[PATCH 1/2] [security] Add new hook to compare new mount to an existing mount

[Casey Schaufler]

On 2/12/2021 1:19 PM, Olga Kornievskaia wrote:
> From: Olga Kornievskaia <kolga at netapp.com>
>
> Add a new hook that takes an existing super block and a new mount
> with new options and determines if new options confict with an
> existing mount or not.
>
> Signed-off-by: Olga Kornievskaia <kolga at netapp.com>
> ---
>  include/linux/lsm_hook_defs.h |  1 +
>  include/linux/lsm_hooks.h     |  6 ++++
>  include/linux/security.h      |  1 +
>  security/security.c           |  7 +++++
>  security/selinux/hooks.c      | 54 +++++++++++++++++++++++++++++++++++
>  5 files changed, 69 insertions(+)
>
> diff --git a/include/linux/lsm_hook_defs.h b/include/linux/lsm_hook_defs.h
> index 7aaa753b8608..fbfc07d0b3d5 100644
> --- a/include/linux/lsm_hook_defs.h
> +++ b/include/linux/lsm_hook_defs.h
> @@ -62,6 +62,7 @@ LSM_HOOK(int, 0, sb_alloc_security, struct super_block *sb)
>  LSM_HOOK(void, LSM_RET_VOID, sb_free_security, struct super_block *sb)
>  LSM_HOOK(void, LSM_RET_VOID, sb_free_mnt_opts, void *mnt_opts)
>  LSM_HOOK(int, 0, sb_eat_lsm_opts, char *orig, void **mnt_opts)
> +LSM_HOOK(int, 0, sb_do_mnt_opts_match, struct super_block *sb, void *mnt_opts)

Don't you want this to be sb_mnt_opts_compatible ?
They may not have to match. They just need to be acceptable
to the security module. SELinux doesn't appear to require
that they "match" in all respects.


>  LSM_HOOK(int, 0, sb_remount, struct super_block *sb, void *mnt_opts)
>  LSM_HOOK(int, 0, sb_kern_mount, struct super_block *sb)
>  LSM_HOOK(int, 0, sb_show_options, struct seq_file *m, struct super_block *sb)
> diff --git a/include/linux/lsm_hooks.h b/include/linux/lsm_hooks.h
> index a19adef1f088..a11b062c1847 100644
> --- a/include/linux/lsm_hooks.h
> +++ b/include/linux/lsm_hooks.h
> @@ -142,6 +142,12 @@
>   *	@orig the original mount data copied from userspace.
>   *	@copy copied data which will be passed to the security module.
>   *	Returns 0 if the copy was successful.
> + * @sb_do_mnt_opts_match:
> + *	Determine if the existing mount options are compatible with the new
> + *	mount options being used.
> + *	@sb superblock being compared
> + *	@mnt_opts new mount options
> + *	Return 1 if options are the same.

This is inconsistent with the convention for returns from LSM interfaces.
Return 0 on success and -ESOMETHINGOROTHER if the operation would be
denied. Your implementation of security_sb_do_mnt_opts_match() will
always return 0 if there's no module supplying the hook. I seriously
doubt that you want the mounts to fail 100% of the time if there isn't
an LSM.

Also, "options are the same" isn't the right description, even for
SELinux.

