| Message ID | 355576.1587996734@warthog.procyon.org.uk (mailing list archive) |
|---|---|
| State | New |
| Headers | show |
| Series | selinux: Fix use of KEY_NEED_* instead of KEY__* perms | expand |
On Mon, Apr 27, 2020 at 10:13 AM David Howells <dhowells@redhat.com> wrote: > > Paul Moore <paul@paul-moore.com> wrote: > > > Okay, can you send the next version of the patch to the SELinux list for > > review? > > Here you go. Note that I did this a few days ago and I actually used EACCES > rather than EPERM. Which one is one preferred for this? Generally SELinux returns EACCES unless the hook normally returns EPERM (e.g. capable). Should we use a build-time or runtime guard to catch introduction of new KEY_NEED values without corresponding SELinux permissions? > > David > --- > selinux: Fix use of KEY_NEED_* instead of KEY__* perms > > selinux_key_getsecurity() is passing the KEY_NEED_* permissions to > security_sid_to_context() instead of the KEY__* values. It happens to work s/security_sid_to_context/avc_has_perm > because the values are all coincident. Shrug. That was just a requirement on key permissions when they were introduced; same is true of capabilities. Not opposed to explicitly mapping them now but it isn't really a bug. > > Fixes: d720024e94de ("[PATCH] selinux: add hooks for key subsystem") > Reported-by: Paul Moore <paul@paul-moore.com> > Signed-off-by: David Howells <dhowells@redhat.com> > --- > security/selinux/hooks.c | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-) > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index 0b4e32161b77..6087955b49d8 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -6539,20 +6539,38 @@ static void selinux_key_free(struct key *k) > kfree(ksec); > } > > +static unsigned int selinux_keyperm_to_av(unsigned int need_perm) > +{ > + switch (need_perm) { > + case KEY_NEED_VIEW: return KEY__VIEW; > + case KEY_NEED_READ: return KEY__READ; > + case KEY_NEED_WRITE: return KEY__WRITE; > + case KEY_NEED_SEARCH: return KEY__SEARCH; > + case KEY_NEED_LINK: return KEY__LINK; > + case KEY_NEED_SETATTR: return KEY__SETATTR; > + default: > + return 0; > + } > +} > + > static int selinux_key_permission(key_ref_t key_ref, > const struct cred *cred, > - unsigned perm) > + unsigned need_perm) > { > struct key *key; > struct key_security_struct *ksec; > + unsigned int perm; > u32 sid; > > /* if no specific permissions are requested, we skip the > permission check. No serious, additional covert channels > appear to be created. */ > - if (perm == 0) > + if (need_perm == 0) > return 0; > > + perm = selinux_keyperm_to_av(need_perm); > + if (perm == 0) > + return -EACCES; > sid = cred_sid(cred); > > key = key_ref_to_ptr(key_ref); >
On Mon, Apr 27, 2020 at 10:36 AM Stephen Smalley <stephen.smalley.work@gmail.com> wrote: > On Mon, Apr 27, 2020 at 10:13 AM David Howells <dhowells@redhat.com> wrote: > > > > Paul Moore <paul@paul-moore.com> wrote: > > > > > Okay, can you send the next version of the patch to the SELinux list for > > > review? > > > > Here you go. Note that I did this a few days ago and I actually used EACCES > > rather than EPERM. Which one is one preferred for this? > > Generally SELinux returns EACCES unless the hook normally returns > EPERM (e.g. capable). > Should we use a build-time or runtime guard to catch introduction of > new KEY_NEED values without corresponding SELinux > permissions? > > > > > David > > --- > > selinux: Fix use of KEY_NEED_* instead of KEY__* perms > > > > selinux_key_getsecurity() is passing the KEY_NEED_* permissions to > > security_sid_to_context() instead of the KEY__* values. It happens to work > > s/security_sid_to_context/avc_has_perm > > > because the values are all coincident. > > Shrug. That was just a requirement on key permissions when they were > introduced; same is true of capabilities. > Not opposed to explicitly mapping them now but it isn't really a bug. I haven't looked at the rest of the patch yet, but I wanted to make a quick comment on this ... over the years I've seen a number of problems that crop up because of cross-subsytem assumptions, unless there is some performance critical path where the mapping is problematic I would prefer to see a translation layer to help protect SELinux.
