Re: [PATCH v4] efivarfs: Add uid/gid mount options

[Ard Biesheuvel]

(cc Christian)

Hi,

On Thu, 7 Sept 2023 at 18:43, Jiao Zhou <jiaozhou@xxxxxxxxxx> wrote:
>
> Allow UEFI variables to be modified by non-root processes
> in order to run sandboxed code. This doesn't change the behavior
> of mounting efivarfs unless uid/gid are specified;
> by default both are set to root.
>
> Signed-off-by: Jiao Zhou <jiaozhou@xxxxxxxxxx>
> Acked-by: Matthew Garrett <mgarrett@xxxxxxxxxxx>
> ---
> Changelog since v1:
> - Add missing sentinel entry in fs_parameter_spec[] array.
> - Fix a NULL pointer dereference.
>
> Changelog since v2:
> - Format the patch description.
>
> Changelog since v3:
> - Use sizeof(*sfi) to allocate memory to avoids future problems if sfi ever changes type.
> - Add gid and uid check to make sure that ids are valid.
> - Drop the indentation for one block.
>
>  fs/efivarfs/inode.c    |  4 +++
>  fs/efivarfs/internal.h |  9 +++++
>  fs/efivarfs/super.c    | 75 ++++++++++++++++++++++++++++++++++++++++--
>  3 files changed, 85 insertions(+), 3 deletions(-)
>

The commit log looks fine now - thanks for taking the time to dive
into the docs.


