Re: [PATCH] scripts/selinux: modernize mdp

[Stephen Smalley <sds@tycho.nsa.gov>]

On 2/21/19 2:44 PM, Dominick Grift wrote:
> On Thu, Feb 21, 2019 at 02:34:38PM -0500, Stephen Smalley wrote:
>> On 2/21/19 1:42 PM, Stephen Smalley wrote:
>>> Derived in part from a patch by Dominick Grift.
>>>
>>> The MDP example no longer works on modern systems.  Fix it.
>>> While we are at it, add MLS support and enable it.
>>>
>>> NB This still does not work on systems using dbus-daemon instead of
>>> dbus-broker because dbus-daemon does not yet gracefully handle unknown
>>> classes/permissions.  This is a deficiency in libselinux's
>>> selinux_set_mapping() interface and underlying implementation,
>>> which was never fully updated to deal with unknown classes/permissions
>>> unlike the kernel.  Programs that instead use selinux_check_access()
>>> like dbus-broker do not have this problem.
>>
>> We could perhaps avoid this problem by having mdp always include at least a
>> core set of userspace classes/permissions in the policy it generates.  We
>> could also fix libselinux but that won't help on any existing distro.
>>
>>>
>>> Other known issues:
>>> - Not everything appears to be relabeled, so some files are left with invalid
>>> contexts and remapped to the unlabeled SID/context.
>>
>> This appears to be partly due to overuse of <<none>> in file_contexts (.fc)
>> files.  That excludes those parts of the filesystem from being relabeled at
>> all.  This was used to exclude pseudo filesystems (obsoleted by seclabel
>> mount option detection) or runtime directories/files whose labels were
>> derived from the creating process and couldn't be statically specified by
>> file_contexts.  To get my system back into working order even with targeted
>> policy, I had to strip all <<none>> entries out of my file_contexts* files
>> and then run setfiles -F with the list of filesystem mounts to relabel.
>> Otherwise, I'd have files left in the old contexts and the system wouldn't
>> even come up to user login, even if permissive.
> 
> What <<none>> spec(s) in fedora would be so important that it causes the system to not come up to user login, even in permissive?
> Does the unlabeled isid not address these particular scenario's? and why not?

Yes, I don't fully understand it myself; I just know that certain 
services won't start successfully and it never reaches the point where I 
can login locally or remotely.  But stripping the <<none>> entries and 
running setfiles -F did fix it for me.  NB This was for converting back 
from mdp to the Fedora targeted policy.  It wouldn't be an issue for 
converting to mdp since that file_contexts has no <<none>> entries and 
has a default match for /.*.

Maybe it is a case of processes with CAP_MAC_ADMIN fetching the raw 
context (which is invalid under the current policy) and then trying to 
feed them back to the kernel via a selinuxfs interface, e.g. 
security_compute_create() or similar.  The kernel would reject those.

> 
> I am testing the patch now here (but in my scenario its a very minimal fedora with dssp2-standard policy)
> 
>>
>> FWIW, Android policy doesn't use <<none>> at all.  But they also don't have
>> a /.* or equivalent entry as a default match, so anything not covered by a
>> more specific match is likewise not labeled. seapp_contexts handles the more
>> dynamic aspect of app directory labeling for Android.
>>
>> The other problem case for relabeling is the mount point directories, which
>> requires unmounting them all and relabeling them if we care. Otherwise
>> they'll just get the unlabeled context and as long as we allow mounting on
>> that, it should be ok.
>>
>>> - X will fail due to lack of a x_contexts file
>>> - libvirtd will fail due to lack of a virtual_domain_context file
>>
>> We could easily add these to the mdp policy.
>>
>>> - crond reports an error with "No security context"
>>
>> This is probably due to the lack of a contexts/default_contexts or any
>> contexts/users/ files in the dummy policy.
>>
>>>
>>> Changes to mdp:
>>> Add support for devtmpfs, required by modern Linux distributions.
>>> Add MLS support, with sample sensitivities, categories, and constraints.
>>> Generate fs_use and genfscon rules based on kernel configuration.
>>> Update list of filesystem types for fs_use and genfscon rules.
>>> Use object_r for object contexts.
