neverallow rules

neverallow rules

[Joe Nall]

Looking at neverallow rules, the semanage.conf file says
"# expand-check check neverallow rules when executing all semanage commands.
 # Large penalty in time if you turn this on. "

If I don't set expand-check=1, are the neverallow rules actually enforced?
If so, when?

An semodule -i of a policy module with neverallow rules that are violated by the existing binary policy succeeds without complaint unless expand-check=1 in RHEL 7.6. This is not what I expected.

The time taken by a trivial module installation goes from ~.3 seconds to ~14 seconds, so the time hit for expand-check is pretty serious.

We are trying to establish some policy invariants to protect against unexpected/unnoticed RHEL upstream policy changes, some of which have bitten us recently. Any suggestions are welcome.

joe

Re: neverallow rules

[Chris PeBenito]

On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
> Looking at neverallow rules, the semanage.conf file says
> "# expand-check check neverallow rules when executing all semanage
> commands.
>  # Large penalty in time if you turn this on. "
> 
> If I don't set expand-check=1, are the neverallow rules actually
> enforced?

Nope.

> If so, when?
> 
> An semodule -i of a policy module with neverallow rules that are
> violated by the existing binary policy succeeds without complaint
> unless expand-check=1 in RHEL 7.6. This is not what I expected.
> 
> The time taken by a trivial module installation goes from ~.3 seconds
> to ~14 seconds, so the time hit for expand-check is pretty serious.

The reason for adding the expand-check option is because the neverallow
checking is so expensive.

> We are trying to establish some policy invariants to protect against
> unexpected/unnoticed RHEL upstream policy changes, some of which have
> bitten us recently. Any suggestions are welcome.

One alternative would be to use the setools API to code up some policy
searches in Python, then process the results to find things you
do/don't want in your policy.

-- 
Chris PeBenito

Re: neverallow rules

[Stephen Smalley]

On 2/26/19 10:38 PM, Chris PeBenito wrote:
> On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
>> Looking at neverallow rules, the semanage.conf file says
>> "# expand-check check neverallow rules when executing all semanage
>> commands.
>>   # Large penalty in time if you turn this on. "
>>
>> If I don't set expand-check=1, are the neverallow rules actually
>> enforced?
> 
> Nope.
> 
>> If so, when?
>>
>> An semodule -i of a policy module with neverallow rules that are
>> violated by the existing binary policy succeeds without complaint
>> unless expand-check=1 in RHEL 7.6. This is not what I expected.
>>
>> The time taken by a trivial module installation goes from ~.3 seconds
>> to ~14 seconds, so the time hit for expand-check is pretty serious.
> 
> The reason for adding the expand-check option is because the neverallow
> checking is so expensive.
> 
>> We are trying to establish some policy invariants to protect against
>> unexpected/unnoticed RHEL upstream policy changes, some of which have
>> bitten us recently. Any suggestions are welcome.
> 
> One alternative would be to use the setools API to code up some policy
> searches in Python, then process the results to find things you
> do/don't want in your policy.

The approach taken in Fedora/RHEL is to run make validate as part of the 
selinux-policy.spec recipe, which uses the validate target in the 
refpolicy Makefile, which manually runs semodule_link and 
semodule_expand to manually link and expand the modules and performs 
full checking.  Then the cost is only paid at policy build time and is 
only applied for the neverallow rules and allow rules in the Fedora/RHEL 
policy, not for third party modules.  If you are rebuilding the RHEL 
policy from source, you could add your neverallows to it there and the 
make validate should catch the violations.

In Android, neverallow checking is done both at policy build time and as 
part of device certification.  The latter is done using a 
sepolicy-analyze tool we originally contributed; it can take a separate 
neverallow rule configuration in source form and an already compiled 
kernel binary policy and check them against each other.  That's useful 
for cases where you don't have policy sources.

You can have libsemanage apply your own checker against policy as part 
of module transactions by specifying a [verify kernel] stanza in 
semanage.conf, see:
http://selinuxproject.org/page/PolicyValidate

Re: neverallow rules

[Stephen Smalley]

