On Tue, Jul 11, 2017 at 04:23:29PM -0400, Stephen Smalley wrote: > On Tue, 2017-07-11 at 22:10 +0200, Dominick Grift wrote: > > On Tue, Jul 11, 2017 at 10:05:36PM +0200, Dominick Grift wrote: > > > On Tue, Jul 11, 2017 at 03:52:52PM -0400, Stephen Smalley wrote: > > > > On Mon, 2017-07-10 at 16:25 -0400, Stephen Smalley wrote: > > > > > As systemd ramps up enabling NoNewPrivileges (either explicitly > > > > > in > > > > > service unit files or as a side effect of other security- > > > > > related > > > > > settings in service unit files), we're increasingly running > > > > > afoul of > > > > > its interactions with SELinux. > > > > > > > > > > The end result is bad for the security of both SELinux-disabled > > > > > and > > > > > SELinux-enabled systems. Packagers have to turn off these > > > > > options in the unit files to preserve SELinux domain > > > > > transitions. For > > > > > users who choose to disable SELinux, this means that they miss > > > > > out on > > > > > at least having the systemd-supported protections. For users > > > > > who > > > > > keep > > > > > SELinux enabled, they may still be missing out on some > > > > > protections > > > > > because it isn't necessarily guaranteed that the SELinux policy > > > > > for > > > > > that service provides the same protections in all cases. > > > > > > > > > > Our options seem to be: > > > > > > > > > > 1) Just keep on the way we are now, i.e. packagers have to > > > > > remove > > > > > default protection settings from upstream package unit files in > > > > > order > > > > > to have them work with SELinux (and not just NoNewPrivileges= > > > > > itself; increasingly systemd is enabling NNP as a side effect > > > > > of > > > > > other > > > > > unit file options, even seemingly unrelated ones like > > > > > PrivateDevices). > > > > > SELinux-disabled users lose entirely, SELinux-enabled users may > > > > > lose > > > > > (depending on whether SELinux policy provides equivalent or > > > > > better guarantees). > > > > > > > > > > 2) Change systemd to automatically disable NNP on SELinux- > > > > > enabled > > > > > systems. Unit files can be left unmodified from > > > > > upstream. SELinux- > > > > > disabled users win. SELinux-enabled users may lose. > > > > > > > > > > 3) Try to use typebounds, since we allow bounded transitions > > > > > under > > > > > NNP. > > > > > Unit files can be left unmodified from upstream. SELinux- > > > > > disabled > > > > > users > > > > > win. SELinux-enabled users get to benefit from systemd- > > > > > provided > > > > > protections. However, this option is impractical to implement > > > > > in > > > > > policy > > > > > currently, since typebounds requires us to ensure that each > > > > > domain is > > > > > allowed everything all of its descendant domains are allowed, > > > > > and > > > > > this > > > > > has to be repeated for the entire chain of domain > > > > > transitions. There > > > > > is > > > > > no way to clone all allow rules from children to the parent in > > > > > policy > > > > > currently, and it is doubtful that doing so would be desirable > > > > > even > > > > > if > > > > > it were practical, as it requires leaking permissions to > > > > > objects and > > > > > operations into parent domains that could weaken their own > > > > > security > > > > > in > > > > > order to allow them to the children (e.g. if a child requires > > > > > execmem > > > > > permission, then so does the parent; if a child requires read > > > > > to a > > > > > symbolic > > > > > link or temporary file that it can write, then so does the > > > > > parent, > > > > > ...). > > > > > > > > > > 4) Decouple NNP from SELinux transitions, so that we don't have > > > > > to > > > > > make a