>>>
>>> Changes to install_policy.sh:
>>> Bail immediately on any errors.
>>> Provide more helpful error messages when unable to find userspace tools.
>>> Refuse to run if SELinux is already enabled.
>>> Unconditionally move aside /etc/selinux/config and create a new one.
>>> Build policy with -U allow so that userspace object managers do not break.
>>> Build policy with MLS enabled by default.
>>> Add default seusers mapping and failsafe context for use by
>>> pam_selinux / libselinux.
>>> Set to permissive mode rather than enforcing to permit initial autorelabel.
>>> Update the list of filesystem types to be relabeled.
>>> Create /.autorelabel to trigger an autorelabel on reboot.
>>> Drop broken attempt to relabel the /dev mountpoint directory.
>>>
>>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
>>> ---
>>>    scripts/selinux/install_policy.sh |  82 ++++++++-------
>>>    scripts/selinux/mdp/mdp.c         | 164 +++++++++++++++++++++++++-----
>>>    2 files changed, 183 insertions(+), 63 deletions(-)
>>>
>>> diff --git a/scripts/selinux/install_policy.sh b/scripts/selinux/install_policy.sh
>>> index 0b86c47baf7d..09eab4d0da5c 100755
>>> --- a/scripts/selinux/install_policy.sh
>>> +++ b/scripts/selinux/install_policy.sh
>>> @@ -1,30 +1,51 @@
>>>    #!/bin/sh
>>>    # SPDX-License-Identifier: GPL-2.0
>>> +set -e
>>>    if [ `id -u` -ne 0 ]; then
>>>    	echo "$0: must be root to install the selinux policy"
>>>    	exit 1
>>>    fi
>>> +
>>>    SF=`which setfiles`
>>>    if [ $? -eq 1 ]; then
>>> -	if [ -f /sbin/setfiles ]; then
>>> -		SF="/usr/setfiles"
>>> -	else
>>> -		echo "no selinux tools installed: setfiles"
>>> -		exit 1
>>> -	fi
>>> +	echo "Could not find setfiles"
>>> +	echo "Do you have policycoreutils installed?"
>>> +	exit 1
>>>    fi
>>> -cd mdp
>>> -
>>>    CP=`which checkpolicy`
>>> +if [ $? -eq 1 ]; then
>>> +	echo "Could not find checkpolicy"
>>> +	echo "Do you have checkpolicy installed?"
>>> +	exit 1
>>> +fi
>>>    VERS=`$CP -V | awk '{print $1}'`
>>> -./mdp policy.conf file_contexts
>>> -$CP -o policy.$VERS policy.conf
>>> +ENABLED=`which selinuxenabled`
>>> +if [ $? -eq 1 ]; then
>>> +	echo "Could not find selinuxenabled"
>>> +	echo "Do you have libselinux-utils installed?"
>>> +	exit 1
>>> +fi
>>> +
>>> +if selinuxenabled; then
>>> +    echo "SELinux is already enabled"
>>> +    echo "This prevents safely relabeling all files."
>>> +    echo "Boot with selinux=0 on the kernel command-line or"
>>> +    echo "SELINUX=disabled in /etc/selinux/config."
>>> +    exit 1
>>> +fi
>>> +
>>> +cd mdp
>>> +./mdp -m policy.conf file_contexts
>>> +$CP -U allow -M -o policy.$VERS policy.conf
>>>    mkdir -p /etc/selinux/dummy/policy
>>>    mkdir -p /etc/selinux/dummy/contexts/files
>>> +echo "__default__:user_u" > /etc/selinux/dummy/seusers
>>> +echo "base_r:base_t" > /etc/selinux/dummy/contexts/failsafe_context
>>> +
>>>    cp file_contexts /etc/selinux/dummy/contexts/files
>>>    cp dbus_contexts /etc/selinux/dummy/contexts
>>>    cp policy.$VERS /etc/selinux/dummy/policy
>>> @@ -33,37 +54,22 @@ FC_FILE=/etc/selinux/dummy/contexts/files/file_contexts
>>>    if [ ! -d /etc/selinux ]; then
>>>    	mkdir -p /etc/selinux
>>>    fi
>>> -if [ ! -f /etc/selinux/config ]; then
>>> -	cat > /etc/selinux/config << EOF
>>> -SELINUX=enforcing
>>> +if [ -f /etc/selinux/config ]; then
>>> +    echo "/etc/selinux/config exists, moving to /etc/selinux/config.bak."
