[RFC][PATCH] apparmor: Enforce progressively tighter permissions for no_new_privs
Eric W. Biederman
ebiederm at xmission.com
Wed Jan 20 22:56:16 UTC 2021
TL;DR ????selinux and apparmor ignore no_new_privs????
What?????
John Johansen <john.johansen at 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 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
>>
Eric
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