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herbert@gondor.apana.org.au, jarkko@kernel.org, jmorris@namei.org, jstancek@redhat.com, justinstitt@google.com, keyrings@vger.kernel.org, linux-crypto@vger.kernel.org, linux-doc@vger.kernel.org, linux-kbuild@vger.kernel.org, linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org, linux-security-module@vger.kernel.org, llvm@lists.linux.dev, masahiroy@kernel.org, mic@digikod.net, morbo@google.com, nathan@kernel.org, neal@gompa.dev, nick.desaulniers+lkml@gmail.com, nicolas@fjasle.eu, nkapron@google.com, roberto.sassu@huawei.com, serge@hallyn.com, shuah@kernel.org, teknoraver@meta.com, xiyou.wangcong@gmail.com, kysrinivasan@gmail.com, Linus Torvalds Date: Wed, 14 May 2025 16:31:55 -0400 In-Reply-To: References: <20250502184421.1424368-1-bboscaccy@linux.microsoft.com> <20250502210034.284051-1-kpsingh@kernel.org> <4f92fcfaeffd179ff6ae265822dc79856310d6a3.camel@HansenPartnership.com> Autocrypt: addr=James.Bottomley@HansenPartnership.com; prefer-encrypt=mutual; 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 8mT2Eh9SkH+YVOmaKsajgZgrBxA7fWmGoxXswEVxJIFj3vW7yNc0C5HaUdYa5iGOMs4kg2ht4s7yy7NRQuh7BifWjo6BQ6k4S1H+6axZucxhSV1L6zN9d+lr3Xo/vy1unzA== Content-Type: text/plain; charset="UTF-8" Content-Transfer-Encoding: quoted-printable User-Agent: Evolution 3.50.3 Precedence: bulk X-Mailing-List: linux-kbuild@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Wed, 2025-05-14 at 20:35 +0200, KP Singh wrote: > On Wed, May 14, 2025 at 7:45=E2=80=AFPM James Bottomley > wrote: > >=20 > > On Wed, 2025-05-14 at 19:17 +0200, KP Singh wrote: > > > On Wed, May 14, 2025 at 5:39=E2=80=AFPM James Bottomley > > > wrote: > > > > On Sun, 2025-05-11 at 04:01 +0200, KP Singh wrote: > > [...] > > > > > This implicitly makes the payload equivalent to the signed > > > > > block > > > > > (B_signed) > > > > >=20 > > > > > =C2=A0=C2=A0=C2=A0 I_loader || H_meta > > > > >=20 > > > > > bpftool then generates the signature of this I_loader payload > > > > > (which now contains the expected H_meta) using a key (system > > > > > or > > > > > user) with new flags that work in combination with bpftool -L > > > >=20 > > > > Could I just push back a bit on this.=C2=A0 The theory of hash > > > > chains > > > > (which I've cut to shorten) is about pure data structures.=C2=A0 Th= e > > > > reason for that is that the entire hash chain is supposed to be > > > > easily independently verifiable in any environment because > > > > anything > > > > can compute the hashes of the blocks and links.=C2=A0 This > > > > independent > > > > verification of the chain is key to formally proving hash > > > > chains to > > > > be correct.=C2=A0 In your proposal we lose the easy verifiability > > > > because the link hash is embedded in the ebpf loader program > > > > which > > > > has to be disassembled to do the extraction of the hash and > > > > verify > > > > the loader is actually checking it. > > >=20 > > > I am not sure I understand your concern. This is something that > > > can > > > easily be built into tooling / annotations. > > >=20 > > > =C2=A0=C2=A0=C2=A0 bpftool -S -v > > >=20 > > > Could you explain what's the use-case for "easy verifiability". > >=20 > > I mean verifiability of the hash chain link.=C2=A0 Given a signed > > program, (i.e. a .h file which is generated by bpftool) which is a > > signature over the loader only how would one use simple > > cryptographic operations to verify it? > >=20 >=20 > I literally just said it above the hash can be extracted if you > really want offline verification. Are you saying this code is hard to > write? or is the tooling hard to write? Do you have some definition > of "simple cryptographic operations".=C2=A0 All operations use tooling. As I said, you have a gap in that you not only have to extract the hash and verify it against the map (which I agree is fairly simple) but also verify the loader program actually checks it correctly. That latter operation is not a simple cryptographic one and represents a security gap between this proposal and the hash linked chains you introduced in your first email in this thread. > > > > I was looking at ways we could use a pure hash chain (i.e. > > > > signature over loader and real map hash) and it does strike me > > > > that the above ebpf hash verification code is pretty invariant > > > > and easy to construct, so it could run as a separate BPF > > > > fragment that then jumps to the real loader.=C2=A0 In that case, it > > > > could be constructed on the fly in a trusted environment, like > > > > the kernel, from the link hash in the signature and the > > > > signature could just be Sig(loader || map hash) which can then > > > > be > > >=20 > > > The design I proposed does the same thing: > > >=20 > > > =C2=A0=C2=A0=C2=A0 Sig(loader || H_metadata) > > >=20 > > > metadata is actually the data (programs, context etc) that's > > > passed in the map. The verification just happens in the loader > > > program and the loader || H_metadata is implemented elegantly to > > > avoid any separate payloads. > >=20 > > OK, so I think this is the crux of the problem:=C2=A0 In formal methods > > proving the validity of a data based hash link is an easy set of > > cryptographic operations.=C2=A0 You can assert that's equivalent to a > > signature over a program that verifies the hash, but formally > > proving it requires a formal analysis of the program to show that > > 1) it contains the correct hash and 2) it correctly checks the hash > > against the map.=C2=A0 That makes the task of someone receiving the .h > > file containing the signed skeleton way harder: it's easy to prove > > the signature matches the loader instructions, but they still have > > to prove the instructions contain and verify the correct map hash. > >=20 >=20 > I don't see this as a problem for 2 reasons: >=20 > 1. It's not hard it requires disassembling the first 20 or so BPF instructions and verifying their operation, so that's harder than simply calculating hashes and signatures. > 2. Your typical user does not want to do formal verification and > extract signatures etc. Users don't want to do formal verification, agreed ... but they do want to know that security experts have verified the algorithms they're using. That's why I was thinking, since the loader preamble that verifies the hash is easy to construct, that the scheme could use a real hash linked chain, which has already been formally verified and is well understood, then construct the preamble for the loader you want in a trusted environment based on the hashes, meaning there's no security gap. Regards, James