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Subject: Re: [RFC][PATCH] apparmor: Enforce progressively tighter permissions
 for no_new_privs
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-security-module@vger.kernel.org,
        Andy Lutomirski <luto@amacapital.net>,
        Kees Cook <keescook@chromium.org>,
        James Morris <james.l.morris@oracle.com>,
        apparmor@lists.ubuntu.com, linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        James Morris <jmorris@namei.org>
References: <87lfcn5mfz.fsf@x220.int.ebiederm.org>
 <c887b7f1-7c50-cd9d-fd97-b89274c2765c@canonical.com>
 <878s8n43qn.fsf@x220.int.ebiederm.org>
From:   John Johansen <john.johansen@canonical.com>
Autocrypt: addr=john.johansen@canonical.com; prefer-encrypt=mutual; keydata=
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Organization: Canonical
Message-ID: <3ccb2c45-2836-bce5-3cb3-0320e66b9656@canonical.com>
Date:   Wed, 20 Jan 2021 15:05:25 -0800
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On 1/20/21 2:56 PM, Eric W. Biederman wrote:
> 
> TL;DR ????selinux and apparmor ignore no_new_privs????
> 
> What?????
> 

AppArmor does not ignore no_new_privs. Its mediation is bounded
and it doesn't grant anything that wasn't allowed when NNP was
set.


> 
> John Johansen <john.johansen@canonical.com> writes:
> 
>> On 1/20/21 1:26 PM, Eric W. Biederman wrote:
>>>
>>> The current understanding of apparmor with respect to no_new_privs is at
>>> odds with how no_new_privs is implemented and understood by the rest of
>>> the kernel.
>>>
>>> The documentation of no_new_privs states:
>>>> With ``no_new_privs`` set, ``execve()`` promises not to grant the
>>>> privilege to do anything that could not have been done without the
>>>> execve call.
>>>
>>> And reading through the kernel except for apparmor that description
>>> matches what is implemented.
>>>
>>
>> That is not correct.
>>
>> commit 7b0d0b40cd78 ("selinux: Permit bounded transitions under
>>     NO_NEW_PRIVS or NOSUID.")
>>
>> Allows for bound transitions under selinux
>> and
> 
> As I understand a bound transition it is a transition to a state with
> a set of permissions that are a subset of what was previously held.
> Which is consistent with the mandate of no_new_privs.
> 
>> commit af63f4193f9f selinux: Generalize support for NNP/nosuid SELinux
>>     domain transitions
>>
>> goes further and "Decouple NNP/nosuid from SELinux transitions".
> 
> Yes.  Looking at that commit I do see that selinux appears to be
> deliberately ignoring no_new_privs in specific cases.
> 
> WTF.
> 
>>> There are two major divergences of apparmor from this definition:
>>> - proc_setattr enforces limitations when no_new_privs are set.
>>> - the limitation is enforced from the apparent time when no_new_privs is
>>>   set instead of guaranteeing that each execve has progressively more
>>>   narrow permissions.
>>>
>>> The code in apparmor that attempts to discover the apparmor label at the
>>> point where no_new_privs is set is not robust.  The capture happens a
>>> long time after no_new_privs is set.
>>>
>>
>> yes, but that shouldn't matter. As apparmor has not changed its label
>> at any point between when no_new_privs was set and when the check is
>> done. AppArmor is attempting to change it label, and if it finds NNP
>> has been set we capture what the confinement was.
>>
>>> Capturing the label at the point where no_new_privs is set is
>>> practically impossible to implement robustly.  Today the rule is struct
>>> cred can only be changed by it's current task.  Today
>>
>> right, and apparmor only ever has the task update its own label.
>>
>>> SECCOMP_FILTER_FLAG_TSYNC sets no_new_privs from another thread.  A
>>> robust implementation would require changing something fundamental in
>>> how creds are managed for SECCOMP_FILTER_FLAG_TSYNC to be able to
>>> capture the cred at the point it is set.
>>>
>> I am open to supporting something like that.
> 
> I can't see how it would be possible to be robust without completely
> changing the locking.  Locking that right now in a simpler model we have
> not figured out how to make obviously correct.
> 
>>> Futhermore given the consistent documentation and how everything else
>>> implements no_new_privs, not having the permissions get progressively
>>
>> Again see above
> 
> Except where selinux deliberately ignores no_new_privs this is
> consitent.
> 
>>> tighter is a footgun aimed at userspace.  I fully expect it to break any
>>
>> tighter is somewhat relative, nor is it only progressively tighter it
>> is bounded against the snapshot of the label that was on the task.
> 
> Which is the BUG I am reporting.  It should be progressingly tighter.
> 
>>> security sensitive software that uses no_new_privs and was not
>>> deliberately designed and tested against apparmor.
