Re: [PATCH v2] Documentation: crypto: add info about "fips=" boot option

Eric Biggers
2021-03-30T22:44:41Z

On Tue, Mar 30, 2021 at 09:38:55AM -0700, Randy Dunlap wrote:
> On 3/29/21 10:29 PM, Eric Biggers wrote:
> > On Mon, Mar 29, 2021 at 10:06:51PM -0700, Randy Dunlap wrote:
> >> Having just seen a report of using "fips=1" on the kernel command line,
> >> I could not find it documented anywhere, so add some help for it.
> >>
> >> Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
> >> Cc: Dexuan Cui <decui@microsoft.com>
> >> Cc: linux-crypto@vger.kernel.org
> >> Cc: Eric Biggers <ebiggers@kernel.org>
> >> Cc: Herbert Xu <herbert@gondor.apana.org.au>
> >> Cc: "David S. Miller" <davem@davemloft.net>
> >> Cc: Jonathan Corbet <corbet@lwn.net>
> >> Cc: linux-doc@vger.kernel.org
> >> ---
> >> Updates/corrections welcome.
> >>
> >> v2: drop comment that "fips_enabled can cause some tests to be skipped".
> >>
> >> Documentation/admin-guide/kernel-parameters.txt | 14 ++++++++++++++
> >> 1 file changed, 14 insertions(+)
> >>
> >> --- linux-next-20210329.orig/Documentation/admin-guide/kernel-parameters.txt
> >> +++ linux-next-20210329/Documentation/admin-guide/kernel-parameters.txt
> >> @@ -1370,6 +1370,20 @@
> >> See Documentation/admin-guide/sysctl/net.rst for
> >> fb_tunnels_only_for_init_ns
> >>
> >> + fips= Format: { 0 | 1}
> >> + Use to disable (0) or enable (1) FIPS mode.
> >> + If enabled, any process that is waiting on the
> >> + 'fips_fail_notif_chain' will be notified of fips
> >> + failures.
> >> + This setting can also be modified via sysctl at
> >> + /proc/sysctl/crypto/fips_enabled, i.e.,
> >> + crypto.fips_enabled.
> >> + If fips_enabled = 1 and a test fails, it will cause a
> >> + kernel panic.
> >> + If fips_enabled = 1, RSA test requires a key size of
> >> + 2K or larger.
> >> + It can also effect which ECC curve is used.
> >
> > This doesn't really explain why anyone would want to give this option.
> > What high-level thing is this option meant to be accomplishing?
> > That's what the documentation should explain.
>
> Yes, clearly, even to me.
>
> But I could not find anything in the kernel source tree that would help me
> explain that. So to repeat:
>
> >> Updates/corrections welcome.
>
> thanks.
> --

I'm by no means an expert on this, but the main thing I have in mind is that
(IIUC) the "fips" option is only useful if your whole kernel binary is certified
as a "FIPS cryptographic module", *and* you actually need the FIPS compliance.
And the upstream kernel doesn't have a FIPS certification out of the box; that's
a task for specific Linux distributors like Red Hat, SUSE, Ubuntu, who get
specific kernel binaries certified.

So, compiling a kernel and using the "fips" option is useless by itself, as your
kernel image won't actually have a FIPS certification in that case anyway.

So, I would expect an explanation like that about under what circumstances the
"fips" option is actually useful and intended for.

The people who actually use this option should be able to explain it properly
though; the above is just my understanding...

- Eric