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

[Olga Kornievskaia <olga.kornievskaia@gmail.com>]

Thank you for comment Casey. Comments are in line.

On Fri, Feb 12, 2021 at 5:37 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>
> On 2/12/2021 1:19 PM, Olga Kornievskaia wrote:
> > From: Olga Kornievskaia <kolga@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@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.

You are right, it was a poor naming choice. Compatible is better. I
can change it.
>
>
> >  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.

The mounts are not failing. This allows a file system to decide
whether or not to share the superblocks or not. Ok a good point, I
haven't tested building a kernel where there is LSM but SElinux is not
compiled into the kernel (ie, the only user of that hook). I'm not
sure that's a possible or an interesting option.

Ok, I can flip the returns, I wasn't aware that SElinux is so
restrictive in its return function meaning. Returning 0 on a success
when you are looking for a match/capability seems backwards.

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

Again compatible is better, I can change it.

> >   * @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),
>