>>>
>>
>> Currently the situation has become either an either or choice between
>> the LSM and NNP. We are trying to walk a balance. Ideally apparmor
>> would like to do something similar to selinux and decouple the label
>> transition from NNP and nosuid via an internal capability, but we
>> have not gone there yet.
> 
> Why do you need to escape no_new_privs.  Why does anyone need to escape
> no_new_privs?
> 
>>> Avoid the questionable and hard to fix implementation and the
>>> potential to confuse userspace by having no_new_privs enforce
>>> progressinvely tighter permissions.
>>>
>>
>> This would completely break several use cases.
> 
> Enforcing no_new_privs as documented would break userspace?
> 
> Isn't the opposite true that you are breaking people by not enforcing
> it?
> 
>>> Fixes: 9fcf78cca198 ("apparmor: update domain transitions that are subsets of confinement at nnp")
>>> Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
>>> ---
>>>
>>> I came accross this while examining the places cred_guard_mutex is
>>> used and trying to find a way to make those code paths less insane.
>>>
>>> If it would be more pallatable I would not mind removing the
>>> task_no_new_privs test entirely from aa_change_hat and aa_change_profile
>>> as those are not part of exec, so arguably no_new_privs should not care
>>> about them at all.
>>>
>>> Can we please get rid of the huge semantic wart and pain in the implementation?
>>>
>>>  security/apparmor/domain.c       | 39 ++++----------------------------
>>>  security/apparmor/include/task.h |  4 ----
>>>  security/apparmor/task.c         |  7 ------
>>>  3 files changed, 4 insertions(+), 46 deletions(-)
>>>
>>> diff --git a/security/apparmor/domain.c b/security/apparmor/domain.c
>>> index f919ebd042fd..8f77059bf890 100644
>>> --- a/security/apparmor/domain.c
>>> +++ b/security/apparmor/domain.c
>>> @@ -869,17 +869,6 @@ int apparmor_bprm_creds_for_exec(struct linux_binprm *bprm)
>>>  
>>>  	label = aa_get_newest_label(cred_label(bprm->cred));
>>>  
>>> -	/*
>>> -	 * Detect no new privs being set, and store the label it
>>> -	 * occurred under. Ideally this would happen when nnp
>>> -	 * is set but there isn't a good way to do that yet.
>>> -	 *
>>> -	 * Testing for unconfined must be done before the subset test
>>> -	 */
>>> -	if ((bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS) && !unconfined(label) &&
>>> -	    !ctx->nnp)
>>> -		ctx->nnp = aa_get_label(label);
>>> -
>>>  	/* buffer freed below, name is pointer into buffer */
>>>  	buffer = aa_get_buffer(false);
>>>  	if (!buffer) {
>>> @@ -915,7 +904,7 @@ int apparmor_bprm_creds_for_exec(struct linux_binprm *bprm)
>>>  	 */
>>>  	if ((bprm->unsafe & LSM_UNSAFE_NO_NEW_PRIVS) &&
>>>  	    !unconfined(label) &&
>>> -	    !aa_label_is_unconfined_subset(new, ctx->nnp)) {
>>> +	    !aa_label_is_unconfined_subset(new, label)) {
>>>  		error = -EPERM;
>>>  		info = "no new privs";
>>>  		goto audit;
>>> @@ -1158,16 +1147,6 @@ int aa_change_hat(const char *hats[], int count, u64 token, int flags)
>>>  	label = aa_get_newest_cred_label(cred);
>>>  	previous = aa_get_newest_label(ctx->previous);
>>>  
>>> -	/*
>>> -	 * Detect no new privs being set, and store the label it
>>> -	 * occurred under. Ideally this would happen when nnp
>>> -	 * is set but there isn't a good way to do that yet.
>>> -	 *
>>> -	 * Testing for unconfined must be done before the subset test
>>> -	 */
>>> -	if (task_no_new_privs(current) && !unconfined(label) && !ctx->nnp)
>>> -		ctx->nnp = aa_get_label(label);
>>> -
>>>  	if (unconfined(label)) {
>>>  		info = "unconfined can not change_hat";
>>>  		error = -EPERM;
>>> @@ -1193,7 +1172,7 @@ int aa_change_hat(const char *hats[], int count, u64 token, int flags)
>>>  		 * reduce restrictions.
>>>  		 */
>>>  		if (task_no_new_privs(current) && !unconfined(label) &&
>>> -		    !aa_label_is_unconfined_subset(new, ctx->nnp)) {
>>> +		    !aa_label_is_unconfined_subset(new, label)) {
>>>  			/* not an apparmor denial per se, so don't log it */
>>>  			AA_DEBUG("no_new_privs - change_hat denied");
>>>  			error = -EPERM;
>>> @@ -1214,7 +1193,7 @@ int aa_change_hat(const char *hats[], int count, u64 token, int flags)
>>>  		 * reduce restrictions.