choice between them. Introduce a new policy capability > > > > > that > > > > > causes the ability to transition under NNP to be based on a new > > > > > permission > > > > > check between the old and new contexts rather than > > > > > typebounds. Domain > > > > > transitions can then be allowed in policy without requiring the > > > > > parent > > > > > to be a strict superset of all of its children. The rationale > > > > > for > > > > > this > > > > > divergence from NNP behavior for capabilities is that SELinux > > > > > permissions > > > > > are substantially broader than just capabilities (they express > > > > > a full > > > > > access matrix, not just privileges) and can only be used to > > > > > further > > > > > restrict capabilities, not grant them beyond what is already > > > > > permitted. > > > > > Unit files can be left unmodified from upstream. SELinux- > > > > > disabled > > > > > users > > > > > win. SELinux-enabled users can benefit from systemd-provided > > > > > protections > > > > > and policy won't need to radically change. > > > > > > > > > > This change takes the last approach above, as it seems to be > > > > > the > > > > > best option. > > > > > > > > > > Signed-off-by: Stephen Smalley <s...@tycho.nsa.gov> > > > > > --- > > > > > security/selinux/hooks.c | 41 > > > > > ++++++++++++++++++++++++--- > > > > > ---------- > > > > > security/selinux/include/classmap.h | 2 +- > > > > > security/selinux/include/security.h | 2 ++ > > > > > security/selinux/ss/services.c | 7 ++++++- > > > > > 4 files changed, 36 insertions(+), 16 deletions(-) > > > > > > > > > > diff --git a/security/selinux/hooks.c > > > > > b/security/selinux/hooks.c > > > > > index 3a06afb..f0c11c2 100644 > > > > > --- a/security/selinux/hooks.c > > > > > +++ b/security/selinux/hooks.c > > > > > @@ -2326,24 +2326,37 @@ static int check_nnp_nosuid(const > > > > > struct > > > > > linux_binprm *bprm, > > > > > return 0; /* No change in credentials */ > > > > > > > > > > /* > > > > > - * The only transitions we permit under NNP or nosuid > > > > > - * are transitions to bounded SIDs, i.e. SIDs that are > > > > > - * guaranteed to only be allowed a subset of the > > > > > permissions > > > > > - * of the current SID. > > > > > + * If the policy enables the nnp_transition policy > > > > > capability, > > > > > + * then we permit transitions under NNP or nosuid if > > > > > the > > > > > + * policy explicitly allows nnp_transition permission > > > > > between > > > > > + * the old and new contexts. > > > > > */ > > > > > - rc = security_bounded_transition(old_tsec->sid, > > > > > new_tsec- > > > > > > sid); > > > > > > > > > > - if (rc) { > > > > > + if (selinux_policycap_nnptransition) { > > > > > + rc = avc_has_perm(old_tsec->sid, new_tsec- > > > > > >sid, > > > > > + SECCLASS_PROCESS, > > > > > + PROCESS__NNP_TRANSITION, > > > > > NULL); > > > > > + if (!rc) > > > > > + return 0; > > > > > + } else { > > > > > /* > > > > > - * On failure, preserve the errno values for > > > > > NNP vs > > > > > nosuid. > > > > > - * NNP: Operation not permitted for caller. > > > > > - * nosuid: Permission denied to file. > > > > > + * Otherwise, the only transitions we permit > > > > > under > > > > > NNP or nosuid > > > > > + * are transitions to bounded SIDs, i.e. SIDs > > > > > that > > > > > are > > > > > + * guaranteed to only be allowed a subset of > > > > > the > > > > > permissions > > > > > + * of the current SID. > > > > > */ > > > > > - if (nnp) > > > > > - return -EPERM; > > > > > - else > > > > > - return -EACCES; > > > > > + rc = security_bounded_transition(old_tsec- > > > > > >sid, > > > > > new_tsec->sid); > > > > > + if (!rc) > > > > > + return 0; > > > > > > > > NB: As currently written, this