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

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

On 2/22/19 3:44 PM, Stephen Smalley wrote:
> On 2/22/19 3:26 PM, Dominick Grift wrote:
>> On Fri, Feb 22, 2019 at 03:17:10PM -0500, Stephen Smalley wrote:
>>> 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?
>>
>> No I do not believe that this is fixed in rawhide, but might be able 
>> to work around that by removing the the env_params option from the 
>> pam_selinux call in /etc/pam.d/sshd.
>>
>> I know, not ideal.
> 
> (cc Petr)
> 
> Any chance we could get pam_selinux to use selinux_check_access() 
> instead, or, if not, to check security_deny_unknown() and handle 
> failures of string_to_security_class()/string_to_av_perm() accordingly?

The latter might be preferable to avoid the overhead of creating an AVC 
just for a single permission check.  Should be just a matter of adding 
calls to security_deny_unknown() in the error handling paths for both 
string_*() mapping functions and returning success instead of failure if 
it returns 0.

> 
>>
>>>
>>>>
>>>>>
>>>>> 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
>>>>>
>>>>
>>>
>>
>