On 2/27/19 8:29 AM, Stephen Smalley wrote:
> On 2/26/19 10:38 PM, Chris PeBenito wrote:
>> On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
>>> Looking at neverallow rules, the semanage.conf file says
>>> "# expand-check check neverallow rules when executing all semanage
>>> commands.
>>>   # Large penalty in time if you turn this on. "
>>>
>>> If I don't set expand-check=1, are the neverallow rules actually
>>> enforced?
>>
>> Nope.
>>
>>> If so, when?
>>>
>>> An semodule -i of a policy module with neverallow rules that are
>>> violated by the existing binary policy succeeds without complaint
>>> unless expand-check=1 in RHEL 7.6. This is not what I expected.
>>>
>>> The time taken by a trivial module installation goes from ~.3 seconds
>>> to ~14 seconds, so the time hit for expand-check is pretty serious.
>>
>> The reason for adding the expand-check option is because the neverallow
>> checking is so expensive.
>>
>>> We are trying to establish some policy invariants to protect against
>>> unexpected/unnoticed RHEL upstream policy changes, some of which have
>>> bitten us recently. Any suggestions are welcome.
>>
>> One alternative would be to use the setools API to code up some policy
>> searches in Python, then process the results to find things you
>> do/don't want in your policy.
> 
> The approach taken in Fedora/RHEL is to run make validate as part of the 
> selinux-policy.spec recipe, which uses the validate target in the 
> refpolicy Makefile, which manually runs semodule_link and 
> semodule_expand to manually link and expand the modules and performs 
> full checking.  Then the cost is only paid at policy build time and is 
> only applied for the neverallow rules and allow rules in the Fedora/RHEL 
> policy, not for third party modules.  If you are rebuilding the RHEL 
> policy from source, you could add your neverallows to it there and the 
> make validate should catch the violations.
> 
> In Android, neverallow checking is done both at policy build time and as 
> part of device certification.  The latter is done using a 
> sepolicy-analyze tool we originally contributed; it can take a separate 
> neverallow rule configuration in source form and an already compiled 
> kernel binary policy and check them against each other.  That's useful 
> for cases where you don't have policy sources.
> 
> You can have libsemanage apply your own checker against policy as part 
> of module transactions by specifying a [verify kernel] stanza in 
> semanage.conf, see:
> http://selinuxproject.org/page/PolicyValidate

Also, FWIW, you can obtain, build, and use sepolicy-analyze outside of 
the Android build system as follows. If you are on RHEL 7, you likely 
need/want to obtain the upstream selinux userspace and build its version 
of libsepol.a for use below instead of using the libsepol-static package 
from RHEL since that is likely too old.

# Get source and build.
$ git clone https://android.googlesource.com/platform/system/sepolicy
$ cd sepolicy/tools/sepolicy-analyze
$ gcc -o sepolicy-analyze *.c /path/to/libsepol.a

# Run with neverallows specified on the command-line.
$ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow -n 
"neverallow user_t self:process signal;"

# Run with neverallows from a separate file.
$ cat > neverallows.conf <<EOF
neverallow user_t self:process signal;
EOF
$ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow -f 
neverallows.conf

There have been improvements to neverallow checking time upstream since 
RHEL 7, but it will still be a function of policy size and obviously 
RHEL has a much larger policy than Android...

Re: neverallow rules

[Stephen Smalley]