logic means that if you enable the > > > > new > > > > policy capability, the only way to allow a transition under NNP > > > > is by > > > > allowing nnp_transition permission between the old and new > > > > domains. The > > > > alternative would be to fall back to checking for a bounded SID > > > > if > > > > nnp_transition permission is not allowed. The former approach has > > > > the > > > > advantage of being simpler (only a single check with a single > > > > audit > > > > message), but means that you can't mix usage of bounded SIDs and > > > > nnp_transition permission as a means of allowing a transitions > > > > under > > > > NNP within the same policy. The latter approach provides more > > > > flexibility / compatibility but will end up producing two audit > > > > messages in the case where the transition is denied by both > > > > checks: an > > > > AVC message for the permission denial and a SELINUX_ERR message > > > > for the > > > > security_bounded_transition failure, which might be confusing to > > > > users > > > > (but probably not; they'll likely just feed the AVC through > > > > audit2allow > > > > and be done with it). Thoughts? > > > > > > I think the current implementation is fine if i understand > > > correctly: > > > > > > 1. With the policy capability disabled the behavior doesnt change, > > > so if you dont want the current behavior (type_bounds) then just to > > > enable the polcap > > > 2. If you enable the policy capability then you decided to leverage > > > nnp_transition instead of the current behavior > > > > > > Theres plenty flexibility with this approach i would argue > > > > Hmm. that came out wrong: > > > > 1. without nnptransition polcap: everything stays the same > > 2. with nnptransition polcap: you choose nnp_transition over current > > type_bounds behavior > > Yes, that's correct. It is somewhat limiting in that if you want to > leverage nnp_transition at all, you have to give up using typebounds > entirely for allowing NNP transitions. So, let's say there is an > existing policy module that defines a typebounds in order to allow a > NNP transition, and then another policy module decides to enable the > policy capability and leverage nnp_transition permission to allow > another NNP transition that wouldn't work under typebounds. The first > one will break under the former approach, while it would keep working > under the latter approach. Not sure if that's a real concern or just a > contrived one. Let's say someone writes a third party policy module > using typebounds for this purpose on Fedora today, and then updates to > a new version of Fedora that enables the policy capability and > leverages nnp_transition in its policy to allow such transitions. Now > that third party policy module will stop working (it would need to be > changed to allow nnp_transition explicitly).
Would this affect the requirement of typebounds by for example mod_selinux? I think that apache module also requires typebounds but not for NNP AFAIK (that stuff predates it i believe) I prefer this implementation if this implementation is reasonably possible. I dont believe that there are any third party modules that support NNP (its too un-usable and hard to write) I wont be leveraging nnp_transition or type_bounds for NNP BTW. I will just contrinue removing the NNP= for systemd service units. > > > > > > > > > > > > > > > } > > > > > - return 0; > > > > > + > > > > > + /* > > > > > + * On failure, preserve the errno values for NNP vs > > > > > nosuid. > > > > > + * NNP: Operation not permitted for caller. > > > > > + * nosuid: Permission denied to file. > > > > > + */ > > > > > + if (nnp) > > > > > + return -EPERM; > > > > > + return -EACCES; > > > > > } > > > > > > > > > > static