>>>  		 */
>>>  		if (task_no_new_privs(current) && !unconfined(label) &&
>>> -		    !aa_label_is_unconfined_subset(previous, ctx->nnp)) {
>>> +		    !aa_label_is_unconfined_subset(previous, label)) {
>>>  			/* not an apparmor denial per se, so don't log it */
>>>  			AA_DEBUG("no_new_privs - change_hat denied");
>>>  			error = -EPERM;
>>> @@ -1303,16 +1282,6 @@ int aa_change_profile(const char *fqname, int flags)
>>>  
>>>  	label = aa_get_current_label();
>>>  
>>> -	/*
>>> -	 * Detect no new privs being set, and store the label it
>>> -	 * occurred under. Ideally this would happen when nnp
>>> -	 * is set but there isn't a good way to do that yet.
>>> -	 *
>>> -	 * Testing for unconfined must be done before the subset test
>>> -	 */
>>> -	if (task_no_new_privs(current) && !unconfined(label) && !ctx->nnp)
>>> -		ctx->nnp = aa_get_label(label);
>>> -
>>>  	if (!fqname || !*fqname) {
>>>  		aa_put_label(label);
>>>  		AA_DEBUG("no profile name");
>>> @@ -1409,7 +1378,7 @@ int aa_change_profile(const char *fqname, int flags)
>>>  		 * reduce restrictions.
>>>  		 */
>>>  		if (task_no_new_privs(current) && !unconfined(label) &&
>>> -		    !aa_label_is_unconfined_subset(new, ctx->nnp)) {
>>> +		    !aa_label_is_unconfined_subset(new, label)) {
>>>  			/* not an apparmor denial per se, so don't log it */
>>>  			AA_DEBUG("no_new_privs - change_hat denied");
>>>  			error = -EPERM;
>>> diff --git a/security/apparmor/include/task.h b/security/apparmor/include/task.h
>>> index f13d12373b25..8a9c258e2018 100644
>>> --- a/security/apparmor/include/task.h
>>> +++ b/security/apparmor/include/task.h
>>> @@ -17,13 +17,11 @@ static inline struct aa_task_ctx *task_ctx(struct task_struct *task)
>>>  
>>>  /*
>>>   * struct aa_task_ctx - information for current task label change
>>> - * @nnp: snapshot of label at time of no_new_privs
>>>   * @onexec: profile to transition to on next exec  (MAY BE NULL)
>>>   * @previous: profile the task may return to     (MAY BE NULL)
>>>   * @token: magic value the task must know for returning to @previous_profile
>>>   */
>>>  struct aa_task_ctx {
>>> -	struct aa_label *nnp;
>>>  	struct aa_label *onexec;
>>>  	struct aa_label *previous;
>>>  	u64 token;
>>> @@ -42,7 +40,6 @@ struct aa_label *aa_get_task_label(struct task_struct *task);
>>>  static inline void aa_free_task_ctx(struct aa_task_ctx *ctx)
>>>  {
>>>  	if (ctx) {
>>> -		aa_put_label(ctx->nnp);
>>>  		aa_put_label(ctx->previous);
>>>  		aa_put_label(ctx->onexec);
>>>  	}
>>> @@ -57,7 +54,6 @@ static inline void aa_dup_task_ctx(struct aa_task_ctx *new,
>>>  				   const struct aa_task_ctx *old)
>>>  {
>>>  	*new = *old;
>>> -	aa_get_label(new->nnp);
>>>  	aa_get_label(new->previous);
>>>  	aa_get_label(new->onexec);
>>>  }
>>> diff --git a/security/apparmor/task.c b/security/apparmor/task.c
>>> index d17130ee6795..4b9ec370a171 100644
>>> --- a/security/apparmor/task.c
>>> +++ b/security/apparmor/task.c
>>> @@ -41,7 +41,6 @@ struct aa_label *aa_get_task_label(struct task_struct *task)
>>>  int aa_replace_current_label(struct aa_label *label)
>>>  {
>>>  	struct aa_label *old = aa_current_raw_label();
>>> -	struct aa_task_ctx *ctx = task_ctx(current);
>>>  	struct cred *new;
>>>  
>>>  	AA_BUG(!label);
>>> @@ -56,12 +55,6 @@ int aa_replace_current_label(struct aa_label *label)
>>>  	if (!new)
>>>  		return -ENOMEM;
>>>  
>>> -	if (ctx->nnp && label_is_stale(ctx->nnp)) {
>>> -		struct aa_label *tmp = ctx->nnp;
>>> -
>>> -		ctx->nnp = aa_get_newest_label(tmp);
>>> -		aa_put_label(tmp);
>>> -	}
>>>  	if (unconfined(label) || (labels_ns(old) != labels_ns(label)))
>>>  		/*
>>>  		 * if switching to unconfined or a different label namespace
>>>
> 
> Eric
>