Re: seccomp feature development

[Christian Brauner]

On Mon, May 18, 2020 at 02:04:57PM -0700, Kees Cook wrote:
> Hi!
> 
> This is my attempt at a brain-dump on my plans for nearish-term seccomp
> features. Welcome to my TED talk... ;)
> 
> These are the things I've been thinking about:
> 
> - fd passing
> - deep argument inspection
> - changing structure sizes
> - syscall bitmasks
> 
> So, diving right in:
> 
> 
> ## fd passing
> 
> Background: seccomp users want to be able to install an fd in a
> monitored process during a user_notif to emulate "open" calls (or
> similar), possibly across security boundaries, etc.
> 
> On the fd passing front, it seems that gaining pidfd_addfd() is the way
> to go as it allows for generic use not tied to seccomp in particular.
> I expect this feature will be developed orthogonally to seccomp (where
> does this stand, BTW?). However, as Sargun has shown[1], seccomp could
> be friendlier to help with using it. Things that need to be resolved:
> 
> - report pidnr, or pidfd? It seems the consensus is to pass pidnr, but
>   if we're going to step back and make some design choices here, is
>   there a place for pidfds in seccomp user_notif, in order to avoid
>   needing the user_notif cookie? I think probably not: it's a rather lot
>   of overhead for notifications. It seems it's safe to perform an fd
>   installation with these steps:
> 	- get pidnr from user_notif_recv
> 	- open pidfd from pidnr
> 	- re-verify user_notif cookie is still valid
> 	- send new fd via pidfd
> 	- reply with user_notif_send
> 	- close pidfd

Yes, that sounds good and should be sufficient for all users.

I don't want to have a range of interfaces being able to create pidfds
as this might make it more difficult when we want to implement new
features on top of pidfds (privilege delegation etc., pidfds for
specific threads).

> 
> - how to deal with changing sizes of the user_notif structures to
>   include a pidnr. (Which will be its own topic below.)
> 
> 
> ## deep argument inspection
> 
> Background: seccomp users would like to write filters that traverse
> the user pointers passed into many syscalls, but seccomp can't do this
> dereference for a variety of reasons (mostly involving race conditions and
> rearchitecting the entire kernel syscall and copy_from_user() code flows).
> 
> During the last plumbers and in conversations since, the grudging
> consensus was reached that having seccomp do this for ALL syscalls was
> likely going to be extremely disruptive for very little gain (i.e.
> many things, like pathnames, have differing lifetimes, aliases, unstable
> kernel object references, etc[6]), but that there were a small subset of
> syscalls for which this WOULD be beneficial, and those are the newly
> created "Extensible Argument" syscalls (is there a better name for this
> design? I'm calling it "EA" for the rest of the email), like clone3(),

esyscalls? :)

> openat2(), etc, which pass a pointer and a size:
> 
> long clone3(struct clone_args *cl_args, size_t size);
> 
> I think it should be possible to extend seccomp to examine this structure
> by appending it to seccomp_data, and allowing filters to examine the
> contents. This means that no BPF language extensions are required for

Whatever design go with ultimately should probably also be able to
handle pointers within the struct as that's just something that's going
to happen no matter how much we fight it, i.e.:

foo(struct a, size_t size_a)

struct a {
	int first;
	int second;
	struct *b;
	size_t size_b;
};

will likely be inevitable.

> seccomp, as I'd still prefer to avoid making the eBPF jump (I don't think
> seccomp's design principles work well with maps, kernel helpers, etc,
> and I think the earlier the examination of using eBPF for user_notif
> bares this out).

Agreed. I would like this to minimally disruptive for userspace so
various projects can simply adapt openat2() and clone3() without having
to either opt for a new LSM-type thing or ebpf. Ideally it'll feel very
natural to extend their seccomp filters to handle these esyscalls.

