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 4/17/24 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.
>
> /M