Re: [PATCH] Secure login and sulogin on S390x

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Date: Tue, 5 May 2026 11:39:41 +0200
From: "Dr. Werner Fink" <[email protected]>
To: Chris Hofstaedtler <[email protected]>
Cc: [email protected]
Subject: Re: [PATCH] Secure login and sulogin on S390x

On 2026/05/04 20:53:30 +0200, Chris Hofstaedtler wrote:
> * Werner Fink <[email protected]> [260429 11:30]:
> > Some remarks: on S390x architecture of modern zSeries the hypervisor
> > does log the console I/O.  For both the 3215 half duplex line mode as
> > well as the 3270 full-screen/block mode console type the I/O is logged
> > on the hypervisor's side.  To control this there are command send via
> > /dev/vmcp to tell the  z/VM control program of the hypervisor not to
> > log during entering the password.  For the 3215 console also automatic
> > scroll is enabled which avoid to press CLEAR to get the password prompt
> > if on the next block.
>=20
> [..]
> > 9 files changed, 436 insertions(+), 12 deletions(-)
>=20
> This seems to add a lot of code to util-linux. I was wondering if sulogin=
 is
> the only place that needs this log filtering. What about su, sudo, doas a=
nd
> so on? Note that there are different su (and probably doas) versions in u=
se
> across distributions.
>=20
> Wouldn't it be better if the hypervisor would not log input if the termin=
al
> output was disabled? That seems like behaviour commonly found in other
> virtual terminal emulators.
>=20
> Best,
> Chris

Maybe other hypervisors have the same behaviour.  Nevertheless the
3215 console is half duplex printer like a punchcard based teletypewriter
running within the hypervisor of an zSystem firmware.  Means every zOS and
Linux instances are VMs on such zSystem.  The 3270 console type is a bit
better as it understands escape sequences and is a block mode console type,
but still you have primitiv local prompt within the hypervisor and not on
the linux side its self.   You can compare this with qemu within virt-manag=
er,
but AFAIK qemu does not log keystrokes.

The vmcp.c is a copy of the version I use for my blogd in the showconsole
project at https://github.com/bitstreamout/showconsole

Werner

--=20
  "Having a smoking section in a restaurant is like having
          a peeing section in a swimming pool." -- Edward Burr

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