From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from out-172.mta1.migadu.com (out-172.mta1.migadu.com [95.215.58.172]) (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 208F578C90 for ; Mon, 15 Apr 2024 16:43:52 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=95.215.58.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713199435; cv=none; b=FLsKMxUucKGCpY01n7leRZEb7b0bJ5E03XIXWtf4t/Br5RimtQ5rTkA6/1V9IQ6OhGn/RwK4Ubdj92c1mm5EF26eTVDGriU5qOM8M9gudHcJNVL9CneZ+rl2oZ1sNXDoV2Y3f6jVISG4Oql/cfM/t7htuS4Su2PtIk58fOVV9YA= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1713199435; c=relaxed/simple; bh=bBp+uUmmVDhT832s3JQLyRxP7kGtMXcRWiR69xAgCbE=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=H7dRcbd7zG5lNfXfhOBQDozwmjL39RGV8GoKs4ZqkQYjTJ31Ey6JF95ws3YXuBCXHGnJ/nqxD5WfMni8Y8k9plot4LsJ2GfHEgFFIdgB/0qcRFN/DTZxTeorzsVqYG3h7O7T0mquYi8eitRHN+kZa3GKgDdBXkz1AbCEq6YL/qI= 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=Z60rwn3B; arc=none smtp.client-ip=95.215.58.172 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="Z60rwn3B" Message-ID: <3f8a481e-0dfe-468f-8c87-6610528f9009@linux.dev> DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1713199430; 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=PuhSZY3KXGUidE5TgEZnuy3C7ySf3U1hECsjHBEsEnY=; b=Z60rwn3BuNmHCProBYroqlh8ywXCa5qX50AnhC5xc+pBV+zi3QGGktkk0H+M45PvJTTI0m HGq5uKQLOUSB4anwU239nQFdo2fT2JU/NJoCOV3ppV1hXq+tILC9Ono7mxmS4xIrt79Zei fTqn6B1sxOoANUbFDU//rTmGnLQNw+A= Date: Mon, 15 Apr 2024 09:43:42 -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 , bpf@vger.kernel.org Cc: 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: 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 4/12/24 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 >