On Wed, May 19, 2021 at 1:08 PM Olga Kornievskaia [off-list ref] wrote:

On Mon, May 17, 2021 at 9:42 AM Ondrej Mosnacek [off-list ref] wrote:

quoted

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 <redacted> (both patches).

Is this stalled on getting an Ack from vfs maintainers?

quoted

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