On Wed, Apr 17, 2024 at 5:11 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
>
>
> On 4/17/24 1:19 PM, Matt Bobrowski wrote:
> > On Mon, Apr 15, 2024 at 09:43:42AM -0700, Yonghong Song wrote:
> >> On 4/12/24 4:31 AM, Matt Bobrowski wrote:
> >>> Hi,
> >>>
> >>> Currently, if a BPF kfunc has been annotated with KF_TRUSTED_ARGS, any
> >>> supplied PTR_TO_BTF_ID | PTR_TRUSTED argument to that BPF kfunc must
> >>> have it's fixed offset set to zero, or else the BPF program being
> >>> loaded will be outright rejected by the BPF verifier.
> >>>
> >>> This non-zero fixed offset restriction in most cases makes a lot of
> >>> sense, as it's considered to be a robust means of assuring that the
> >>> supplied PTR_TO_BTF_ID to the KF_TRUSTED_ARGS annotated BPF kfunc
> >>> upholds it's PTR_TRUSTED property. However, I believe that there are
> >>> also cases out there whereby a PTR_TO_BTF_ID | PTR_TRUSTED w/ a fixed
> >>> offset can still be considered as something which posses the
> >>> PTR_TRUSTED property, and could be safely passed to a BPF kfunc that
> >>> is annotated w/ KF_TRUSTED_ARGS. I believe that this can particularly
> >>> hold true for selected embedded data structure members present within
> >>> given PTR_TO_BTF_ID | PTR_TRUSTED types i.e. struct
> >>> task_struct.thread_info, struct file.nf_path.
> >>>
> >>> Take for example the struct thread_info which is embedded within
> >>> struct task_struct. In a BPF program, if we happened to acquire a
> >>> PTR_TO_BTF_ID | PTR_TRUSTED for a struct task_struct via
> >>> bpf_get_current_task_btf(), and then constructed a pointer of type
> >>> struct thread_info which was assigned the address of the embedded
> >>> struct task_struct.thread_info member, we'd have ourselves a
> >>> PTR_TO_BTF_ID | PTR_TRUSTED w/ a fixed offset. Now, let's
> >>> hypothetically also say that we had a BPF kfunc that took a struct
> >>> thread_info pointer as an argument and the BPF kfunc was also
> >>> annotated w/ KF_TRUSTED_ARGS. If we attempted to pass the constructed
> >>> PTR_TO_BTF_ID | PTR_TRUSTED w/ fixed offset to this hypothetical BPF
> >>> kfunc, the BPF program would be rejected by the BPF verifier. This is
> >>> irrespective of the fact that supplying pointers to such embedded data
> >>> structure members of a PTR_TO_BTF_ID | PTR_TRUSTED may be considered
> >>> to be safe.
> >>>
> >>> One of the ideas that I had in mind to workaround the non-zero fixed
> >>> offset restriction was to simply introduce a new BPF kfunc annotation
> >>> i.e. __offset_allowed that could be applied on selected BPF kfunc
> >>> arguments that are expected to be KF_TRUSTED_ARGS. Such an annotation
> >>> would effectively control whether we enforce the non-zero offset
> >>> restriction or not in check_kfunc_args(), check_func_arg_reg_off(),
> >>> and __check_ptr_off_reg(). Although, now I'm second guessing myself
> >>> and I am wondering whether introducing something like the
> >>> __offset_allowed annotation for BPF kfunc arguments could lead to
> >>> compromising any of the safety guarantees that are provided by the BPF
> >>> verifier. Does anyone see an immediate problem with using such an
> >>> approach? I raise concerns, because it feels like we're effectively
> >>> punching a hole in the BPF verifier, but it may also be perfectly safe
> >>> to do on carefully selected PTR_TO_BTF_ID | PTR_TRUSTED types
> >>> i.e. struct thread_info, struct file, and it's just my paranoia
> >>> getting the better of me. Or, maybe someone has another idea to
> >>> support PTR_TO_BTF_ID | PTR_TRUSTED w/ fixed offset safely and a
> >>> little more generally without the need to actually make use of any
> >>> other BPF kfunc annotations?
> >> In verifier.c, we have BTF_TYPE_SAFE_TRUSTED to indidate that
> >> a pointer of a particular struct is safe and trusted if the point
> >> of that struct is trusted, e.g.,
> >>
> >> BTF_TYPE_SAFE_TRUSTED(struct file) {
> >> struct inode *f_inode;
> >> };
> >>
> >> We do the above since gcc does not support btf_tag yet.
> > Yes, I'm rather familiar with this construct.
> >
> >> I guess you could do
> >>
> >> BTF_TYPE_SAFE_TRUSTED(struct file) {
> >> struct path f_path;
> >> };
> >>
> >> and enhance verifier with the above information.
> >>
> >> But the above 'struct path f_path' may unnecessary
> >> consume extra memory since we only care about field
> >> 'f_path'. Maybe create a new construct like
> >>
> >> /* pointee is a field of the struct */
> >> BTF_TYPE_SAFE_FIELD_TRUSTED(struct file) {
> >> struct path *f_path;
> >> };
> > I don't fully understand how something like
> > BTF_TYPE_SAFE_FIELD_TRUSTED could work in practice. Do you mind
> > elaborating on that a little?
> >
> > What I'm currently thinking is that with something like
> > BTF_TYPE_SAFE_FIELD_TRUSTED, if the BPF verifier sees a PTR_TO_BTF_ID
> > | PTR_TRUSTED w/ a fixed offset supplied to a BPF kfunc, then the BPF
> > verifier can also check that fixed offset for the supplied
> > PTR_TO_BTF_ID | PTR_TRUSTED actually accesses a member that has been
> > explicitly annotated as being trusted via
> > BTF_TYPE_SAFE_FIELD_TRUSTED. Maybe that would be better then making
> > use of an __offset_allowed annotation, which would solely rely on the
> > btf_struct_ids_match() check for its safety.
> Right. What you described in the above is what I think as well.
I believe BTF_TYPE_SAFE_* or __offset_allowed annotations
are not necessary.
In this case thread_info is the first field of struct task_struct
and I suspect the verifier already allows:
bpf_kfunc void do_stuff_with_thread(struct thread_info *ti) KF_TRUSTED_ARGS
and use it as:
task = bpf_get_current_task_btf();
do_stuff_with_thread(&task->thread_info);
We have similar setup with:
struct bpf_cpumask {
cpumask_t cpumask;
...
};
and kfunc that accepts trusted cpumask_t * will accept
trusted struct bpf_cpumask *.
The other way around should be rejected, of course.
Similar approach should work with file/path.
The only difference is that the offset will be non-zero.
process_kf_arg_ptr_to_btf_id() needs to get smarter.
David Vernet added that check:
WARN_ON_ONCE(is_kfunc_trusted_args(meta) && reg->off);
as part of commit b613d335a743c.
iirc the reg->off==0 check is there, as an extra caution.
We can allow off!=0 and it won't confuse btf_type_ids_nocast_alias.
struct nf_conn___init {
int another_field_at_off_zero;
struct nf_conn ct;
};
will still trigger strict_type_match as expected.
Maybe other places in the verifier need to get smarter too
to allow non-zero offset into kf_trusted_args.
