linux-kernel - Re: [PATCH v6 07/15] digest_cache: Allow registration of digest list parsers

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Message-ID: <10c8fd4b53f946c2d7e933a35c6eb36557e8c592.camel@huaweicloud.com>
Date: Thu, 28 Nov 2024 09:23:57 +0100
From: Roberto Sassu <roberto.sassu@...weicloud.com>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: zohar@...ux.ibm.com, dmitry.kasatkin@...il.com,
 eric.snowberg@...cle.com,  corbet@....net, petr.pavlu@...e.com,
 samitolvanen@...gle.com, da.gomez@...sung.com,  akpm@...ux-foundation.org,
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 jikos@...nel.org, mkoutny@...e.com, ppavlu@...e.com, petr.vorel@...il.com, 
 mzerqung@...inter.de, kgold@...ux.ibm.com, Roberto Sassu
 <roberto.sassu@...wei.com>
Subject: Re: [PATCH v6 07/15] digest_cache: Allow registration of digest
 list parsers

On Wed, 2024-11-27 at 11:53 -0800, Luis Chamberlain wrote:
> On Wed, Nov 27, 2024 at 10:51:11AM +0100, Roberto Sassu wrote:
> > For eBPF programs we are also in a need for a better way to
> > measure/appraise them.
> 
> I am confused now, I was under the impression this "Integrity Digest
> Cache" is just a special thing for LSMs, and so I was under the
> impression that kernel_read_file() lsm hook already would take care
> of eBPF programs.

Yes, the problem is that eBPF programs are transformed in user space
before they are sent to the kernel:

https://lwn.net/Articles/977394/

The Integrity Digest Cache can be used for the measurement/appraisal of
the initial eBPF ELF file, when they are accessed from the filesystem,
but the resulting blob sent to the kernel will be different.

> > Now, I'm trying to follow you on the additional kernel_read_file()
> > calls. I agree with you, if a parser tries to open again the file that
> > is being verified it would cause a deadlock in IMA (since the inode
> > mutex is already locked for verifying the original file).
> 
> Just document this on the parser as a requirement.

Ok, will do.

> > > > Supporting kernel modules opened the road for new deadlocks, since one
> > > > can ask a digest list to verify a kernel module, but that digest list
> > > > requires the same kernel module. That is why the in-kernel mechanism is
> > > > 100% reliable,
> > > 
> > > Are users of this infrastructure really in need of modules for these
> > > parsers?
> > 
> > I planned to postpone this to later, and introduced two parsers built-
> > in (TLV and RPM). However, due to Linus's concern regarding the RPM
> > parser, I moved it out in a kernel module.
> 
> OK this should be part of the commit log, ie that it is not desirable to
> have an rpm parser in-kernel for some users.

I understand. Will add in the commit log.

Just to clarify, we are not talking about the full blown librpm in the
kernel, but a 243 LOC that I rewrote to obtain only the information I
need. I also formally verified it with pseudo/totally random data with
Frama-C:

https://github.com/robertosassu/rpm-formal/blob/main/validate_rpm.c

Thanks

Roberto