>   * @sb_remount:
>   *	Extracts security system specific mount options and verifies no changes
>   *	are being made to those options.
> diff --git a/include/linux/security.h b/include/linux/security.h
> index c35ea0ffccd9..07026db7304d 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -291,6 +291,7 @@ int security_sb_alloc(struct super_block *sb);
>  void security_sb_free(struct super_block *sb);
>  void security_free_mnt_opts(void **mnt_opts);
>  int security_sb_eat_lsm_opts(char *options, void **mnt_opts);
> +int security_sb_do_mnt_opts_match(struct super_block *sb, void *mnt_opts);
>  int security_sb_remount(struct super_block *sb, void *mnt_opts);
>  int security_sb_kern_mount(struct super_block *sb);
>  int security_sb_show_options(struct seq_file *m, struct super_block *sb);
> diff --git a/security/security.c b/security/security.c
> index 7b09cfbae94f..dae380916c6a 100644
> --- a/security/security.c
> +++ b/security/security.c
> @@ -890,6 +890,13 @@ int security_sb_eat_lsm_opts(char *options, void **mnt_opts)
>  }
>  EXPORT_SYMBOL(security_sb_eat_lsm_opts);
>  
> +int security_sb_do_mnt_opts_match(struct super_block *sb,
> +				 void *mnt_opts)
> +{
> +	return call_int_hook(sb_do_mnt_opts_match, 0, sb, mnt_opts);
> +}
> +EXPORT_SYMBOL(security_sb_do_mnt_opts_match);
> +
>  int security_sb_remount(struct super_block *sb,
>  			void *mnt_opts)
>  {
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 644b17ec9e63..aaa3a725da94 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2656,6 +2656,59 @@ static int selinux_sb_eat_lsm_opts(char *options, void **mnt_opts)
>  	return rc;
>  }
>  
> +static int selinux_sb_do_mnt_opts_match(struct super_block *sb, void *mnt_opts)
> +{
> +	struct selinux_mnt_opts *opts = mnt_opts;
> +	struct superblock_security_struct *sbsec = sb->s_security;
> +	u32 sid;
> +	int rc;
> +
> +	/* superblock not initialized (i.e. no options) - reject if any
> +	 * options specified, otherwise accept
> +	 */
> +	if (!(sbsec->flags & SE_SBINITIALIZED))
> +		return opts ? 0 : 1;
> +
> +	/* superblock initialized and no options specified - reject if
> +	 * superblock has any options set, otherwise accept
> +	 */
> +	if (!opts)
> +		return (sbsec->flags & SE_MNTMASK) ? 0 : 1;
> +
> +	if (opts->fscontext) {
> +		rc = parse_sid(sb, opts->fscontext, &sid);
> +		if (rc)
> +			return 0;
> +		if (bad_option(sbsec, FSCONTEXT_MNT, sbsec->sid, sid))
> +			return 0;
> +	}
> +	if (opts->context) {
> +		rc = parse_sid(sb, opts->context, &sid);
> +		if (rc)
> +			return 0;
> +		if (bad_option(sbsec, CONTEXT_MNT, sbsec->mntpoint_sid, sid))
> +			return 0;
> +	}
> +	if (opts->rootcontext) {
> +		struct inode_security_struct *root_isec;
> +
> +		root_isec = backing_inode_security(sb->s_root);
> +		rc = parse_sid(sb, opts->rootcontext, &sid);
> +		if (rc)
> +			return 0;
> +		if (bad_option(sbsec, ROOTCONTEXT_MNT, root_isec->sid, sid))
> +			return 0;
> +	}
> +	if (opts->defcontext) {
> +		rc = parse_sid(sb, opts->defcontext, &sid);
> +		if (rc)
> +			return 0;
> +		if (bad_option(sbsec, DEFCONTEXT_MNT, sbsec->def_sid, sid))
> +			return 0;
> +	}
> +	return 1;
> +}
> +
>  static int selinux_sb_remount(struct super_block *sb, void *mnt_opts)
>  {
>  	struct selinux_mnt_opts *opts = mnt_opts;
> @@ -6984,6 +7037,7 @@ static struct security_hook_list selinux_hooks[] __lsm_ro_after_init = {
>  
>  	LSM_HOOK_INIT(sb_free_security, selinux_sb_free_security),
>  	LSM_HOOK_INIT(sb_free_mnt_opts, selinux_free_mnt_opts),
> +	LSM_HOOK_INIT(sb_do_mnt_opts_match, selinux_sb_do_mnt_opts_match),
>  	LSM_HOOK_INIT(sb_remount, selinux_sb_remount),
>  	LSM_HOOK_INIT(sb_kern_mount, selinux_sb_kern_mount),
>  	LSM_HOOK_INIT(sb_show_options, selinux_sb_show_options),