> 
> In order for this to work, there are a number of prerequisites:
> 
> - argument caching, in two halves: syscall side and seccomp side:
>   - the EA syscalls needs to include awareness of potential seccomp
>     hooking. i.e. seccomp may have done the copy_from_user() already and
>     kept a cached copy.
>   - seccomp needs to potentially DO the copy_from_user() itself when it
>     hits these syscalls for a given filter, and put it somewhere for
>     later use by the syscall.
> - the sizes of these EA structs are, by design, growable over time.
>   seccomp and its users need to be handle this in a forward and backward
>   compatible way, similar to the design of the EA syscall interface
>   itself.

We could provide a wrapper similar to SYSCALL_DEFINE*(). So we'd have -
idk - ESYSCALL_DEFINE*(foo, T1, T1_NAME, T2, T2_NAME, <foo_copy_from_user>)
Each such syscall that wants to do
ESYSCALL_DEFINE*() would need to define copy-from-user function for it's
struct argument that seccomp can call to get a full cached copy of the
arguments. So the ESYSCALL_DEFINE*() would register the copy function
with seccomp so seccomp knows what it needs to call. This way seccomp
doesn't need to worry about what the function does or always be careful
to catch up with extensions to these syscalls.

> 
> The argument caching bit is, I think, rather mechanical in nature since
> it's all "just" internal to the kernel: seccomp can likely adjust how it
> allocates seccomp_data (maybe going so far as to have it split across two
> pages with the syscall argument struct always starting on the 2nd page
> boundary), and copying the EA struct into that page, which will be both
> used by the filter and by the syscall. I imagine state tracking ("is
> there a cached EA?", "what is the address of seccomp_data?", "what is
> the address of the EA?") can be associated with the thread struct.

thread_struct or task_struct? The latter feels like it would make more
sense.

> 
> The growing size of the EA struct will need some API design. For filters
> to operate on the contiguous seccomp_data+EA struct, the filter will
> need to know how large seccomp_data is (more on this later), and how
> large the EA struct is. When the filter is written in userspace, it can
> do the math, point into the expected offsets, and get what it needs. For
> this to work correctly in the kernel, though, the seccomp BPF verifier
> needs to know the size of the EA struct as well, so it can correctly
> perform the offset checking (as it currently does for just the
> seccomp_data struct size).
> 
> Since there is not really any caller-based "seccomp state" associated
> across seccomp(2) calls, I don't think we can add a new command to tell
> the kernel "I'm expecting the EA struct size to be $foo bytes", since
> the kernel doesn't track who "I" is besides just being "current", which
> doesn't take into account the thread lifetime -- if a process launcher
> knows about one size and the child knows about another, things will get
> confused. The sizes really are just associated with individual filters,
> based on the syscalls they're examining. So, I have thoughts on possible
> solutions:
> 
> - create a new seccomp command SECCOMP_SET_MODE_FILTER2 which uses the
>   EA style so we can pass in more than a filter and include also an
>   array of syscall to size mappings. (I don't like this...)
> - create a new filter flag, SECCOMP_FILTER_FLAG_EXTENSIBLE, which changes
>   the meaning of the uarg from "filter" to a EA-style structure with
>   sizes and pointers to the filter and an array of syscall to size
>   mappings. (I like this slightly better, but I still don't like it.)
> - leverage the EA design and just accept anything <= PAGE_SIZE, record
>   the "max offset" value seen during filter verification, and zero-fill
>   the EA struct with zeros to that size when constructing the
>   seccomp_data + EA struct that the filter will examine. Then the seccomp
>   filter doesn't care what any of the sizes are, and userspace doesn't
>   care what any of the sizes are. (I like this as it makes the problems
>   to solve contained entirely by the seccomp infrastructure and does not
>   touch user API, but I worry I'm missing some gotcha I haven't
>   considered.)

