linux-kernel - Re: [PATCH] selinux: log anon inode class name

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Date:   Thu, 24 Feb 2022 19:25:29 -0500
From:   Paul Moore <paul@...l-moore.com>
To:     Christian Göttsche <cgzones@...glemail.com>
Cc:     selinux@...r.kernel.org, James Morris <jmorris@...ei.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        Richard Guy Briggs <rgb@...hat.com>,
        Ondrej Mosnacek <omosnace@...hat.com>,
        linux-kernel@...r.kernel.org, linux-security-module@...r.kernel.org
Subject: Re: [PATCH] selinux: log anon inode class name

On Thu, Feb 17, 2022 at 9:35 AM Christian Göttsche
<cgzones@...glemail.com> wrote:
>
> Log the anonymous inode class name in the security hook
> inode_init_security_anon.  This name is the key for name based type
> transitions on the anon_inode security class on creation.  Example:
>
>     type=AVC msg=audit(02/16/22 22:02:50.585:216) : avc:  granted  { create } for  pid=2136 comm=mariadbd anonclass="[io_uring]" dev="anon_inodefs" ino=6871 scontext=system_u:system_r:mysqld_t:s0 tcontext=system_u:system_r:mysqld_iouring_t:s0 tclass=anon_inode
>
> Add a new LSM audit data type holding the inode and the class name.
>
> Also warn if the security hook gets called with no name set; currently
> the only caller fs/anon_inodes.c:anon_inode_make_secure_inode() passes
> one.
>
> Signed-off-by: Christian Göttsche <cgzones@...glemail.com>
> ---
>  include/linux/lsm_audit.h |  5 +++++
>  security/lsm_audit.c      | 21 +++++++++++++++++++++
>  security/selinux/hooks.c  |  7 +++++--
>  3 files changed, 31 insertions(+), 2 deletions(-)

...

> diff --git a/include/linux/lsm_audit.h b/include/linux/lsm_audit.h
> index 17d02eda9538..8135a88d6d82 100644
> --- a/include/linux/lsm_audit.h
> +++ b/include/linux/lsm_audit.h
> @@ -76,6 +76,7 @@ struct common_audit_data {
>  #define LSM_AUDIT_DATA_IBENDPORT 14
>  #define LSM_AUDIT_DATA_LOCKDOWN 15
>  #define LSM_AUDIT_DATA_NOTIFICATION 16
> +#define LSM_AUDIT_DATA_ANONINODE       17
>         union   {
>                 struct path path;
>                 struct dentry *dentry;
> @@ -96,6 +97,10 @@ struct common_audit_data {
>                 struct lsm_ibpkey_audit *ibpkey;
>                 struct lsm_ibendport_audit *ibendport;
>                 int reason;
> +               struct {
> +                       struct inode *inode;
> +                       const char *anonclass;
> +               } anoninode_struct;
>         } u;
>         /* this union contains LSM specific data */
>         union {
> diff --git a/security/lsm_audit.c b/security/lsm_audit.c
> index 1897cbf6fc69..5545fed35539 100644
> --- a/security/lsm_audit.c
> +++ b/security/lsm_audit.c
> @@ -433,6 +433,27 @@ static void dump_common_audit_data(struct audit_buffer *ab,
>                 audit_log_format(ab, " lockdown_reason=\"%s\"",
>                                  lockdown_reasons[a->u.reason]);
>                 break;
> +       case LSM_AUDIT_DATA_ANONINODE: {
> +               struct dentry *dentry;
> +               struct inode *inode;
> +
> +               rcu_read_lock();
> +               inode = a->u.anoninode_struct.inode;
> +               dentry = d_find_alias_rcu(inode);
> +               if (dentry) {
> +                       audit_log_format(ab, " name=");
> +                       spin_lock(&dentry->d_lock);
> +                       audit_log_untrustedstring(ab, dentry->d_name.name);
> +                       spin_unlock(&dentry->d_lock);
> +               }

I'm not sure we are ever going to get a useful dentry name for
anonymous inodes, I think we can probably drop this.  The "anonclass="
field will likely be much more interesting and helpful.

> +               audit_log_format(ab, " anonclass=");
> +               audit_log_untrustedstring(ab, a->u.anoninode_struct.anonclass);
> +               audit_log_format(ab, " dev=");
> +               audit_log_untrustedstring(ab, inode->i_sb->s_id);

I'm pretty sure this is always going to end up being "anon_inodefs"
and thus not very useful.

> +               audit_log_format(ab, " ino=%lu", inode->i_ino);

Similarly, I'm not sure how useful the inode number is in practice.
I've never tried, but can a user lookup an anonymous inode via the
inode number?

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index dafabb4dcc64..19c831d94d9b 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -2932,6 +2932,8 @@ static int selinux_inode_init_security_anon(struct inode *inode,
>         if (unlikely(!selinux_initialized(&selinux_state)))
>                 return 0;
>
> +       WARN_ON(!name);
> +
>         isec = selinux_inode(inode);
>
>         /*
> @@ -2965,8 +2967,9 @@ static int selinux_inode_init_security_anon(struct inode *inode,
>          * allowed to actually create this type of anonymous inode.
>          */
>
> -       ad.type = LSM_AUDIT_DATA_INODE;
> -       ad.u.inode = inode;
> +       ad.type = LSM_AUDIT_DATA_ANONINODE;
> +       ad.u.anoninode_struct.inode = inode;
> +       ad.u.anoninode_struct.anonclass = name ? (const char *)name->name : "unknown(null)";

This doesn't seem to match well with the newly added WARN_ON()
assertion above.  I would suggest dropping the WARN_ON() assertion as
security_transition_sid() can already handle that safely, and leaving
the tertiary statement above; however I think we should probably
change the anonclass string to "?" as that is the common unset field
value used by audit.

-- 
paul-moore.com