>>> +    mv /etc/selinux/config /etc/selinux/config.bak
>>> +fi
>>> +echo "Creating new /etc/selinux/config for dummy policy."
>>> +cat > /etc/selinux/config << EOF
>>> +SELINUX=permissive
>>>    SELINUXTYPE=dummy
>>>    EOF
>>> -else
>>> -	TYPE=`cat /etc/selinux/config | grep "^SELINUXTYPE" | tail -1 | awk -F= '{ print $2 '}`
>>> -	if [ "eq$TYPE" != "eqdummy" ]; then
>>> -		selinuxenabled
>>> -		if [ $? -eq 0 ]; then
>>> -			echo "SELinux already enabled with a non-dummy policy."
>>> -			echo "Exiting.  Please install policy by hand if that"
>>> -			echo "is what you REALLY want."
>>> -			exit 1
>>> -		fi
>>> -		mv /etc/selinux/config /etc/selinux/config.mdpbak
>>> -		grep -v "^SELINUXTYPE" /etc/selinux/config.mdpbak >> /etc/selinux/config
>>> -		echo "SELINUXTYPE=dummy" >> /etc/selinux/config
>>> -	fi
>>> -fi
>>>    cd /etc/selinux/dummy/contexts/files
>>> -$SF file_contexts /
>>> +$SF -F file_contexts /
>>> -mounts=`cat /proc/$$/mounts | egrep "ext2|ext3|xfs|jfs|ext4|ext4dev|gfs2" | awk '{ print $2 '}`
>>> -$SF file_contexts $mounts
>>> +mounts=`cat /proc/$$/mounts | \
>>> +	egrep "ext[234]|jfs|xfs|reiserfs|jffs2|gfs2|btrfs|f2fs|ocfs2" | \
>>> +	awk '{ print $2 '}`
>>> +$SF -F file_contexts $mounts
>>> -
>>> -dodev=`cat /proc/$$/mounts | grep "/dev "`
>>> -if [ "eq$dodev" != "eq" ]; then
>>> -	mount --move /dev /mnt
>>> -	$SF file_contexts /dev
>>> -	mount --move /mnt /dev
>>> -fi
>>> +touch /.autorelabel
>>> diff --git a/scripts/selinux/mdp/mdp.c b/scripts/selinux/mdp/mdp.c
>>> index 073fe7537f6c..daad333c7252 100644
>>> --- a/scripts/selinux/mdp/mdp.c
>>> +++ b/scripts/selinux/mdp/mdp.c
>>> @@ -33,6 +33,7 @@
>>>    #include <unistd.h>
>>>    #include <string.h>
>>>    #include <sys/socket.h>
>>> +#include <linux/kconfig.h>
>>>    static void usage(char *name)
>>>    {
>>> @@ -95,10 +96,31 @@ int main(int argc, char *argv[])
>>>    	}
>>>    	fprintf(fout, "\n");
>>> -	/* NOW PRINT OUT MLS STUFF */
>>> +	/* print out mls declarations and constraints */
>>>    	if (mls) {
>>> -		printf("MLS not yet implemented\n");
>>> -		exit(1);
>>> +		fprintf(fout, "sensitivity s0;\n");
>>> +		fprintf(fout, "sensitivity s1;\n");
>>> +		fprintf(fout, "dominance { s0 s1 }\n");
>>> +		fprintf(fout, "category c0;\n");
>>> +		fprintf(fout, "category c1;\n");
>>> +		fprintf(fout, "level s0:c0.c1;\n");
>>> +		fprintf(fout, "level s1:c0.c1;\n");
>>> +#define SYSTEMLOW "s0"
>>> +#define SYSTEMHIGH "s1:c0.c1"
>>> +		for (i = 0; secclass_map[i].name; i++) {
>>> +			struct security_class_mapping *map = &secclass_map[i];
>>> +
>>> +			fprintf(fout, "mlsconstrain %s {\n", map->name);
>>> +			for (j = 0; map->perms[j]; j++)
>>> +				fprintf(fout, "\t%s\n", map->perms[j]);
>>> +			/*
>>> +			 * This requires all subjects and objects to be
>>> +			 * single-level (l2 eq h2), and that the subject
>>> +			 * level dominate the object level (h1 dom h2)
>>> +			 * in order to have any permissions to it.
