Re: [PATCH bpf-next v7 1/3] bpf: Allow trusted pointers to be passed to KF_TRUSTED_ARGS kfuncs

[Alexei Starovoitov <alexei.starovoitov@gmail.com>]

On Fri, Nov 18, 2022 at 11:14:03PM -0600, David Vernet wrote:
> On Fri, Nov 18, 2022 at 08:13:37PM -0800, Alexei Starovoitov wrote:
> > On Fri, Nov 18, 2022 at 03:44:42PM -0600, David Vernet wrote:
> > > > > if it's a release arg it should always have a refcount on it.
> > > > > PTR_UNTRUSTED | PTR_TRUSTED would also make no sense. MEM_FIXED_SIZE
> > > > > though seems fine? In general, I thought it was prudent for us to take
> > > > > the most conservative possible approach here, which is that PTR_TRUSTED
> > > > > only applies when no other modifiers are present, and it applies for all
> > > > > obj_ptr types (other than PTR_TO_CTX which does its own thing).
> > > > 
> > > > Probably worth refining when PTR_TRUSTED is cleared.
> > > > For example adding PTR_UNTRUSTED should definitely clear it.
> 
> 
> 
> > > 
> > > That makes sense for PTR_UNTRUSTED, what about the other type modifiers
> > > like PTR_MAYBE_NULL? We set and unset if a ptr is NULL throughout a
> > > function, so we'd have to record if it was previously trusted in order
> > > to properly re-OR after a NULL check.
> > 
> > PTR_MAYBE_NULL is another bit and I don't think it conflicts with PTR_TRUSTED.
> > PTR_TO_BTF_ID | PTR_TRUSTED is a valid pointer.
> > PTR_TO_BTF_ID | PTR_TRUSTED | PTR_MAYBE_NULL is a valid pointer or NULL.
> > 
> > PTR_TO_BTF_ID | PTR_MAYBE_NULL is a legacy "valid pointer" or NULL.
> > That legacy pointer cannot be passed to KF_TRUSTED_ARGS kfuncs.
> > 
> > KF_TRUSTED_ARGS kfuncs should not accept PTR_TO_BTF_ID | PTR_TRUSTED | PTR_MAYBE_NULL.
> 
> Indeed -- my point was that I don't think e.g. clearing PTR_TRUSTED when
> we set PTR_UNTRUSTED will work, at least not yet. It's still too tricky
> to find all the places where we'd have to &= ~PTR_TRUSTED or |=
> PTR_TRUSTED when setting specific type modifiers. As described below, we
> first have to clarify the general workflow to enable the presence of
> PTR_TRUSTED to be the single source of truth for trust.

Agree. A reg->type with both PTR_TRUSTED and PTR_UNTRUSTED would be a bug,
but let's fix it when we get there.
Even if such bug hits us we can protect from it by make sure that
we treat PTR_UNTRUSTED as logically stronger flag.

> > It's a job of the prog to do != NULL check.
> > Otherwise all such != NULL checks would need to move inside kfuncs which is not good.
> > 
> > > > MEM_ALLOC flag is probably equivalent to PTR_TRUSTED.
> > > > Maybe the bit:
> > > > regs[BPF_REG_0].type = PTR_TO_BTF_ID | MEM_ALLOC;
> > > > should set PTR_TRUSTED as well?
> > > 
> > > We could, but that changes the meaning of PTR_TRUSTED and IMO makes it
> > > harder to reason about. Before it was just "the kernel passed this arg
> > > to the program and promises the program that it was trusted when it was
> > > first passed". Now it's that plus it could mean that it points to an
> > > allocated object from bpf_obj_new()". IMO we should keep all of these
> > > modifiers separate so that the presence of a modifier has a well-defined
> > > meaning that we can interpret in each context as needed.  In this case,
> > > we can make trust opt-in, so a KF_TRUSTED_ARGS BTF pointer either of the
> > > following:
> > > 
> > > 1. reg->ref_obj_id > 0
> > > 2. Either one of PTR_TRUSTED | MEM_ALLOC type modifiers are set, and no
> > >    others.
> > 
> > I don't think MEM_ALLOC conflicts with PTR_TRUSTED.
> > MEM_ALLOC flags means that it came from bpf_obj_new() and that's what
> > bpf_spin_lock and bpf_obj_drop() want to see.
> > 
> > Adding PTR_TRUSTED to MEM_ALLOC looks necessary to me.
> > It doesn't have to be done right now, but eventually feels right.
> 
> I think I agree. MEM_ALLOC should always imply PTR_TRUSTED. Ideally we
> shouldn't have to check MEM_ALLOC for KF_TRUSTED_ARGS at all, and
> PTR_TRUSTED should be the only modifier representing if something is
> safe. 

exactly.

> For now I'd prefer to keep them separate until we have a clear
> plan, especially with respect to clearing PTR_TRUSTED for when something
> unsafe happens like PTR_UNTRUSTED or PTR_MAYBE_NULL. It's all too
> muddied still.

sure. we can do that in the follow up.
A bit more technical debt to address later.

> 
> > I've been thinking whether reg->ref_obj_id > 0 condition should be converted
> > to PTR_TRUSTED too...
> > On one side it will simplify the check for KF_TRUSTED_ARGS.
> > The only thing check_kfunc_args() would need to do is:
> > is_kfunc_trusted_args(meta)
> > && type_flag(reg->type) & PTR_TRUSTED
> > && !(type_flag(reg->type) & PTR_MAYBE_NULL)
> > 
> > On the other side fixing all places where we set ref_obj_id
> > and adding |= PTR_TRUSTED may be too cumbersome ?
> 
> I think it's probably too cumbersome now, but yeah, as mentioned above,
> I think it's the right direction. I think it will require a lot of
> thought to do it right, though. With the code the way that it is now, I
> can't convince myself that we wouldn't do something like |= PTR_TRUSTED
> when we observe ref_obj_id > 0, and then later &= ~PTR_TRUSTED when
> setting PTR_MAYBE_NULL. I think Kumar's latest patch set is a nice step
> towards achieving this clearer state. Hopefully we can continue to
> improve.
> 
> > Right now we're saying PTR_TO_CTX is implicitly trusted, but we can OR
> > PTR_TO_CTX with PTR_TRUSTED to make it explicit and truly generalize the check.
> 
> Further agreed, this is the correct longer-term direction.
> 
> > > Agreed that after the rebase this would no longer be correct. I think we
> > > should make it opt-in, though. PTR_TRUSTED | MEM_ALLOC is fine.
> > > PTR_TRUSTED | MEM_ALLOC | PTR_MAYBE_NULL would not be.
> > 
> > to pass into KF_TRUSTED_ARGS kfunc? Agree.
> > I guess we can tighten the check a bit:
> > is_kfunc_trusted_args(meta)
> > && type_flag(reg->type) & PTR_TRUSTED
> > && !(type_flag(reg->type) & ~(PTR_TRUSTED | MEM_ALLOC))
> > 
> > In english: the pointer should have PTR_TRUSTED flag and
> > no other flags other than PTR_TRUSTED and MEM_ALLOC should be set.
> 
> Yeah, I think this is the correct way to model this for now. Later on
> once we refactor things, the presence of PTR_TRUSTED on its own should
> be sufficient. A good north star to aim towards.
> 
> I'll send this out as v8 tomorrow.

Perfect. Looking forward.