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.johan...@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 <ebied...@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
>