>>> +			 */
>>> +			fprintf(fout, "} (l2 eq h2 and h1 dom h2);\n\n");
>>> +		}
>>>    	}
>>>    	/* types, roles, and allows */
>>> @@ -108,34 +130,126 @@ int main(int argc, char *argv[])
>>>    	for (i = 0; secclass_map[i].name; i++)
>>>    		fprintf(fout, "allow base_t base_t:%s *;\n",
>>>    			secclass_map[i].name);
>>> -	fprintf(fout, "user user_u roles { base_r };\n");
>>> -	fprintf(fout, "\n");
>>> +	fprintf(fout, "user user_u roles { base_r }");
>>> +	if (mls)
>>> +		fprintf(fout, " level %s range %s - %s", SYSTEMLOW,
>>> +			SYSTEMLOW, SYSTEMHIGH);
>>> +	fprintf(fout, ";\n");
>>> +
>>> +#define SUBJUSERROLETYPE "user_u:base_r:base_t"
>>> +#define OBJUSERROLETYPE "user_u:object_r:base_t"
>>>    	/* default sids */
>>>    	for (i = 1; i < initial_sid_to_string_len; i++)
>>> -		fprintf(fout, "sid %s user_u:base_r:base_t\n", initial_sid_to_string[i]);
>>> +		fprintf(fout, "sid %s " SUBJUSERROLETYPE "%s\n",
>>> +			initial_sid_to_string[i], mls ? ":" SYSTEMLOW : "");
>>>    	fprintf(fout, "\n");
>>> -	fprintf(fout, "fs_use_xattr ext2 user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr ext3 user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr ext4 user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr jfs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr xfs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr reiserfs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr jffs2 user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_xattr gfs2 user_u:base_r:base_t;\n");
>>> +#define FS_USE(behavior, fstype)			    \
>>> +	fprintf(fout, "fs_use_%s %s " OBJUSERROLETYPE "%s;\n", \
>>> +		behavior, fstype, mls ? ":" SYSTEMLOW : "")
>>> +
>>> +	/*
>>> +	 * Filesystems whose inode labels can be fetched via getxattr.
>>> +	 */
>>> +#ifdef CONFIG_EXT2_FS_SECURITY
>>> +	FS_USE("xattr", "ext2");
>>> +#endif
>>> +#ifdef CONFIG_EXT3_FS_SECURITY
>>> +	FS_USE("xattr", "ext3");
>>> +#endif
>>> +#ifdef CONFIG_EXT4_FS_SECURITY
>>> +	FS_USE("xattr", "ext4");
>>> +#endif
>>> +#ifdef CONFIG_JFS_SECURITY
>>> +	FS_USE("xattr", "jfs");
>>> +#endif
>>> +#ifdef CONFIG_REISERFS_FS_SECURITY
>>> +	FS_USE("xattr", "reiserfs");
>>> +#endif
>>> +#ifdef CONFIG_JFFS2_FS_SECURITY
>>> +	FS_USE("xattr", "jffs2");
>>> +#endif
>>> +#ifdef CONFIG_XFS_FS
>>> +	FS_USE("xattr", "xfs");
>>> +#endif
>>> +#ifdef CONFIG_GFS2_FS
>>> +	FS_USE("xattr", "gfs2");
>>> +#endif
>>> +#ifdef CONFIG_BTRFS_FS
>>> +	FS_USE("xattr", "btrfs");
>>> +#endif
>>> +#ifdef CONFIG_F2FS_FS_SECURITY
>>> +	FS_USE("xattr", "f2fs");
>>> +#endif
>>> +#ifdef CONFIG_OCFS2_FS
>>> +	FS_USE("xattr", "ocsfs2");
>>> +#endif
>>> +#ifdef CONFIG_OVERLAY_FS
>>> +	FS_USE("xattr", "overlay");
>>> +#endif
>>> +#ifdef CONFIG_SQUASHFS_XATTR
>>> +	FS_USE("xattr", "squashfs");
>>> +#endif
>>> +
>>> +	/*
>>> +	 * Filesystems whose inodes are labeled from allocating task.
