Re: RFC PATCH: apply security_syslog() only to the syslog() syscall, not to /proc/kmsg

[Stephen Smalley <sds@tycho.nsa.gov>]

On Thu, 2006-11-09 at 08:08 -0800, Zack Weinberg wrote:
> [some of you have seen this already in off-list discussion, but it
> really should have gone to the list, apologies for the noise]
> 
> On 11/9/06, Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > On Wed, 2006-11-08 at 20:14 -0800, Chris Wright wrote:
> > > * Sergey Vlasov (vsu@altlinux.ru) wrote:
> > > > Then what would you think about another solution:
> > > >
> > > >  1) When sys_syslog() is called with commands 2 (read) or 9 (get unread
> > > >     count), additionally call security_syslog(1) to check that the
> > > >     process has permissions to open the kernel log.  This change by
> > > >     itself will not make any difference, because all existing
> > > >     implementations of the security_ops->syslog hook treat the operation
> > > >     codes 1, 2 and 9 the same way.
> > > >
> > > >  2) Change cap_syslog() and dummy_syslog() to permit commands 2 and 9
> > > >     for unprivileged users, in addition to 3 and 10 which are currently
> > > >     permitted.  This will not really permit access through sys_syslog()
> > > >     due to the added security_syslog(1) check, but if a process somehow
> > > >     got access to an open file descriptor for /proc/kmsg, it would be
> > > >     able to read from it.  Also, because selinux_syslog() is not
> > > >     changed, under SELinux the process will still need to have
> > > >     additional privileges even if it has /proc/kmsg open.
> > >
> > > It's a bit clumsy in the extra caveats for sys_syslog and cap_syslog,
> > > but does achieve what you're after.  We lose default checking on the
> > > actual read access, but perhaps this is a fair tradeoff.  Stephen,
> > > James do you have any issues with this for SELinux?
> >
> > It makes the already unfortunate coupling between the security modules
> > and the core syslog code even worse, by making assumptions about how the
> > security modules treat different type codes.  If you were to go down
> > this route, I think you would want to map the type codes to abstract
> > permissions in the core syslog code and only pass the abstract
> > permissions to the security modules (so that they no longer see 2, 9,
> > and 1 separately but as a single permission).  Might be nice to have
> > actual #define's for the type codes too.
> >
> > We wouldn't want the SELinux checking changed; we can already run klogd
> > confined by policy, and decomposition into privileged and unprivileged
> > components is preferable to privilege bracketing within a single
> > component.
> 
> I've been working on an alternate patch.  It is not ready yet, but
> tell me how this sounds:
> 
> 1) Move the security check from do_syslog to sys_syslog, as in my
> original patch.
> 2) Add a call to security_syslog() to kmsg_open.
> 3) sys_syslog() operations 0 and 1 ("close" and "open" respectively -
> both do precisely nothing) cease to call the security hook.  (This
> part is so that a security module knows that a call of its ->syslog
> hook with code 1 is an attempt to open /proc/kmsg, not a no-op
> syslog(1, 0, 0).  Security modules never see code 0, on the principle
> that close shouldn't ever fail.)
> 
> -- At this point the user visible semantics (in the default security
> framework) are that a process without CAP_SYS_ADMIN cannot open
> /proc/kmsg and cannot call any of the syslog() interfaces that
> actually do stuff [except the one "dmesg" uses].  However, a process
> without CAP_SYS_ADMIN will be able to issue a read() on a fd open on
> /proc/kmsg if it somehow gets hold of one.  (This is a tidier way of
> getting the semantics that Sergey's patches provide.)
> 
> Then I also want to have:
> 
> 4) Change the default security policy modules (commoncap.c and
> dummy.c) so that a process without CAP_SYS_ADMIN is allowed to open
> /proc/kmsg provided that DAC allows it (i.e. provided that root has
> gone and chowned it to the appropriate uid).  No change to SELinux is
> required.
> 
> Part 4 allows me to run an *unmodified* klogd as an unprivileged user,
> and just chown /proc/kmsg in the init script, rather than having to
> modify klogd to open /proc/kmsg and then drop privileges.  (A SELinux
> environment still requires modifications to klogd, but we were going
> to have to do that anyway.)
> 
> [Possibly as long as we are futzing with syslog(), we ought to
> consider renaming it to match glibc [which calls it klogctl()] and
> defining some symbolic constants for all those opcode numbers.  Also I
> really don't like the looks of the code for operations 3 and 4.  But
> this is all tangential.]
> 
> [re Stephen's comment: I don't like the idea of mapping type codes to
> abstract permissions in the core; that seems like more of a layering
> violation than what we have now.  Surely it is the core's business to
> say _what_ it is about to do, and the security module's to decide what
> that means in terms of its privilege model.  I don't propose to change
> SELinux, except insofar as (in my above plan) it would no longer see
> certain operations on the syscall interface that don't do anything
> anyway.]

Unless I missed something, your plan above would disable SELinux
syslog-related permission checking upon reads of a previously opened
file descriptor to /proc/kmsg.  So it would change SELinux behavior in a
way that is directly contrary to the notion of mandatory access control.

Part 4 appears to further expose /proc/kmsg to access by any uid 0
process even if it has no capabilities (think privilege shedding or
containers).

I think the function name is immaterial, but having symbolic constants
for the type codes would be helpful if we have to refer to them in the
security modules directly.  But having a mapping in the core to a much
smaller set of permissions would be even better, and help with
maintenance; the next time someone added a new code, they would more
likely see the mapping table in the core and update it than go digging
into the individual security modules.

-- 
Stephen Smalley
National Security Agency

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