Re: [RFC PATCH] selinux: add a fallback to defcontext for native labeling

[Paul Moore]

On Wed, Sep 19, 2018 at 12:52 PM Taras Kondratiuk <takondra@xxxxxxxxx> wrote:
> When files on NFSv4 server are not properly labeled (label doesn't match
> a policy on a client) they will end up with unlabeled_t type which is
> too generic. We would like to be able to set a default context per
> mount. 'defcontext' mount option looks like a nice solution, but it
> doesn't seem to be fully implemented for native labeling. Default
> context is stored, but is never used.
>
> The patch adds a fallback to a default context if a received context is
> invalid. If the inode context is already initialized, then it is left
> untouched to preserve a context set locally on a client.
>
> Signed-off-by: Taras Kondratiuk <takondra@xxxxxxxxx>
> ---
>  security/selinux/hooks.c | 25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)

The idea seems reasonable to me; if we want to treat labeled NFS like
we treat local filesystems we should also support the defcontext mount
option.

However, I wonder if we are better off generalizing some of the
SECURITY_FS_USE_XATTR based code in inode_doinit_with_dentry() such
that it can be used both in selinux_inode_notifysecctx() and in
inode_doinit_with_dentry().  Or maybe we just need a sbsec->def_sid
variant of selinux_inode_setsecurity().  Regardless, the key is the
call to security_context_to_sid_default(), the
selinux_inode_setsecurity() function only calls
security_context_to_sid().

Just in case anyone is wondering, I'm not sure I want to update
selinux_inode_setsecurity() to call security_context_to_sid_default();
at least not without more investigation.

> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index ad9a9b8e9979..f7debe798bf5 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6598,7 +6598,30 @@ static void selinux_inode_invalidate_secctx(struct inode *inode)
>   */
>  static int selinux_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen)
>  {
> -       return selinux_inode_setsecurity(inode, XATTR_SELINUX_SUFFIX, ctx, ctxlen, 0);
> +       struct superblock_security_struct *sbsec;
> +       struct inode_security_struct *isec;
> +       int rc;
> +
> +       rc = selinux_inode_setsecurity(inode, XATTR_SELINUX_SUFFIX, ctx, ctxlen, 0);
> +
> +       /*
> +        * In case of Native labeling with defcontext mount option fall back
> +        * to a default SID if received context is invalid.
> +        */
> +       if (rc == -EINVAL) {
> +               sbsec = inode->i_sb->s_security;
> +               if (sbsec->behavior == SECURITY_FS_USE_NATIVE &&
> +                   sbsec->flags & DEFCONTEXT_MNT) {
> +                       isec = inode->i_security;
> +                       if (!isec->initialized) {
> +                               isec->sclass = inode_mode_to_security_class(inode->i_mode);
> +                               isec->sid = sbsec->def_sid;
> +                               isec->initialized = 1;
> +                       }
> +                       rc = 0;
> +               }
> +       }
> +       return rc;
>  }
>
>  /*
> --
> 2.10.3.dirty
>


-- 
paul moore
www.paul-moore.com