Debian SID kernel doesn't boot on PowerBook 3400c

[Christophe Leroy]

Le 06/08/2021 à 11:43, Finn Thain a écrit :
> On Fri, 6 Aug 2021, Christophe Leroy wrote:
> 
>>>>>>
>>>>>> Can you check if they DO NOT happen at preceding commit c16728835~
>>>>>>
>>>>
>>>> $ git checkout c16728835~
>>>> Previous HEAD position was c16728835eec powerpc/32: Manage KUAP in C
>>>> HEAD is now at 0b45359aa2df powerpc/8xx: Create C version of kuap
>>>> save/restore/check helpers
>>>> $ git am ../message.mbox
>>>> warning: Patch sent with format=flowed; space at the end of lines might be
>>>> lost.
>>>> Applying: powerpc/32: Dismantle EXC_XFER_STD/LITE/TEMPLATE
>>>> $ cp ../dot-config-powermac-5.13 .config
>>>> $ make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- -j4 clean
>>>> olddefconfig vmlinux
>>>>
>>>> Linux version 5.12.0-rc3-pmac-00077-gc9f6e8dd045
>>>>
>>>> 3) PB 3400c
>>>> Hangs at boot (Mac OS screen)
>>>>
>>>> 4) Wallstreet
>>>> X fails, errors in console log (different than test 2), see
>>>> Wallstreet_console-2.txt.
>>>>
>>>
>>> This log shows that the errors "xfce4-session[1775]: bus error (7)" and
>>> "kernel BUG at arch/powerpc/kernel/interrupt.c:49!" happen prior to commit
>>> c16728835eec ("powerpc/32: Manage KUAP in C").
>>
>> As mentionned by Nic, this is due to r11 being cloberred. For the time being
>> the only r11 clobber identified is the one I have provided a fix for. I'm
>> wondering whether it was applied for all further tests or not.
>>
> 
> Your fix was applied to this build with "git am ../message.mbox".

Ok good.

> 
>> ...
>>>>
>>>>>
>>>>>> Could you test with CONFIG_PPC_KUAP and CONFIG_PPC_KUAP_DEBUG
>>>> ...
>>>>
>>>> $scripts/config -e CONFIG_PPC_KUAP
>>>> $ scripts/config -e CONFIG_PPC_KUAP_DEBUG
>>>> $ make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- -j4 clean
>>>> olddefconfig vmlinux
>>>> $ grep CONFIG_PPC_KUAP .config
>>>> CONFIG_PPC_KUAP=y
>>>> CONFIG_PPC_KUAP_DEBUG=y
>>>>
>>>> Linux version 5.12.0-rc3-pmac-00078-g5cac2bc3752
>>>>
>>>> 9) PB 3400c
>>>> Hangs at boot (Mac OS screen)
>>>>
>>>> 10) Wallstreet
>>>> X failed at first login, worked at second login, one error in console
>>>> log ("BUG: Unable to handle kernel instruction fetch"), see
>>>> Wallstreet_console-5.txt.
>>>>
>>>
>>> One might expect to see "Kernel attempted to write user page (b3399774) -
>>> exploit attempt?" again here (see c16728835eec build above) but instead
>>> this log says "Oops: Kernel access of bad area, sig: 11".
>>
>> Maybe the test should be done a second time. As r11 is garbage it may or
>> may not be a user address. If it is a user address the we get "Kernel
>> attempted to write user page". If it is a random kernel address, we
>> likely get "Kernel access of bad area" instead.
>>
> 
> Your fix was applied here also.
> 

Anyway, it would be worth trying to boot a few times more with the same kernel, because as I said 
the value is random, so it may or may not hit userspace, hence the possible difference of message, 
either "Kernel attempted to write user page" or "Kernel access of bad area" depending on whether the 
address is a user address or not.

I have cooked a tentative fix for that KUAP stuff.
Could you try the branch 'bugtest' at https://github.com/chleroy/linux.git

Thanks
Christophe