Re: [RFC] bpf: allowing PTR_TO_BTF_ID | PTR_TRUSTED w/ non-zero fixed offset to selected KF_TRUSTED_ARGS BPF kfuncs

[Yonghong Song <yonghong.song@linux.dev>]

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.

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;
};


>
> /M
>