From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-184.mta1.migadu.com (out-184.mta1.migadu.com [95.215.58.184]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D01A2181 for ; Thu, 18 Apr 2024 00:11:29 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.184 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713399091; cv=none; b=kVOqSeNC36MOpOB+aXWS/zusUWOvsaJpofOUVrb7PRH4AamjXa3qp/+LPby/1ODQ95aoup/OfkjxYeKKgE47GkWw+bTLC/15w0kX00OLuwYqkNu7tQhjociWjwkX3A6Ey2nQmMAZ9TFcxWvkgh0QhsLoO3unjLb7VeH0NyGke74= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713399091; c=relaxed/simple; bh=LvnzemrN1h4DVJsCgnk8FHAN3zqmD4zd4N/7lSSpxZE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Ae9rwJ9bYwvauoswRI2FKA9S8vYve03kouGv67ETKIo8h3qGLTlcrLYYV5e0PJoOCgqj6LlYYin1NBf/WUeyQVwPIo3TSbLdQkNIkFEgQZNomw2/GWb2HrSygAn5KSDqHo4g1bn+YDr/aIdGm8xxMnW8cA2v+PVuoSwFRkcHyNA= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev; spf=pass smtp.mailfrom=linux.dev; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b=KTP1yUDo; arc=none smtp.client-ip=95.215.58.184 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=linux.dev Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=linux.dev Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux.dev header.i=@linux.dev header.b="KTP1yUDo" Message-ID: DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1713399087; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=DMqBtc9FOHb9l5xEy1bAUb1ypvNz3uDiAsqNwUVTLcA=; b=KTP1yUDokO6Ss8xOaxQ3dU1RAe8Yd5SN/g1XG7atIyRIIuaqkLJhxKrahz046WFm8U7jms NSg92qCP++YC1XaeiQ2j5Q2KG4bVPcqITjUKdr9pr3IdkD8AYZKwYLCI24JILeKgqL1gM5 KOhzIstAwsXw0UMSUkkiC94YcthslK0= Date: Wed, 17 Apr 2024 17:11:22 -0700 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Subject: Re: [RFC] bpf: allowing PTR_TO_BTF_ID | PTR_TRUSTED w/ non-zero fixed offset to selected KF_TRUSTED_ARGS BPF kfuncs Content-Language: en-GB To: Matt Bobrowski Cc: bpf@vger.kernel.org, ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, song@kernel.org, kpsingh@kernel.org, sdf@google.com, haoluo@google.com, memxor@gmail.com, void@manifault.com, jolsa@kernel.org References: <3f8a481e-0dfe-468f-8c87-6610528f9009@linux.dev> X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Yonghong Song In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Migadu-Flow: FLOW_OUT 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