Re: [PATCH v6] scripts/selinux: modernize mdp

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

On 2/22/19 3:55 AM, Dominick Grift wrote:
> On Thu, Feb 21, 2019 at 04:31:47PM -0500, 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 appears to be 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.  The same problem also occurs with XSELinux.
>> Programs that instead use selinux_check_access() like dbus-broker
>> should not have this problem.
>>
>> 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.
>> Create seusers, failsafe_context, and default_contexts for use by
>> pam_selinux / libselinux.
>> Create x_contexts for the SELinux X extension.
>> Create virtual_domain_context and virtual_image_context for libvirtd.
>> Set to permissive mode rather than enforcing to permit initial autorelabel.
>> Update the list of filesystem types to be relabeled.
>> Write -F to /.autorelabel to cause a forced autorelabel on reboot.
>> Drop broken attempt to relabel the /dev mountpoint directory.
>>
>> Signed-off-by: Stephen Smalley <sds@tycho.nsa.gov>
>> ---
>> v6 fixes the seusers and failsafe_contexts to include :s0
>> as per Dominick's comments. It also adds a default_contexts
>> configuration for good measure, although this might not be
>> necessary. It creates a minimal working x_contexts configuration
>> to appease XSELinux, although XSELinux still has problems due
>> to the lack of the userspace class/perms definitions. It
>> creates empty virtual_*_context files to make libvirtd happy.
>> It writes -F to /.autorelabel as per Dominick's comments to
>> trigger a forced relabel. It also fixes mdp to correctly generate
>> fs_use rules for ext3 and ext2 when using ext4 as the driver.
>> These days ext4 is always used to handle ext3 mounts and it can
>> also be used for ext2.  This version is called v6 to distinguish it from
>> Dominick's earlier patch sequence, which went up to v4, and then
>> my previous coalesced patch is logically v5.
>>
>>   scripts/selinux/install_policy.sh |  92 ++++++++++-------
>>   scripts/selinux/mdp/mdp.c         | 165 +++++++++++++++++++++++++-----
>>   2 files changed, 194 insertions(+), 63 deletions(-)
> 
> Acked-by: Dominick Grift <dominick.grift@defensec.nl>
> 
> I was pretty sure that seusers entries need both current as well as clearance (s0-s0 instead of just s0) but just s0 works in this scenario atleast
> I did not test the X and virt_contexts aspects.

Hmm...so even with the fix for libselinux selinux_set_mapping (which 
addresses the dbus-daemon issue), I can't login under the dummy policy 
if the system is set to enforcing.  I get the following from pam_selinux:

sshd[2015]: pam_selinux(sshd:session): Failed to translate security 
class context. Invalid argument
sshd[2015]: pam_selinux(sshd:session): Security context 
user_u:base_r:base_t:s0 is not allowed for user_u:base_r:base_t:s0
sshd[2015]: pam_selinux(sshd:session): Unable to get valid context for sds

Looking at the pam_selinux code, it performs a userspace permission 
check on the context class, contains permission, in order to validate 
that the MLS level is authorized for the Linux user. The normal context 
validation is not sufficient for this purpose since the SELinux user in 
the security context to which the Linux username is mapped may be 
authorized for a wider range than the Linux user. pam_selinux directly 
calls string_to_security_class(), string_to_av_perm(), and 
security_compute_av() instead of using selinux_check_access(), so it 
doesn't honor allow_unknown presently.  Is this fixed in rawhide?

> 
>>
>> diff --git a/scripts/selinux/install_policy.sh b/scripts/selinux/install_policy.sh
>> index 0b86c47baf7d..2dccf141241d 100755
>> --- a/scripts/selinux/install_policy.sh
>> +++ b/scripts/selinux/install_policy.sh
>> @@ -1,30 +1,61 @@
>>   #!/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.$VERS policy.conf
>>   
>>   mkdir -p /etc/selinux/dummy/policy
>>   mkdir -p /etc/selinux/dummy/contexts/files
>>   
>> +echo "__default__:user_u:s0" > /etc/selinux/dummy/seusers
>> +echo "base_r:base_t:s0" > /etc/selinux/dummy/contexts/failsafe_context
>> +echo "base_r:base_t:s0 base_r:base_t:s0" > /etc/selinux/dummy/default_contexts
>> +cat > /etc/selinux/dummy/contexts/x_contexts <<EOF
>> +client * user_u:base_r:base_t:s0
>> +property * user_u:object_r:base_t:s0
>> +extension * user_u:object_r:base_t:s0
>> +selection * user_u:object_r:base_t:s0
>> +event * user_u:object_r:base_t:s0
>> +EOF
>> +touch /etc/selinux/dummy/contexts/virtual_domain_context
>> +touch /etc/selinux/dummy/contexts/virtual_image_context
>> +
>>   cp file_contexts /etc/selinux/dummy/contexts/files
>>   cp dbus_contexts /etc/selinux/dummy/contexts
>>   cp policy.$VERS /etc/selinux/dummy/policy
>> @@ -33,37 +64,22 @@ 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
>> +echo "-F" > /.autorelabel
>> diff --git a/scripts/selinux/mdp/mdp.c b/scripts/selinux/mdp/mdp.c
>> index 073fe7537f6c..edaba8e51651 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 +130,127 @@ 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_EXT4_FS_SECURITY
>> +#ifdef CONFIG_EXT4_USE_FOR_EXT2
>> +	FS_USE("xattr", "ext2");
>> +#endif
>> +	FS_USE("xattr", "ext3");
>> +	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 +259,8 @@ 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;
>> -- 
>> 2.20.1
>>
>