linux-kernel - Re: [PATCH v2 security-next 1/4] security: Hornet LSM

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Message-ID: <CAADnVQJ6SRePz7yc5x3BAz7q-e8DVYq=vRdahxCZ4XzpWtnYpQ@mail.gmail.com>
Date: Thu, 24 Apr 2025 16:41:42 -0700
From: Alexei Starovoitov <alexei.starovoitov@...il.com>
To: James Bottomley <James.Bottomley@...senpartnership.com>
Cc: Blaise Boscaccy <bboscaccy@...ux.microsoft.com>, Jonathan Corbet <corbet@....net>, 
	David Howells <dhowells@...hat.com>, Herbert Xu <herbert@...dor.apana.org.au>, 
	"David S. Miller" <davem@...emloft.net>, Paul Moore <paul@...l-moore.com>, 
	James Morris <jmorris@...ei.org>, "Serge E. Hallyn" <serge@...lyn.com>, 
	Masahiro Yamada <masahiroy@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
	Nicolas Schier <nicolas@...sle.eu>, Shuah Khan <shuah@...nel.org>, 
	Mickaël Salaün <mic@...ikod.net>, 
	Günther Noack <gnoack@...gle.com>, 
	Nick Desaulniers <nick.desaulniers+lkml@...il.com>, Bill Wendling <morbo@...gle.com>, 
	Justin Stitt <justinstitt@...gle.com>, Jarkko Sakkinen <jarkko@...nel.org>, 
	Jan Stancek <jstancek@...hat.com>, Neal Gompa <neal@...pa.dev>, 
	"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>, 
	keyrings@...r.kernel.org, 
	Linux Crypto Mailing List <linux-crypto@...r.kernel.org>, 
	LSM List <linux-security-module@...r.kernel.org>, 
	Linux Kbuild mailing list <linux-kbuild@...r.kernel.org>, 
	"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>, bpf <bpf@...r.kernel.org>, 
	clang-built-linux <llvm@...ts.linux.dev>, nkapron@...gle.com, 
	Matteo Croce <teknoraver@...a.com>, Roberto Sassu <roberto.sassu@...wei.com>, 
	Cong Wang <xiyou.wangcong@...il.com>
Subject: Re: [PATCH v2 security-next 1/4] security: Hornet LSM

On Wed, Apr 23, 2025 at 7:12 AM James Bottomley
<James.Bottomley@...senpartnership.com> wrote:
>
> On Mon, 2025-04-21 at 13:12 -0700, Alexei Starovoitov wrote:
> [...]
> > Calling bpf_map_get() and
> > map->ops->map_lookup_elem() from a module is not ok either.
>
> I don't understand this objection.

Consider an LSM that hooks into security_bprm_*(bprm),
parses something in linux_binprm, then
struct file *file = fd_file(fdget(some_random_file_descriptor_in_current));
file->f_op->read(..);

Would VFS maintainers approve such usage ?

More so, your LSM does
file = get_task_exe_file(current);
kernel_read_file(file, ...);

This is even worse.
You've corrupted the ELF binary with extra garbage at the end.
objdump/elfutils will choke on it and you're lucky that binfmt_elf
still loads it.
The whole approach is a non-starter.

> The program just got passed in to
> bpf_prog_load() as a set of attributes which, for a light skeleton,
> directly contain the code as a blob and have the various BTF
> relocations as a blob in a single element array map.  I think everyone
> agrees that the integrity of the program would be compromised by
> modifications to the relocations, so the security_bpf_prog_load() hook
> can't make an integrity determination without examining both.  If the
> hook can't use the bpf_maps.. APIs directly is there some other API it
> should be using to get the relocations, or are you saying that the
> security_bpf_prog_load() hook isn't fit for purpose and it should be
> called after the bpf core has loaded the relocations so they can be
> provided to the hook as an argument?

No. As I said twice already the only place to verify program
signature is a bpf subsystem itself.
Hacking into bpf internals from LSM, BPF-LSM program,
or any other kernel subsystem is a no go.

> The above, by the way, is independent of signing, because it applies to
> any determination that might be made in the security_bpf_prog_load()
> hook regardless of purpose.

security_bpf_prog_load() should not access bpf internals.
That LSM hook sees the following:
security_bpf_prog_load(struct bpf_prog *prog, union bpf_attr *attr,
                       struct bpf_token *token, bool kernel);

LSM can look into uapi things there.
Like prog->sleepable, prog->tag, prog->aux->name,
but things like prog->aux->jit_data or prog->aux->used_maps
are not ok to access.
If in doubt, ask on the mailing list.