Re: [RFC][PATCH] apparmor: Enforce progressively tighter permissions for no_new_privs

[Eric W. Biederman]

TL;DR ????selinux and apparmor ignore no_new_privs????

What?????


John Johansen <john.johansen@xxxxxxxxxxxxx> 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@xxxxxxxxxxxx>
>> ---
>> 
>> 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