Re: [RFC PATCH 3/3] selinux: do not override context on context mounts

From: Stephen Smalley <sds@tycho.nsa.gov>
To: Ondrej Mosnacek <omosnace@redhat.com>,
	selinux@vger.kernel.org, Paul Moore <paul@paul-moore.com>,
	cgroups@vger.kernel.org, Tejun Heo <tj@kernel.org>
Cc: Li Zefan <lizefan@huawei.com>, Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [RFC PATCH 3/3] selinux: do not override context on context mounts
Date: Thu, 13 Dec 2018 11:27:46 -0500	[thread overview]
Message-ID: <a779b0e5-79d5-8a10-8372-488de5f8345f@tycho.nsa.gov> (raw)
In-Reply-To: <20181213141739.8534-4-omosnace@redhat.com>

On 12/13/18 9:17 AM, Ondrej Mosnacek wrote:
> Ignore all selinux_inode_notifysecctx() calls on mounts with the
> SECURITY_FS_USE_MNTPOINT behavior.
> 
> This fixes behavior of kernfs-based filesystems when mounted with the
> 'context=' option. Before this patch, if a node's context had been
> explicitly set to a non-default value and later the filesystem has been
> remounted with the 'context=' option, then this node would show up as
> having a different context.
> 
> Steps to reproduce:
>      # mount -t cgroup2 cgroup2 /sys/fs/cgroup/unified
>      # chcon unconfined_u:object_r:user_home_t:s0 /sys/fs/cgroup/unified/cgroup.stat
>      # ls -lZ /sys/fs/cgroup/unified
>      total 0
>      -r--r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.controllers
>      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.max.depth
>      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.max.descendants
>      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.procs
>      -r--r--r--. 1 root root unconfined_u:object_r:user_home_t:s0 0 Dec 13 10:41 cgroup.stat
>      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.subtree_control
>      -rw-r--r--. 1 root root system_u:object_r:cgroup_t:s0        0 Dec 13 10:41 cgroup.threads
>      # umount /sys/fs/cgroup/unified
>      # mount -o context=system_u:object_r:tmpfs_t:s0 -t cgroup2 cgroup2 /sys/fs/cgroup/unified
> 
> Result before:
>      # ls -lZ /sys/fs/cgroup/unified
>      total 0
>      -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.controllers
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.max.depth
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.max.descendants
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.procs
>      -r--r--r--. 1 root root unconfined_u:object_r:user_home_t:s0 0 Dec 13 10:41 cgroup.stat
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.subtree_control
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0         0 Dec 13 10:41 cgroup.threads
> 
> Result after:
>      # ls -lZ /sys/fs/cgroup/unified
>      total 0
>      -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.controllers
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.max.depth
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.max.descendants
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.procs
>      -r--r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.stat
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.subtree_control
>      -rw-r--r--. 1 root root system_u:object_r:tmpfs_t:s0 0 Dec 13 10:41 cgroup.threads
> 
> Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> ---
>   security/selinux/hooks.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> index d6d29ec54eab..0ca5ed30afe1 100644
> --- a/security/selinux/hooks.c
> +++ b/security/selinux/hooks.c
> @@ -6620,6 +6620,13 @@ static void selinux_inode_invalidate_secctx(struct inode *inode)
>    */
>   static int selinux_inode_notifysecctx(struct inode *inode, void *ctx, u32 ctxlen)
>   {
> +	struct superblock_security_struct *sbsec = inode->i_sb->s_security;
> +
> +	/* Do not change context in SECURITY_FS_USE_MNTPOINT case */
> +	if ((sbsec->flags & SE_SBINITIALIZED) &&
> +	    (sbsec->behavior == SECURITY_FS_USE_MNTPOINT))
> +		return 0;
> +
>   	return selinux_inode_setsecurity(inode, XATTR_SELINUX_SUFFIX, ctx, ctxlen, 0);
>   }

Wondering if we ought to take this into selinux_inode_setsecurity() and 
return -EOPNOTSUPP in that case.  We already return -EOPNOTSUPP from 
selinux_inode_setxattr() if (!(sbsec->flags & SBLABEL_MNT)) and that 
should precede other calls to selinux_inode_setsecurity() IIRC.  Should 
we just be checking SBLABEL_MNT here instead?  And do we need to 
separately check SE_SBINITIALIZED?