On Mon, Apr 27, 2020 at 10:13 AM David Howells <dhowells@redhat.com> wrote: > > Paul Moore <paul@paul-moore.com> wrote: > > > Okay, can you send the next version of the patch to the SELinux list for > > review? > > Here you go. Note that I did this a few days ago and I actually used EACCES > rather than EPERM. Which one is one preferred for this? > > David > --- > selinux: Fix use of KEY_NEED_* instead of KEY__* perms > > selinux_key_getsecurity() is passing the KEY_NEED_* permissions to > security_sid_to_context() instead of the KEY__* values. It happens to work > because the values are all coincident. Both function names in the above description are wrong. > Fixes: d720024e94de ("[PATCH] selinux: add hooks for key subsystem") > Reported-by: Paul Moore <paul@paul-moore.com> > Signed-off-by: David Howells <dhowells@redhat.com> > --- > security/selinux/hooks.c | 22 ++++++++++++++++++++-- > 1 file changed, 20 insertions(+), 2 deletions(-) > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > index 0b4e32161b77..6087955b49d8 100644 > --- a/security/selinux/hooks.c > +++ b/security/selinux/hooks.c > @@ -6539,20 +6539,38 @@ static void selinux_key_free(struct key *k) > kfree(ksec); > } > > +static unsigned int selinux_keyperm_to_av(unsigned int need_perm) > +{ > + switch (need_perm) { > + case KEY_NEED_VIEW: return KEY__VIEW; > + case KEY_NEED_READ: return KEY__READ; > + case KEY_NEED_WRITE: return KEY__WRITE; > + case KEY_NEED_SEARCH: return KEY__SEARCH; > + case KEY_NEED_LINK: return KEY__LINK; > + case KEY_NEED_SETATTR: return KEY__SETATTR; > + default: Possibly WARN() or BUG() here? Or BUILD_BUG_ON(KEY_NEED_ALL != 0x3f) to force an update here whenever a new key permission is defined? > + return 0; > + } > +} > + > static int selinux_key_permission(key_ref_t key_ref, > const struct cred *cred, > - unsigned perm) > + unsigned need_perm) > { > struct key *key; > struct key_security_struct *ksec; > + unsigned int perm; > u32 sid; > > /* if no specific permissions are requested, we skip the > permission check. No serious, additional covert channels > appear to be created. */ > - if (perm == 0) > + if (need_perm == 0) > return 0; > > + perm = selinux_keyperm_to_av(need_perm); > + if (perm == 0) > + return -EACCES; We should log or audit some kind of message here, whether via WARN(), audit_log(), or something, to avoid silent denials. > sid = cred_sid(cred); > > key = key_ref_to_ptr(key_ref); >
On Mon, Apr 27, 2020 at 1:02 PM Stephen Smalley <stephen.smalley.work@gmail.com> wrote: > On Mon, Apr 27, 2020 at 10:13 AM David Howells <dhowells@redhat.com> wrote: ... > > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c > > index 0b4e32161b77..6087955b49d8 100644 > > --- a/security/selinux/hooks.c > > +++ b/security/selinux/hooks.c > > @@ -6539,20 +6539,38 @@ static void selinux_key_free(struct key *k) > > kfree(ksec); > > } > > > > +static unsigned int selinux_keyperm_to_av(unsigned int need_perm) > > +{ > > + switch (need_perm) { > > + case KEY_NEED_VIEW: return KEY__VIEW; > > + case KEY_NEED_READ: return KEY__READ; > > + case KEY_NEED_WRITE: return KEY__WRITE; > > + case KEY_NEED_SEARCH: return KEY__SEARCH; > > + case KEY_NEED_LINK: return KEY__LINK; > > + case KEY_NEED_SETATTR: return KEY__SETATTR; > > + default: > > Possibly WARN() or BUG() here? Or BUILD_BUG_ON(KEY_NEED_ALL != 0x3f) > to force an update here > whenever a new key permission is defined? My vote is for a build time check via BUILD_BUG_ON() or similar mechanism. It's worked really well in other places, I'm thinking specifically of the SELinux netlink mapping. > > + return 0; > > + } > > +} > > + > > static int selinux_key_permission(key_ref_t key_ref, > > const struct cred *cred, > > - unsigned perm) > > + unsigned need_perm) > > { > > struct key *key; > > struct key_security_struct *ksec; > > + unsigned int perm; > > u32 sid; > > > > /* if no specific permissions are requested, we skip the > > permission check. No serious, additional covert channels > > appear to be created. */ > > - if (perm == 0) > > + if (need_perm == 0) > > return 0; > > > > + perm = selinux_keyperm_to_av(need_perm); > > + if (perm == 0) > > + return -EACCES; > > We should log or audit some kind of message here, whether via WARN(), > audit_log(), or something, to avoid silent denials. Assuming we add a build time check (see above), I think a WARN here is okay.
diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c index 0b4e32161b77..6087955b49d8 100644 --- a/security/selinux/hooks.c +++ b/security/selinux/hooks.c @@ -6539,20 +6539,38 @@ static void selinux_key_free(struct key *k) kfree(ksec); } +static unsigned int selinux_keyperm_to_av(unsigned int need_perm) +{ + switch (need_perm) { + case KEY_NEED_VIEW: return KEY__VIEW; + case KEY_NEED_READ: return KEY__READ; + case KEY_NEED_WRITE: return KEY__WRITE; + case KEY_NEED_SEARCH: return KEY__SEARCH; + case KEY_NEED_LINK: return KEY__LINK; + case KEY_NEED_SETATTR: return KEY__SETATTR; + default: + return 0; + } +} + static int selinux_key_permission(key_ref_t key_ref, const struct cred *cred, - unsigned perm) + unsigned need_perm) { struct key *key; struct key_security_struct *ksec; + unsigned int perm; u32 sid; /* if no specific permissions are requested, we skip the permission check. No serious, additional covert channels appear to be created. */ - if (perm == 0) + if (need_perm == 0) return 0; + perm = selinux_keyperm_to_av(need_perm); + if (perm == 0) + return -EACCES; sid = cred_sid(cred); key = key_ref_to_ptr(key_ref);