> diff --git a/fs/efivarfs/inode.c b/fs/efivarfs/inode.c
> index db9231f0e77b..06dfc73fda04 100644
> --- a/fs/efivarfs/inode.c
> +++ b/fs/efivarfs/inode.c
> @@ -20,9 +20,13 @@ struct inode *efivarfs_get_inode(struct super_block *sb,
>                                 const struct inode *dir, int mode,
>                                 dev_t dev, bool is_removable)
>  {
> +       struct efivarfs_fs_info *fsi = sb->s_fs_info;
>         struct inode *inode = new_inode(sb);
> +       struct efivarfs_mount_opts *opts = &fsi->mount_opts;
>
>         if (inode) {
> +               inode->i_uid = opts->uid;
> +               inode->i_gid = opts->gid;
>                 inode->i_ino = get_next_ino();
>                 inode->i_mode = mode;
>                 inode->i_atime = inode->i_mtime = inode_set_ctime_current(inode);
> diff --git a/fs/efivarfs/internal.h b/fs/efivarfs/internal.h
> index 8ebf3a6a8aa2..c66647f5c0bd 100644
> --- a/fs/efivarfs/internal.h
> +++ b/fs/efivarfs/internal.h
> @@ -9,6 +9,15 @@
>  #include <linux/list.h>
>  #include <linux/efi.h>
>
> +struct efivarfs_mount_opts {
> +       kuid_t uid;
> +       kgid_t gid;
> +};
> +
> +struct efivarfs_fs_info {
> +       struct efivarfs_mount_opts mount_opts;
> +};
> +
>  struct efi_variable {
>         efi_char16_t  VariableName[EFI_VAR_NAME_LEN/sizeof(efi_char16_t)];
>         efi_guid_t    VendorGuid;
> diff --git a/fs/efivarfs/super.c b/fs/efivarfs/super.c
> index e028fafa04f3..c53cebf45ac5 100644
> --- a/fs/efivarfs/super.c
> +++ b/fs/efivarfs/super.c
> @@ -8,6 +8,7 @@
>  #include <linux/efi.h>
>  #include <linux/fs.h>
>  #include <linux/fs_context.h>
> +#include <linux/fs_parser.h>
>  #include <linux/module.h>
>  #include <linux/pagemap.h>
>  #include <linux/ucs2_string.h>
> @@ -24,6 +25,22 @@ static void efivarfs_evict_inode(struct inode *inode)
>         clear_inode(inode);
>  }
>
> +static int efivarfs_show_options(struct seq_file *m, struct dentry *root)
> +{
> +       struct super_block *sb = root->d_sb;
> +       struct efivarfs_fs_info *sbi = sb->s_fs_info;
> +       struct efivarfs_mount_opts *opts = &sbi->mount_opts;
> +
> +       /* Show partition info */
> +       if (!uid_eq(opts->uid, GLOBAL_ROOT_UID))
> +               seq_printf(m, ",uid=%u",
> +                               from_kuid_munged(&init_user_ns, opts->uid));
> +       if (!gid_eq(opts->gid, GLOBAL_ROOT_GID))
> +               seq_printf(m, ",gid=%u",
> +                               from_kgid_munged(&init_user_ns, opts->gid));
> +       return 0;
> +}
> +
>  static int efivarfs_statfs(struct dentry *dentry, struct kstatfs *buf)
>  {
>         const u32 attr = EFI_VARIABLE_NON_VOLATILE |
> @@ -64,6 +81,7 @@ static const struct super_operations efivarfs_ops = {
>         .statfs = efivarfs_statfs,
>         .drop_inode = generic_delete_inode,
>         .evict_inode = efivarfs_evict_inode,
> +       .show_options = efivarfs_show_options,
>  };
>
>  /*
> @@ -225,6 +243,45 @@ static int efivarfs_destroy(struct efivar_entry *entry, void *data)
>         return 0;
>  }
>
> +enum {
> +       Opt_uid, Opt_gid,
> +};
> +
> +static const struct fs_parameter_spec efivarfs_parameters[] = {
> +       fsparam_u32("uid",                      Opt_uid),
> +       fsparam_u32("gid",                      Opt_gid),
> +       {},
> +};
> +
> +static int efivarfs_parse_param(struct fs_context *fc, struct fs_parameter *param)
> +{
> +       struct efivarfs_fs_info *sbi = fc->s_fs_info;
> +       struct efivarfs_mount_opts *opts = &sbi->mount_opts;
> +       struct fs_parse_result result;
> +       int opt;
> +
> +       opt = fs_parse(fc, efivarfs_parameters, param, &result);
> +       if (opt < 0)
> +               return opt;
> +
> +       switch (opt) {
> +       case Opt_uid:
> +               opts->uid = make_kuid(current_user_ns(), result.uint_32);
> +               if (!uid_valid(opts->uid))
> +                       return -EINVAL;
> +               break;
> +       case Opt_gid:
> +               opts->gid = make_kgid(current_user_ns(), result.uint_32);
> +               if (!gid_valid(opts->gid))
> +                       return -EINVAL;
> +               break;
> +       default:
> +               return -EINVAL;
> +       }
> +
> +       return 0;
> +}
> +
>  static int efivarfs_fill_super(struct super_block *sb, struct fs_context *fc)
>  {
>         struct inode *inode = NULL;
> @@ -270,11 +327,22 @@ static int efivarfs_get_tree(struct fs_context *fc)
>  }
>
>  static const struct fs_context_operations efivarfs_context_ops = {
> -       .get_tree       = efivarfs_get_tree,
> +       .get_tree = efivarfs_get_tree,
> +       .parse_param = efivarfs_parse_param,
>  };
>
>  static int efivarfs_init_fs_context(struct fs_context *fc)
>  {
> +       struct efivarfs_fs_info *sfi;
> +
> +       sfi = kzalloc(sizeof(struct efivarfs_fs_info), GFP_KERNEL);
> +       if (!sizeof(*sfi))

Something went wrong here


> +               return -ENOMEM;
> +
> +       sfi->mount_opts.uid = GLOBAL_ROOT_UID;
> +       sfi->mount_opts.gid = GLOBAL_ROOT_GID;
> +
> +       fc->s_fs_info = sfi;
>         fc->ops = &efivarfs_context_ops;
>         return 0;
>  }
> @@ -291,10 +359,11 @@ static void efivarfs_kill_sb(struct super_block *sb)
>  }
>
>  static struct file_system_type efivarfs_type = {
> -       .owner   = THIS_MODULE,
> -       .name    = "efivarfs",
> +       .owner = THIS_MODULE,
> +       .name = "efivarfs",
>         .init_fs_context = efivarfs_init_fs_context,
>         .kill_sb = efivarfs_kill_sb,
> +       .parameters     = efivarfs_parameters,
>  };
>
>  static __init int efivarfs_init(void)
> --
> 2.42.0.283.g2d96d420d3-goog
>