context,
>         .parameters             = nfs_fs_parameters,
>         .kill_sb                = nfs_kill_super,
> -       .fs_flags               = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA,
> +       .fs_flags               = FS_RENAME_DOES_D_MOVE|FS_BINARY_MOUNTDATA|
> +                                 FS_HANDLES_LSM_OPTS,
>  };
>  MODULE_ALIAS_FS("nfs4");
>  MODULE_ALIAS("nfs4");
> diff --git a/fs/super.c b/fs/super.c
> index 11b7e7213fd1..918c77b8c161 100644
> --- a/fs/super.c
> +++ b/fs/super.c
> @@ -1520,10 +1520,12 @@ int vfs_get_tree(struct fs_context *fc)
>         smp_wmb();
>         sb->s_flags |= SB_BORN;
>
> -       error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
> -       if (unlikely(error)) {
> -               fc_drop_locked(fc);
> -               return error;
> +       if (!(fc->fs_type->fs_flags & FS_HANDLES_LSM_OPTS)) {
> +               error = security_sb_set_mnt_opts(sb, fc->security, 0, NULL);
> +               if (unlikely(error)) {
> +                       fc_drop_locked(fc);
> +                       return error;
> +               }
>         }
>
>         /*
> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index c3c88fdb9b2a..36f9cd37bc83 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -2469,7 +2469,8 @@ struct file_system_type {
>  #define FS_HAS_SUBTYPE         4
>  #define FS_USERNS_MOUNT                8       /* Can be mounted by userns root */
>  #define FS_DISALLOW_NOTIFY_PERM        16      /* Disable fanotify permission events */
> -#define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
> +#define FS_ALLOW_IDMAP         32      /* FS has been updated to handle vfs idmappings. */
> +#define FS_HANDLES_LSM_OPTS    64      /* FS handles LSM opts on its own - skip it in VFS */
>  #define FS_THP_SUPPORT         8192    /* Remove once all fs converted */
>  #define FS_RENAME_DOES_D_MOVE  32768   /* FS will handle d_move() during rename() internally. */
>         int (*init_fs_context)(struct fs_context *);
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index eaea837d89d1..041529cbf214 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -684,21 +684,6 @@ static int selinux_set_mnt_opts(struct super_block *sb,
>                 goto out;
>         }
>
> -       /*
> -        * Binary mount data FS will come through this function twice.  Once
> -        * from an explicit call and once from the generic calls from the vfs.
> -        * Since the generic VFS calls will not contain any security mount data
> -        * we need to skip the double mount verification.
> -        *
> -        * This does open a hole in which we will not notice if the first
> -        * mount using this sb set explict options and a second mount using
> -        * this sb does not set any security options.  (The first options
> -        * will be used for both mounts)
> -        */
> -       if ((sbsec->flags & SE_SBINITIALIZED) && (sb->s_type->fs_flags & FS_BINARY_MOUNTDATA)
> -           && !opts)
> -               goto out;
> -
>         root_isec = backing_inode_security_novalidate(root);
>
>         /*
> --
> 2.31.1
>