Dbus and multiple LSMs (was Preferred subj= with multiple LSMs)

Thanks for considering user-space in this, and sorry if I'm hijacking this thread a bit (but I think some of the things I'm raising might be equally applicable for audit subjects).

On Fri, 19 Jul 2019 at 09:29:17 -0700, Casey Schaufler wrote: > On 7/19/2019 5:15 AM, Simon McVittie wrote: >> However, I think it would be great to have multiple-"big"-LSM-aware >> replacements for those interfaces, which present the various LSMs as >> multiple parallel credentials. > Defining what would go into liblsm* is a task that has fallen to > the chicken/egg paradox. We can't really define how the user-space > should work without knowing how the kernel will work, and we can't > solidify how the kernel will work until we know what user-space > can use. I was hoping the syscall wrappers in glibc would be a viable user-space interface to the small amount of LSM stuff that dbus needs to use in an LSM-agnostic way. That's what we use in dbus at the moment (in practice just getsockopt, but I'd also be reading /proc/self/attr/current if there was a specification for how to normalize it to match SO_PEERSEC results) and it's no harder than the rest of the syscall-level APIs.

A single LSM-agnostic shared library would be the next best thing from my point of view.

> An option that hasn't been discussed is a display option to provide > the Hideous format for applications that know that's what they want. > Write "hideous" into /proc/self/attr/display, and from then on you > get selinux='a:b:c:d',apparmor='z'. This could be used widely in liblsm > interfaces. If the way to parse/split it is documented, then this would be easier for dbus-daemon than continually resetting attr/display. It would be especially good if you can document a way to find out which one of the many labels would have been seen by an older user-space process that never wrote to attr/display ("it's the first one in the list" would be fine), so that we can put that one in our backwards-compatible API to clients.

Or, alternatively, we could pass it on directly to our clients and let *them* parse it (possibly by using liblsm), the same way AppArmor-aware D-Bus clients have to know how to use either aa_splitcon() or their own parsing to go from the raw SO_PEERSEC result "/usr/bin/firefox (enforce)" to the pair ("/usr/bin/firefox", "enforce") that they probably actually wanted. >> Do you mean that if process 11111 writes (for example) "apparmor" into >> /proc/11111/attr/display, and then reads /proc/22222/attr/current >> or queries the SO_PEERSEC of a socket opened by process 22222, >> it will specifically see 22222's AppArmor label and not 22222's SELinux >> label? > Process 11111 would see the AppArmor label when reading > /proc/22222/attr/current. The display value is controlled > by process 11111 so that it can control what data it wants > to see. OK, that's what I'd hoped. > The display is set at the task level, so should be thread safe. OK, good. However, thinking more about this, I have other concerns: * In library code that can be used by a thread (task) that also uses other arbitrary libraries, or in an executable that uses libraries that might be interested in LSMs, the only safe way to deal with attr/display would be this sequence: - write desired value to /proc/self/attr/display - immediately read /proc/other/attr/current or query SO_PEERSEC and it would not be safe to rely on writing /proc/self/attr/display just once at startup, because some other library might have already changed it between startup and the actual read. Paradoxically, this maximizes the chance of breaking a reader that was relying on writing /proc/self/attr/display once during startup. * If an async signal handler needs to know a LSM label for whatever reason, it will break anything in the same thread that was relying on that sequence, because it might have interrupted them between their write and their read: main execution path signal handler ------------------- -------------- write "apparmor" to attr/display (interrupted by async signal) write "selinux" to attr/display read attr/current or SO_PEERSEC do other stuff with SELinux label return (resumes) read attr/current or SO_PEERSEC expect an AppArmor label get a SELinux label sadness ensues Of course it's probably crazy for an async signal handler to do this... but people do lots of odd things in async signal handlers, and open(), read(), write(), getsockopt() are all async-signal-safe functions, so it's at least arguably valid.

> Writing to display does not require privilege, as it affects only > the current process. The display is inherited on fork and reset on > a privileged exec. Another concern here: are you sure it shouldn't be reset on *any* exec?

Lots of programs (including dbus-daemon) fork-and-exec arbitrary child processes that come from a different codebase not under our control and aren't necessarily LSM-stacking-aware. I don't really want to have to reset /proc/self/attr/display in our increasingly crowded after-fork-but-before-exec code path (which, according to POSIX, is not a safe place to invoke any non-async-signal-safe function, so we can't easily do error handling if something goes wrong there).

Is there any possibility of having a parallel kernel API that, if it exists, always returns the whole stack, maybe something like /proc/<pid>/attr/current_stack and the SO_PEERSECLABELS that I suggested previously,

instead of repurposing /proc/<pid>/attr/current and SO_PEERSEC to have contents that vary according to ambient process state in their reader?

(Bonus points if they are documented/defined with a particular syntactic normalization this time, unlike the situation with /proc/<pid>/attr/current and SO_PEERSEC where in principle you need LSM-specific knowledge to know whether a trailing "\n" or "\0" is safe to discard.)

smcv