On 2/27/19 8:48 AM, Stephen Smalley wrote:
> On 2/27/19 8:29 AM, Stephen Smalley wrote:
>> On 2/26/19 10:38 PM, Chris PeBenito wrote:
>>> On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
>>>> Looking at neverallow rules, the semanage.conf file says
>>>> "# expand-check check neverallow rules when executing all semanage
>>>> commands.
>>>>   # Large penalty in time if you turn this on. "
>>>>
>>>> If I don't set expand-check=1, are the neverallow rules actually
>>>> enforced?
>>>
>>> Nope.
>>>
>>>> If so, when?
>>>>
>>>> An semodule -i of a policy module with neverallow rules that are
>>>> violated by the existing binary policy succeeds without complaint
>>>> unless expand-check=1 in RHEL 7.6. This is not what I expected.
>>>>
>>>> The time taken by a trivial module installation goes from ~.3 seconds
>>>> to ~14 seconds, so the time hit for expand-check is pretty serious.
>>>
>>> The reason for adding the expand-check option is because the neverallow
>>> checking is so expensive.
>>>
>>>> We are trying to establish some policy invariants to protect against
>>>> unexpected/unnoticed RHEL upstream policy changes, some of which have
>>>> bitten us recently. Any suggestions are welcome.
>>>
>>> One alternative would be to use the setools API to code up some policy
>>> searches in Python, then process the results to find things you
>>> do/don't want in your policy.
>>
>> The approach taken in Fedora/RHEL is to run make validate as part of 
>> the selinux-policy.spec recipe, which uses the validate target in the 
>> refpolicy Makefile, which manually runs semodule_link and 
>> semodule_expand to manually link and expand the modules and performs 
>> full checking.  Then the cost is only paid at policy build time and is 
>> only applied for the neverallow rules and allow rules in the 
>> Fedora/RHEL policy, not for third party modules.  If you are 
>> rebuilding the RHEL policy from source, you could add your neverallows 
>> to it there and the make validate should catch the violations.
>>
>> In Android, neverallow checking is done both at policy build time and 
>> as part of device certification.  The latter is done using a 
>> sepolicy-analyze tool we originally contributed; it can take a 
>> separate neverallow rule configuration in source form and an already 
>> compiled kernel binary policy and check them against each other.  
>> That's useful for cases where you don't have policy sources.
>>
>> You can have libsemanage apply your own checker against policy as part 
>> of module transactions by specifying a [verify kernel] stanza in 
>> semanage.conf, see:
>> http://selinuxproject.org/page/PolicyValidate
> 
> Also, FWIW, you can obtain, build, and use sepolicy-analyze outside of 
> the Android build system as follows. If you are on RHEL 7, you likely 
> need/want to obtain the upstream selinux userspace and build its version 
> of libsepol.a for use below instead of using the libsepol-static package 
> from RHEL since that is likely too old.
> 
> # Get source and build.
> $ git clone https://android.googlesource.com/platform/system/sepolicy
> $ cd sepolicy/tools/sepolicy-analyze
> $ gcc -o sepolicy-analyze *.c /path/to/libsepol.a
> 
> # Run with neverallows specified on the command-line.
> $ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow -n 
> "neverallow user_t self:process signal;"
> 
> # Run with neverallows from a separate file.
> $ cat > neverallows.conf <<EOF
> neverallow user_t self:process signal;
> EOF
> $ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow -f 
> neverallows.conf
> 
> There have been improvements to neverallow checking time upstream since 
> RHEL 7, but it will still be a function of policy size and obviously 
> RHEL has a much larger policy than Android...

For reference, on Fedora 29:

$ time sepolicy-analyze /etc/selinux/targeted/policy/policy.31 
neverallow -n "neverallow user_t self:capability mac_admin;"

real	0m0.145s
user	0m0.133s
sys	0m0.012s

Re: neverallow rules

[Stephen Smalley]

On 2/27/19 8:54 AM, Stephen Smalley wrote:
> On 2/27/19 8:48 AM, Stephen Smalley wrote:
>> On 2/27/19 8:29 AM, Stephen Smalley wrote:
>>> On 2/26/19 10:38 PM, Chris PeBenito wrote:
>>>> On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
>>>>> Looking at neverallow rules, the semanage.conf file says
>>>>> "# expand-check check neverallow rules when executing all semanage
>>>>> commands.
>>>>>   # Large penalty in time if you turn this on. "
>>>>>
>>>>> If I don't set expand-check=1, are the neverallow rules actually
>>>>> enforced?
>>>>
>>>> Nope.
>>>>
>>>>> If so, when?
>>>>>
>>>>> An semodule -i of a policy module with neverallow rules that are
>>>>> violated by the existing binary policy succeeds without complaint
>>>>> unless expand-check=1 in RHEL 7.6. This is not what I expected.
>>>>>
>>>>> The time taken by a trivial module installation goes from ~.3 seconds
>>>>> to ~14 seconds, so the time hit for expand-check is pretty serious.
>>>>
>>>> The reason for adding the expand-check option is because the neverallow
>>>> checking is so expensive.
>>>>
>>>>> We are trying to establish some policy invariants to protect against
>>>>> unexpected/unnoticed RHEL upstream policy changes, some of which have
>>>>> bitten us recently. Any suggestions are welcome.
>>>>
>>>> One alternative would be to use the setools API to code up some policy
>>>> searches in Python, then process the results to find things you
>>>> do/don't want in your policy.
>>>
>>> The approach taken in Fedora/RHEL is to run make validate as part of 
>>> the selinux-policy.spec recipe, which uses the validate target in the 
>>> refpolicy Makefile, which manually runs semodule_link and 
>>> semodule_expand to manually link and expand the modules and performs 
>>> full checking.  Then the cost is only paid at policy build time and 
>>> is only applied for the neverallow rules and allow rules in the 
>>> Fedora/RHEL policy, not for third party modules.  If you are 
>>> rebuilding the RHEL policy from source, you could add your 
>>> neverallows to it there and the make validate should catch the 
>>> violations.
>>>
>>> In Android, neverallow checking is done both at policy build time and 
>>> as part of device certification.  The latter is done using a 
>>> sepolicy-analyze tool we originally contributed; it can take a 
>>> separate neverallow rule configuration in source form and an already 
>>> compiled kernel binary policy and check them against each other. 
>>> That's useful for cases where you don't have policy sources.
>>>
>>> You can have libsemanage apply your own checker against policy as 
>>> part of module transactions by specifying a [verify kernel] stanza in 
>>> semanage.conf, see:
>>> http://selinuxproject.org/page/PolicyValidate
>>
>> Also, FWIW, you can obtain, build, and use sepolicy-analyze outside of 
>> the Android build system as follows. If you are on RHEL 7, you likely 
>> need/want to obtain the upstream selinux userspace and build its 
>> version of libsepol.a for use below instead of using the 
>> libsepol-static package from RHEL since that is likely too old.
>>
>> # Get source and build.
>> $ git clone https://android.googlesource.com/platform/system/sepolicy
>> $ cd sepolicy/tools/sepolicy-analyze
>> $ gcc -o sepolicy-analyze *.c /path/to/libsepol.a
>>
>> # Run with neverallows specified on the command-line.
>> $ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow 
>> -n "neverallow user_t self:process signal;"
>>
>> # Run with neverallows from a separate file.
>> $ cat > neverallows.conf <<EOF
>> neverallow user_t self:process signal;
>> EOF
>> $ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow 
>> -f neverallows.conf
>>
>> There have been improvements to neverallow checking time upstream 
>> since RHEL 7, but it will still be a function of policy size and 
>> obviously RHEL has a much larger policy than Android...
> 
> For reference, on Fedora 29:
> 
> $ time sepolicy-analyze /etc/selinux/targeted/policy/policy.31 
> neverallow -n "neverallow user_t self:capability mac_admin;"
> 
> real    0m0.145s
> user    0m0.133s
> sys    0m0.012s