int selinux_bprm_set_creds(struct linux_binprm *bprm) > > > > > diff --git a/security/selinux/include/classmap.h > > > > > b/security/selinux/include/classmap.h > > > > > index b9fe343..7fde56d 100644 > > > > > --- a/security/selinux/include/classmap.h > > > > > +++ b/security/selinux/include/classmap.h > > > > > @@ -47,7 +47,7 @@ struct security_class_mapping secclass_map[] > > > > > = { > > > > > "getattr", "setexec", "setfscreate", "noatsecure", > > > > > "siginh", > > > > > "setrlimit", "rlimitinh", "dyntransition", > > > > > "setcurrent", > > > > > "execmem", "execstack", "execheap", > > > > > "setkeycreate", > > > > > - "setsockcreate", "getrlimit", NULL } }, > > > > > + "setsockcreate", "getrlimit", "nnp_transition", > > > > > NULL } > > > > > }, > > > > > { "system", > > > > > { "ipc_info", "syslog_read", "syslog_mod", > > > > > "syslog_console", "module_request", "module_load", > > > > > NULL > > > > > } }, > > > > > diff --git a/security/selinux/include/security.h > > > > > b/security/selinux/include/security.h > > > > > index e91f08c..88efb1b 100644 > > > > > --- a/security/selinux/include/security.h > > > > > +++ b/security/selinux/include/security.h > > > > > @@ -73,6 +73,7 @@ enum { > > > > > POLICYDB_CAPABILITY_EXTSOCKCLASS, > > > > > POLICYDB_CAPABILITY_ALWAYSNETWORK, > > > > > POLICYDB_CAPABILITY_CGROUPSECLABEL, > > > > > + POLICYDB_CAPABILITY_NNPTRANSITION, > > > > > __POLICYDB_CAPABILITY_MAX > > > > > }; > > > > > #define POLICYDB_CAPABILITY_MAX (__POLICYDB_CAPABILITY_MAX - > > > > > 1) > > > > > @@ -84,6 +85,7 @@ extern int selinux_policycap_openperm; > > > > > extern int selinux_policycap_extsockclass; > > > > > extern int selinux_policycap_alwaysnetwork; > > > > > extern int selinux_policycap_cgroupseclabel; > > > > > +extern int selinux_policycap_nnptransition; > > > > > > > > > > /* > > > > > * type_datum properties > > > > > diff --git a/security/selinux/ss/services.c > > > > > b/security/selinux/ss/services.c > > > > > index 2f02fa6..2faf47a 100644 > > > > > --- a/security/selinux/ss/services.c > > > > > +++ b/security/selinux/ss/services.c > > > > > @@ -76,7 +76,8 @@ char > > > > > *selinux_policycap_names[__POLICYDB_CAPABILITY_MAX] = { > > > > > "open_perms", > > > > > "extended_socket_class", > > > > > "always_check_network", > > > > > - "cgroup_seclabel" > > > > > + "cgroup_seclabel", > > > > > + "nnp_transition" > > > > > }; > > > > > > > > > > int selinux_policycap_netpeer; > > > > > @@ -84,6 +85,7 @@ int selinux_policycap_openperm; > > > > > int selinux_policycap_extsockclass; > > > > > int selinux_policycap_alwaysnetwork; > > > > > int selinux_policycap_cgroupseclabel; > > > > > +int selinux_policycap_nnptransition; > > > > > > > > > > static DEFINE_RWLOCK(policy_rwlock); > > > > > > > > > > @@ -2009,6 +2011,9 @@ static void > > > > > security_load_policycaps(void) > > > > > selinux_policycap_cgroupseclabel = > > > > > ebitmap_get_bit(&policydb.policycaps, > > > > > POLICYDB_CAPABILITY_CGROUPSECL > > > > > ABEL); > > > > > + selinux_policycap_nnptransition = > > > > > + ebitmap_get_bit(&policydb.policycaps, > > > > > + POLICYDB_CAPABILITY_NNPTRANSIT > > > > > ION); > > > > > > > > > > for (i = 0; i < ARRAY_SIZE(selinux_policycap_names); > > > > > i++) > > > > > pr_info("SELinux: policy capability %s=%d\n", > > > > > > -- > > > Key fingerprint = 5F4D 3CDB D3F8 3652 FBD8 02D5 3B6C 5F1D 2C7B > > > 6B02 > > > https://sks-keyservers.net/pks/lookup?op=get&search=0x3B6C5F1D2C7B6 > > > B02 > > > Dominick Grift > > > > > > -- Key fingerprint = 5F4D 3CDB D3F8 3652 FBD8 02D5 3B6C 5F1D 2C7B 6B02 https://sks-keyservers.net/pks/lookup?op=get&search=0x3B6C5F1D2C7B6B02 Dominick Grift
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