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

[John Johansen]

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

commit af63f4193f9f selinux: Generalize support for NNP/nosuid SELinux
    domain transitions

goes further and "Decouple NNP/nosuid from SELinux transitions". 

> 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.

> Futhermore given the consistent documentation and how everything else
> implements no_new_privs, not having the permissions get progressively

Again see above

> 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.

> 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.

> 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.

> Fixes: 9fcf78cca198 ("apparmor: update domain transitions that are subsets of confinement at nnp")
> Signed-off-by: Eric W. Biederman <ebiederm at 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
>