New: kernel 4.9.175 fails to boot on a PowerMac G4 3,6 at early stage

https://bugzilla.kernel.org/show_bug.cgi?id=203597

            Bug ID: 203597
           Summary: kernel 4.9.175 fails to boot on a PowerMac G4 3,6 at
                    early stage
           Product: Platform Specific/Hardware
           Version: 2.5
    Kernel Version: 4.9.175
          Hardware: PPC-32
                OS: Linux
              Tree: Mainline
            Status: NEW
          Severity: normal
          Priority: P1
         Component: PPC-32
          Assignee: [email protected]
          Reporter: [email protected]
        Regression: No

Created attachment 282743
--> https://bugzilla.kernel.org/attachment.cgi?id=282743&action=edit
kernel .config (PowerMac G4 MDD)

Trying out older kernels on the G4 MDD I noticed recent 4.9.x freeze the machine. Only message displayed in black letters on a white screen:

done
found display : /pco@f0000000/ATY,AlteracParent@10/ATY,Alterac_B@1,
opening...

It's a hard freeze, RCU_CPU_STALL_TIMEOUT does not kick in.

Tried other stable kernels, which all worked:
4.19.37
4.14.114
4.4.179

So I suppose it's only a 4.9.x issue. Last working 4.9.x kernel I had in service was 4.9.161. First one I spotted freezing was 4.9.171. A bisect gave me the following commit:

1c38a84d45862be06ac418618981631eddbda741 is the first bad commit commit 1c38a84d45862be06ac418618981631eddbda741

Author: Michael Neuling <[email protected]>
Date:   Thu Apr 11 21:45:59 2019 +1000

powerpc: Avoid code patching freed init sections

commit 51c3c62b58b357e8d35e4cc32f7b4ec907426fe3 upstream.

This stops us from doing code patching in init sections after they've been freed.

In this chain:
kvm_guest_init() ->
kvm_use_magic_page() ->
fault_in_pages_readable() ->
__get_user() ->
__get_user_nocheck() ->
barrier_nospec();

We have a code patching location at barrier_nospec() and kvm_guest_init() is an init function. This whole chain gets inlined, so when we free the init section (hence kvm_guest_init()), this code goes away and hence should no longer be patched.

We seen this as userspace memory corruption when using a memory checker while doing partition migration testing on powervm (this starts the code patching post migration via /sys/kernel/mobility/migration). In theory, it could also happen when using /sys/kernel/debug/powerpc/barrier_nospec.

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https://bugzilla.kernel.org/show_bug.cgi?id=203597

--- Comment #1 from Erhard F. ([email protected]) --- Created attachment 282745
--> https://bugzilla.kernel.org/attachment.cgi?id=282745&action=edit
bisect.log

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https://bugzilla.kernel.org/show_bug.cgi?id=203597

Christophe Leroy ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------
CC| |[email protected]

--- Comment #2 from Christophe Leroy ([email protected]) --- You are missing following commit:

https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=b45ba4a51cd

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Hi Greg,

Could you please apply b45ba4a51cde29b2939365ef0c07ad34c8321789 to 4.9 since 51c3c62b58b357e8d35e4cc32f7b4ec907426fe3 was applied allthought marked for #4.13+

Thanks
Christophe

Le 13/05/2019 à 22:18, [email protected] a écrit :

It does not apply there (nor to the 4.4.y queue, which will need it as well), so can you provide a working backport so that I can queue it up?

thanks,

greg k-h

https://bugzilla.kernel.org/show_bug.cgi?id=203597

Erhard F. ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|NEW                         |RESOLVED
         Resolution|---                         |CODE_FIX

--- Comment #3 from Erhard F. ([email protected]) --- (In reply to Christophe Leroy from comment #2)

Your fix landed in 4.9.177 and I can confirm my G4 boots fine now. Thanks!

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Michael Ellerman ([email protected]) changed:

What |Removed |Added
----------------------------------------------------------------------------

             Status|RESOLVED                    |CLOSED
                 CC|                            |[email protected]

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