>>> +	 */
>>> +	FS_USE("task", "pipefs");
>>> +	FS_USE("task", "sockfs");
>>> -	fprintf(fout, "fs_use_task eventpollfs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_task pipefs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_task sockfs user_u:base_r:base_t;\n");
>>> +	/*
>>> +	 * Filesystems whose inode labels are computed from both
>>> +	 * the allocating task and the superblock label.
>>> +	 */
>>> +#ifdef CONFIG_UNIX98_PTYS
>>> +	FS_USE("trans", "devpts");
>>> +#endif
>>> +#ifdef CONFIG_HUGETLBFS
>>> +	FS_USE("trans", "hugetlbfs");
>>> +#endif
>>> +#ifdef CONFIG_TMPFS
>>> +	FS_USE("trans", "tmpfs");
>>> +#endif
>>> +#ifdef CONFIG_DEVTMPFS
>>> +	FS_USE("trans", "devtmpfs");
>>> +#endif
>>> +#ifdef CONFIG_POSIX_MQUEUE
>>> +	FS_USE("trans", "mqueue");
>>> +#endif
>>> -	fprintf(fout, "fs_use_trans mqueue user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_trans devpts user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_trans hugetlbfs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_trans tmpfs user_u:base_r:base_t;\n");
>>> -	fprintf(fout, "fs_use_trans shm user_u:base_r:base_t;\n");
>>> +#define GENFSCON(fstype, prefix)			     \
>>> +	fprintf(fout, "genfscon %s %s " OBJUSERROLETYPE "%s\n", \
>>> +		fstype, prefix, mls ? ":" SYSTEMLOW : "")
>>> -	fprintf(fout, "genfscon proc / user_u:base_r:base_t\n");
>>> +	/*
>>> +	 * Filesystems whose inodes are labeled from path prefix match
>>> +	 * relative to the filesystem root.  Depending on the filesystem,
>>> +	 * only a single label for all inodes may be supported.  Here
>>> +	 * we list the filesystem types for which per-file labeling is
>>> +	 * supported using genfscon; any other filesystem type can also
>>> +	 * be added by only with a single entry for all of its inodes.
>>> +	 */
>>> +#ifdef CONFIG_PROC_FS
>>> +	GENFSCON("proc", "/");
>>> +#endif
>>> +#ifdef CONFIG_SECURITY_SELINUX
>>> +	GENFSCON("selinuxfs", "/");
>>> +#endif
>>> +#ifdef CONFIG_SYSFS
>>> +	GENFSCON("sysfs", "/");
>>> +#endif
>>> +#ifdef CONFIG_DEBUG_FS
>>> +	GENFSCON("debugfs", "/");
>>> +#endif
>>> +#ifdef CONFIG_TRACING
>>> +	GENFSCON("tracefs", "/");
>>> +#endif
>>> +#ifdef CONFIG_PSTORE
>>> +	GENFSCON("pstore", "/");
>>> +#endif
>>> +	GENFSCON("cgroup", "/");
>>> +	GENFSCON("cgroup2", "/");
>>>    	fclose(fout);
>>> @@ -144,8 +258,8 @@ int main(int argc, char *argv[])
>>>    		printf("Wrote policy, but cannot open %s for writing\n", ctxout);
>>>    		usage(argv[0]);
>>>    	}
>>> -	fprintf(fout, "/ user_u:base_r:base_t\n");
>>> -	fprintf(fout, "/.* user_u:base_r:base_t\n");
>>> +	fprintf(fout, "/ " OBJUSERROLETYPE "%s\n", mls ? ":" SYSTEMLOW : "");
>>> +	fprintf(fout, "/.* " OBJUSERROLETYPE "%s\n", mls ? ":" SYSTEMLOW : "");
>>>    	fclose(fout);
>>>    	return 0;
>>>
>>
>