Also, you can put multiple neverallow statements (semicolon separated) 
in the -n option value and have them all checked at once, or similarly 
multiple ones in the file.

Re: neverallow rules

[Stephen Smalley]

On 2/27/19 8:57 AM, Stephen Smalley wrote:
> On 2/27/19 8:54 AM, Stephen Smalley wrote:
>> On 2/27/19 8:48 AM, Stephen Smalley wrote:
>>> On 2/27/19 8:29 AM, Stephen Smalley wrote:
>>>> On 2/26/19 10:38 PM, Chris PeBenito wrote:
>>>>> On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
>>>>>> Looking at neverallow rules, the semanage.conf file says
>>>>>> "# expand-check check neverallow rules when executing all semanage
>>>>>> commands.
>>>>>>   # Large penalty in time if you turn this on. "
>>>>>>
>>>>>> If I don't set expand-check=1, are the neverallow rules actually
>>>>>> enforced?
>>>>>
>>>>> Nope.
>>>>>
>>>>>> If so, when?
>>>>>>
>>>>>> An semodule -i of a policy module with neverallow rules that are
>>>>>> violated by the existing binary policy succeeds without complaint
>>>>>> unless expand-check=1 in RHEL 7.6. This is not what I expected.
>>>>>>
>>>>>> The time taken by a trivial module installation goes from ~.3 seconds
>>>>>> to ~14 seconds, so the time hit for expand-check is pretty serious.
>>>>>
>>>>> The reason for adding the expand-check option is because the 
>>>>> neverallow
>>>>> checking is so expensive.
>>>>>
>>>>>> We are trying to establish some policy invariants to protect against
>>>>>> unexpected/unnoticed RHEL upstream policy changes, some of which have
>>>>>> bitten us recently. Any suggestions are welcome.
>>>>>
>>>>> One alternative would be to use the setools API to code up some policy
>>>>> searches in Python, then process the results to find things you
>>>>> do/don't want in your policy.
>>>>
>>>> The approach taken in Fedora/RHEL is to run make validate as part of 
>>>> the selinux-policy.spec recipe, which uses the validate target in 
>>>> the refpolicy Makefile, which manually runs semodule_link and 
>>>> semodule_expand to manually link and expand the modules and performs 
>>>> full checking.  Then the cost is only paid at policy build time and 
>>>> is only applied for the neverallow rules and allow rules in the 
>>>> Fedora/RHEL policy, not for third party modules.  If you are 
>>>> rebuilding the RHEL policy from source, you could add your 
>>>> neverallows to it there and the make validate should catch the 
>>>> violations.
>>>>
>>>> In Android, neverallow checking is done both at policy build time 
>>>> and as part of device certification.  The latter is done using a 
>>>> sepolicy-analyze tool we originally contributed; it can take a 
>>>> separate neverallow rule configuration in source form and an already 
>>>> compiled kernel binary policy and check them against each other. 
>>>> That's useful for cases where you don't have policy sources.
>>>>
>>>> You can have libsemanage apply your own checker against policy as 
>>>> part of module transactions by specifying a [verify kernel] stanza 
>>>> in semanage.conf, see:
>>>> http://selinuxproject.org/page/PolicyValidate
>>>
>>> Also, FWIW, you can obtain, build, and use sepolicy-analyze outside 
>>> of the Android build system as follows. If you are on RHEL 7, you 
>>> likely need/want to obtain the upstream selinux userspace and build 
>>> its version of libsepol.a for use below instead of using the 
>>> libsepol-static package from RHEL since that is likely too old.
>>>
>>> # Get source and build.
>>> $ git clone https://android.googlesource.com/platform/system/sepolicy
>>> $ cd sepolicy/tools/sepolicy-analyze
>>> $ gcc -o sepolicy-analyze *.c /path/to/libsepol.a
>>>
>>> # Run with neverallows specified on the command-line.
>>> $ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow 
>>> -n "neverallow user_t self:process signal;"
>>>
>>> # Run with neverallows from a separate file.
>>> $ cat > neverallows.conf <<EOF
>>> neverallow user_t self:process signal;
>>> EOF
>>> $ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow 
>>> -f neverallows.conf
>>>
>>> There have been improvements to neverallow checking time upstream 
>>> since RHEL 7, but it will still be a function of policy size and 
>>> obviously RHEL has a much larger policy than Android...
>>
>> For reference, on Fedora 29:
>>
>> $ time sepolicy-analyze /etc/selinux/targeted/policy/policy.31 
>> neverallow -n "neverallow user_t self:capability mac_admin;"
>>
>> real    0m0.145s
>> user    0m0.133s
>> sys    0m0.012s
> 
> Also, you can put multiple neverallow statements (semicolon separated) 
> in the -n option value and have them all checked at once, or similarly 
> multiple ones in the file.

