Re: [PATCH v5 2/9] watchdog/hpwdt: Remove legacy NMI sourcing.

[Guenter Roeck]

On 02/26/2018 05:02 PM, Jerry Hoemann wrote:
> On Mon, Feb 26, 2018 at 06:32:30AM -0800, Guenter Roeck wrote:
> > On 02/26/2018 06:11 AM, Arnd Bergmann wrote:
> > > On Mon, Feb 26, 2018 at 4:22 AM, Jerry Hoemann <jerry.hoemann@xxxxxxx> wrote:
> > > > Gen8 and prior Proliant systems supported the "CRU" interface
> > > > to firmware.  This interfaces allows linux to "call back" into firmware
> > > > to source the cause of an NMI.  This feature isn't fully utilized
> > > > as the actual source of the NMI isn't printed, the driver only
> > > > indicates that the source couldn't be determined when the call
> > > > fails.
> > > >
> > > > With the advent of Gen9, iCRU replaces the CRU. The call back
> > > > feature is no longer available in firmware.  To be compatible and
> > > > not attempt to call back into firmware on system not supporting CRU,
> > > > the SMBIOS table is consulted to determine if it is safe to
> > > > make the call back or not.
> > > >
> > > > This results in about half of the driver code being devoted
> > > > to either making CRU calls or determing if it is safe to make
> > > > CRU calls.  As noted, the driver isn't really using the results of
> > > > the CRU calls.
> > > >
> > > > Furthermore, as a consequence of the Spectre security issue, the
> > > > BIOS/EFI calls are being wrapped into Spectre-disabling section.
> > > > Removing the call back in hpwdt_pretimeout assists in this effort.
> > > >
> > > > As the CRU sourcing of the NMI isn't required for handling the
> > > > NMI and there are security concerns with making the call back, remove
> > > > the legacy (pre Gen9) NMI sourcing and the DMI code to determine if
> > > > the system had the CRU interface.
> > > >
> > > > Signed-off-by: Jerry Hoemann <jerry.hoemann@xxxxxxx>
> > >
> > > This avoids a warning in mainline kernels, so that's great:
> > >
> > > drivers/watchdog/hpwdt.o: warning: objtool: .text+0x24: indirect call
> > > found in RETPOLINE build
> > >
> > > I wonder what we do about stable kernels. Are both this patch and the patch
> > > that added the objtool warning message candidates for backports to
> > > stable kernels?
> >
> > Makes sense to me, but it is really a bit more than a bug fix, so I'll
> > leave it up to Jerry/HPE to make the call in respect to hpwdt.
>
> Generally speaking, HPE customers who run linux do so through a distro
> vendor and pick up patches from them.  But I'm sure there are some
> customers who do things differently.
>
> The distro vendor's have their own repos and we'll work with them
> to back port patches to their code base.  So, I typically don't do a lot
> of kernel.org stable branch work.
>
> Looks like objtool has been enhanced to find Spectre vulnerable code.
> Are the other kernel patches related to Spectre being back ported
> to stable release lines?  If yes, it probably make sense to do
> the hpwdt change as well.

Spectre has been backported to v4.4 and later. I don't know about earlier kernels.

> Is just the patch removing the firmware call back wanted/needed? Or the
> whole driver rewrite?  (The older baseline don't have all the watchdog
> features that the patch set uses.)

We would only want to backport this patch. The rest is really out of scope.

> Which stable baseline(s) would need to be patched?  Priority?
>
> Who does it?  (i.e. do you want me to submit patches to the stable baseline?)
We would tag the patch for stable (and submit it into v4.16-rc). Greg would
take care of the rest unless there are conflicts, in which case we get a note
telling us that a backport is needed.

Guenter