This sounds like the sanest option and again, the least invasive one to
userspace.
I think this just needs to take care to adhere to the current convention
that we established that passing a larger struct than the kernel/seccomp
knows about is fine, as long as the unknown bits are zeroed. For
example, I currently use clone3() like this in LXC on any kernel:

struct lxc_clone_args {
	__aligned_u64 flags;
	__aligned_u64 pidfd;
	__aligned_u64 child_tid;
	__aligned_u64 parent_tid;
	__aligned_u64 exit_signal;
	__aligned_u64 stack;
	__aligned_u64 stack_size;
	__aligned_u64 tls;
	__aligned_u64 set_tid;
	__aligned_u64 set_tid_size;
	__aligned_u64 cgroup;
};

__returns_twice static inline pid_t lxc_clone3(struct lxc_clone_args *args, size_t size)
{
	/* COW stack, don't even think about passing CLONE_VM. */
	if (flags & (CLONE_VM | CLONE_PARENT_SETTID | CLONE_CHILD_SETTID |
		     CLONE_CHILD_CLEARTID | CLONE_SETTLS))
		return -EINVAL;

	return syscall(__NR_clone3, args, size);
}

and there's not even a kernel out with the CLONE_INTO_CGROUP and thus
the .cgroup member in there and things just work fine (TM). I think
seccomp needs to make sure to not reject syscalls with larger but
zeroed structs.

> 
> And then, my age-old concern, that maybe doesn't need a solution... I
> remain plagued by the lack of pathname inspection. But I think the
> ToCToU nature of it means we just cannot do it from seccomp. It does
> make filtering openat2()'s EA struct a bit funny... a filter has no idea
> what path it applies to... but that doesn't matter because the object
> the path points to might change[6] during the syscall. Argh.

Also, people who need to filter open*() now will already need to make
use of LSMs and I think for this specific case it's fine to burden them
with this. So I suggest we simply shrug our shoulders and move on.
The other way of putting this is, openat2() would be a full replacement
for open*() as soon as it's flags argument in the struct can be
filtered, I think and so it's not that we're loosing any feature that
currently is supported. The path-based filtering would be the cherry on
top and as you say it's doubtful whether there's much use to it.

> 
> 
> ## changing structure sizes
> 
> Background: there have been regular needs to add things to various
> seccomp structures. Each come with their own unique pains, and solving
> this as completely as possible in a future-proof way would be very nice.
> 
> As noted in "fd passing" above, there is a desire to add some useful
> things to the user_notif struct (e.g. thread group pid). Similarly,
> there have been requests in the past (though I can't find any good
> references right now, just my indirect comments[3]) to grow seccomp_data.
> Combined with the EA struct work above, there's a clear need for seccomp
> to reexamine how it deals with its API structures (and how this
> interacts with filters).
> 
> First, let's consider seccomp_data. If we grow it, the EA struct offset
> will move, based on the deep arg inspection design above. Alternatively,
> we could instead put seccomp_data offset 0, and EA struct at offset
> PAGE_SIZE, and treat seccomp_data itself as an EA struct where we let
> the filter access whatever it thinks is there, with it being zero-filled
> by the kernel. For any values where 0 is valid, there will just need to
> be a "is that field valid?" bit before it:

That's exactly what clone3() - and openat2() too - I think is doing.
(Case in point, CLONE_INTO_CGROUP with the .cgroup member referencing a
 file descriptor for which 0 is obviously valid.)

> 
> 	unsigned long feature_bits;
> 	unsigned long interesting_thing_1;
> 	unsigned long interesting_thing_2;
> 	unsigned long interesting_thing_3;
> 	...
> 
> and the filter would check feature_bits...
> 
> (However, this needs to be carefully considered given that seccomp_data
> is embedded in user_notif... should the EA struct from userspace also be
> copied into user_notif? More thoughts on this below...)
> 
> For user_notif, I think we need something in and around these options:
> 
> - make a new API that explicitly follows EA struct design
>   (and while read()/write() might be easier[4], I tend to agree with
>   Jann and we need to stick to ioctl(): as Tycho noted, "read/write is
>   for data". Though I wonder if read() could be used for the notifications,
>   which ARE data, and use ioctl() for the responses?)

I'm still pretty convinced that my original suggestion here to switch
user_notif over to esyscall design is the way to go and probably the
least complex.

