linux-kernel - Re: [Non-DoD Source] Re: [PATCH v3 15/15] selinux: delay sid population for rootfs till init is complete

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Date:   Tue, 20 Mar 2018 12:30:55 -0400
From:   Stephen Smalley <sds@...ho.nsa.gov>
To:     Victor Kamensky <kamensky@...co.com>
Cc:     Taras Kondratiuk <takondra@...co.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>, Rob Landley <rob@...dley.net>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Jonathan Corbet <corbet@....net>,
        James McMechan <james.w.mcmechan@...il.com>,
        initramfs@...r.kernel.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        linux-security-module@...r.kernel.org, xe-linux-external@...co.com,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>
Subject: Re: [Non-DoD Source] Re: [PATCH v3 15/15] selinux: delay sid
 population for rootfs till init is complete

On 03/10/2018 10:08 PM, Victor Kamensky wrote:
> 
> 
> On Tue, 20 Feb 2018, Stephen Smalley wrote:
> 
>> On Fri, 2018-02-16 at 20:33 +0000, Taras Kondratiuk wrote:
>>> From: Victor Kamensky <kamensky@...co.com>
>>>
>>> With initramfs cpio format that supports extended attributes
>>> we need to skip sid population on sys_lsetxattr call from
>>> initramfs for rootfs if security server is not initialized yet.
>>>
>>> Otherwise callback in selinux_inode_post_setxattr will try to
>>> translate give security.selinux label into sid context and since
>>> security server is not available yet inode will receive default
>>> sid (typically kernel_t). Note that in the same time proper
>>> label will be stored in inode xattrs. Later, since inode sid
>>> would be already populated system will never look back at
>>> actual xattrs. But if we skip sid population for rootfs and
>>> we have policy that direct use of xattrs for rootfs, proper
>>> sid will be filled in from extended attributes one node is
>>> accessed and server is initialized.
>>>
>>> Note new DELAYAFTERINIT_MNT super block flag is introduced
>>> to only mark rootfs for such behavior. For other types of
>>> tmpfs original logic is still used.
>>
>> (cc selinux maintainers)
>>
>> Wondering if we shouldn't just do this always, for all filesystem
>> types.
> 
> Ok, I think it makes sense. The one that do not support xattrs
> will not reach selinux_inode_post_setxattr anyway. And try
> to cache sid while !ss_initialized is not good idea for any
> filesystem types.
> 
>> Also, I think this should likely also be done in
>> selinux_inode_setsecurity() for consistency.
> 
> I am not sure that I follow selinux_inode_setsecurity suggestion.
> selinux_inode_setsecurity is about permission check. And
> selinux_inode_post_setxattr deals with processing and setting
> side effects if xattr was "security.selinux", it does not
> matter what happens in selinux_inode_setsecurity if it
> returns access_ok, LSM will still call selinux_inode_post_setxattr
> and we would need to check and not produce any sid caching
> side effects if !ss_initialized.

selinux_inode_setsecurity is the vfs fallback for setting security
attributes when the filesystem/inode does not support setxattr itself,
and is also used by kernfs. 
So you need to update both selinux_inode_post_setxattr and selinux_inode_setsecurity
in the same way.

> 
> Sitll keeping logic in selinux_inode_post_setxattr, checked
> that the following with much simple code works too:
> 
>> From bfc54e4805f3059671417ff2cda1266bc68e18f9 Mon Sep 17 00:00:00 2001
> From: Victor Kamensky <kamensky@...co.com>
> Date: Fri, 9 Mar 2018 23:06:08 -0800
> Subject: [PATCH 2/2] selinux: delay sid population in setxattr till policy
>  loaded
> 
> With initramfs cpio format that supports extended attributes
> we need to skip sid population on sys_lsetxattr call from
> initramfs for rootfs if security server is not initialized yet.
> 
> Otherwise callback in selinux_inode_post_setxattr will try to
> translate give security.selinux label into sid context and since
> security server is not available yet inode will receive default
> sid (typically kernel_t). Note that in the same time proper
> label will be stored in inode xattrs. Later, since inode sid
> would be already populated system will never look back at
> actual xattrs. But if we skip sid population for rootfs and
> we have policy that direct use of xattrs for rootfs, proper
> sid will be filled in from extended attributes one node is
> accessed and server is initialized.
> 
> Signed-off-by: Victor Kamensky <kamensky@...co.com>
> ---
>  security/selinux/hooks.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index 31303ed..4c13759 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -3197,6 +3197,10 @@ static void selinux_inode_post_setxattr(struct dentry *dentry, const char *name,
>          return;
>      }
> 
> +        if (!ss_initialized) {
> +                return;
> +        }
> +
>      rc = security_context_to_sid_force(value, size, &newsid);
>      if (rc) {
>          printk(KERN_ERR "SELinux:  unable to map context to SID"