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