There is an error in the example above; mac_admin is a permission in 
capability2 rather than capability.  By default, sepolicy-analyze 
doesn't warn about undefined classes/permissions/types because a given 
binary policy might not define everything used in a neverallow 
configuration and that is ok (i.e. if there is no matching type, class 
or permission, then it isn't possible for the neverallow to be 
violated). If you want it to detect such things, pass -w to neverallow, ala:

$ sepolicy-analyze /etc/selinux/targeted/policy/policy.31 neverallow -w 
-n "neverallow user_t self:capability mac_admin;"
Warning!  Permission mac_admin used in neverallow undefined in class 
capability in policy being checked.
Warning!  Empty permission set

These are only warnings however; they do not yield an error exit.

Re: [Non-DoD Source] Re: neverallow rules

[jwcart2]

On 2/26/19 10:38 PM, Chris PeBenito wrote:
> On Tue, 2019-02-26 at 19:29 -0600, Joe Nall wrote:
>> Looking at neverallow rules, the semanage.conf file says
>> "# expand-check check neverallow rules when executing all semanage
>> commands.
>>   # Large penalty in time if you turn this on. "
>>
>> If I don't set expand-check=1, are the neverallow rules actually
>> enforced?
> 
> Nope.
> 
>> If so, when?
>>
>> An semodule -i of a policy module with neverallow rules that are
>> violated by the existing binary policy succeeds without complaint
>> unless expand-check=1 in RHEL 7.6. This is not what I expected.
>>
>> The time taken by a trivial module installation goes from ~.3 seconds
>> to ~14 seconds, so the time hit for expand-check is pretty serious.
> 
> The reason for adding the expand-check option is because the neverallow
> checking is so expensive.
> 

I guess RHEL 7.6 is using an older version of the libsepol and friends? I 
refactored neverallow checking back in 2015 (see commit9e6840e6) which greatly 
reduced the time and memory usage of neverallow checking.

Jim

>> We are trying to establish some policy invariants to protect against
>> unexpected/unnoticed RHEL upstream policy changes, some of which have
>> bitten us recently. Any suggestions are welcome.
> 
> One alternative would be to use the setools API to code up some policy
> searches in Python, then process the results to find things you
> do/don't want in your policy.
> 


-- 
James Carter <jwcart2@tycho.nsa.gov>
National Security Agency