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 <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 > -- AppArmor mailing list AppArmor@lists.ubuntu.com Modify settings or unsubscribe at: https://lists.ubuntu.com/mailman/listinfo/apparmor