> - make a new API that is perf_event_open()-style where fields are
>   explicitly requested, as Sargun suggested[5]. (This looks like it
>   might be complex to construct, but would get us by far the most
>   extensible API.)
> - jam whatever we pick into the existing API (we'll be forced to do
>   SOMETHING to make the old API still work, so, I dunno what that will
>   look like until we finish the rest of the design).
> 
> If we did a requested-fields approach, what would the user_notif event
> block of bytes look like? Would it be entirely dynamic based on the
> initial ioctl()? Another design consideration here is that we don't want
> the kernel doing tons of work (especially copying) and tossing tons
> of stuff into a huge structure that the user doesn't care about. In
> addition to explicit fields, maybe the EA struct could be included,
> perhaps with specified offset/size, so only the portion the user_notif
> user wanted to inspect was copied?
> 
> The complexity of the per-field API is higher, but I think it might be
> the most robust and have the greatest chance at being performant.
> For example, "send me user_notif but I only care about the pid" would
> mean no syscall arguments are copied, etc.

Before we go with such a flexible design we should ask who's going to
use it that way? At least for our use-cass we always want everything.
Maybe I'm missing other uses though. But most use-cases seem kinda
pointless, e.g. just getting the pid seems not interesting. I could see
a use-case for pid + syscall number without arguments and the notifier
always replying with _CONTINUE but even that seems a little odd to me.

> 
> 
> ## syscall bitmasks
> 
> Background: the number one bit of feedback on seccomp has been performance
> concerns, especially for fast syscalls like futex(). When looking at
> where time is spent, it is very clearly spent running the filters (which
> I found surprising, given that adding TIF_SECCOMP tended to trip the
> "slow path" syscall path (though most architectures these days just
> don't have a fast path any more thanks to Meltdown). It would be nice

Well, ia64 has fsys-syscalls :)
/me ducks

> to make filtering faster without running BPF at all. :)
> 
> Nearly every thread on adding eBPF, for example, has been about trying to
> speed up the if/then nature of BPF for finding a syscall that the filter
> wants to always accept (or reject). The bulk of most seccomp filters
> are pretty simple, usually they're either "reject everything except
> $set-of-syscalls", or "accept everything except $set-of-syscalls". The
> stuff in between tends to be a mix, like "accept $some, process $these
> with argument checks, and reject $remaining".
> 
> In all three cases, the entire seccomp() path could be sped up by having
> a syscall bitmask that could be applied before the filters are ever run,
> with 3 (actually 2) syscall bitmasks: accept, reject, or process. If
> a syscall was in the "accept" bitmask, we immediately exit seccomp and
> continue. Otherwise, if it's in the "reject" bitmask, we mark it rejected
> and immediately exit seccomp. And finally, we run the filters. In all
> ways, doing bitmask math is going to be faster than running the BPF. ;)

That sounds great!

> 
> So how would the API for this work? I have two thoughts, and I don't
> think they're exclusive:
> 
> - new API for "add this syscall to the reject bitmask". We can't really
>   do an "accept" bitmask addition without processing the attached
>   filters...
> - process attached filters! Each time a filter is added, have the
>   BPF verifier do an analysis to determine if there are any static
>   results, and set bits in the various bitmasks to represent it.
>   i.e. when seccomp is first enabled on a thread, the "accept"
>   bitmask is populated with all syscalls, and for each filter, do
>   [math,simulation,magic] and knock each syscall out of "accept" if
>   it isn't always accepted, and further see if there are any syscalls
>   that are always rejected, and mark those in the "reject" bitmask.
> 
> 
> Okay, that's all I had ... what do people think?

Great, in fact, I'd like to strongly encourage you to hand in either a
session for the container's microconference for Virtual-LPC 2020 for
this or a refereed track talk (that's less of a discussion though) for
the combined LPC/Kernel Summit track:

https://www.linuxplumbersconf.org/event/7/abstracts/

:)

(I think we're also going to want to have a talk on extensible syscall
 design. I still want to "standardize" some aspects such as type of flag
 arguments, extensible structures and so on.)

Christian