[PATCH bpf-next v2 0/3] BPF signature hash chains

[James Bottomley]

On Thu, 2025-10-09 at 18:00 -0700, Alexei Starovoitov wrote:
[...]
> James's concern is valid though:
>  
> > However, the rub for LSM is that the verification of the program
> > map by the loader happens *after* the security_bpf_prog_load() hook
> > has been called.
> 
> I understand the discomfort, but that's what the kernel module
> loading process is doing as well, so you should be concerned with
> both. Since both are doing pretty much the same work.

OK, so let me push on this one point because I don't agree with what
you say here.  The way kernel modules and eBPF load is not equivalent.
The kernel module signatures go over a relocateable elf binary which is
subsequently relocated after signature verification in the kernel by
the ELF loader.  You can regard the ELF loader as being equivalent to
the eBPF loader in terms of function, absolutely.  However for security
purposes the ELF loader is a trusted part of the kernel security
envelope and its integrity is part of the kernel integrity and we have
a this single trusted loader for every module.  In security terms
verification of the ELF object signature is sufficient to guarantee
integrity of the module because the integrity of the ELF loader is
already checked.

The eBPF loader, by contrast, because it contains all the relocations,
is different for every eBPF light skeleton.  This means it's not a
trusted part of the kernel and has to be integrity checked as well. 
Thus for eBPF, the integrity check must be over both the loader and the
program; integrity checking is not complete until the integrity of both
has been verified.  If you sign only the loader and embed the hash  of
the program into the loader that is a different way of doing things,
but the integrity check is not complete until the loader does the hash
verification which, as has been stated many times before, is *after*
the load LSM hook has run.

There are two potential solutions to this: complete the integrity check
before running the load hook (Blaise's patch) or add a LSM hook to
collect the integrity information from the run of the loader.  Neither
of these is present in the scheme you put upstream.

Regards,

James