On Mon, May 17, 2021 at 9:42 AM Ondrej Mosnacek <omosnace@xxxxxxxxxx> 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@xxxxxxxxxx> (both patches).
>
> Signed-off-by: Ondrej Mosnacek <omosnace@xxxxxxxxxx>
> ---
> 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
>
