Debian SID kernel doesn't boot on PowerBook 3400c

Fri Aug 6 11:06:27 AEST 2021

(Christophe, you've seen some of this before, however there are new 
results added at the end. I've Cc'd the mailing lists this time.)

On Wed, 4 Aug 2021, Stan Johnson wrote:

> On 8/4/21 8:41 PM, Finn Thain wrote:
> 
> >
> > $ curl https://lore.kernel.org/lkml/9b64dde3-6ebd-b446-41d9-61e8cb0d8c39@csgroup.eu/raw
> > ../message.mbox
> ok
> 
> $ sha1 ../message.mbox
> SHA1 (../message.mbox) = 436ce0adf893c46c84c54607f73c838897caeeea
> 
> >
> > On Wed, 4 Aug 2021, Christophe Leroy wrote:
> >
> >> Can you check if they happen at commit c16728835
> >>
> 
> $ git checkout c16728835eec
> Checking out files: 100% (20728/20728), done.
> Note: checking out 'c16728835eec'.
> 
> You are in 'detached HEAD' state. You can look around, make experimental
> changes and commit them, and you can discard any commits you make in this
> state without impacting any branches by performing another checkout.
> 
> If you want to create a new branch to retain commits you create, you may
> do so (now or later) by using -b with the checkout command again. Example:
> 
>   git checkout -b <new-branch-name>
> 
> HEAD is now at c16728835eec powerpc/32: Manage KUAP in C
> $ 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
> $ strings vmlinux | fgrep 'Linux version'
> Linux version 5.12.0-rc3-pmac-00078-geb51c431b81 (johnson at ThinkPad) (powerpc-linux-gnu-gcc (Debian 8.3.0-2) 8.3.0, GNU ld (GNU Binutils for Debian) 2.31.1) #1 SMP Wed Aug 4 21:50:47 MDT 2021
> 
> 1) PB 3400c
> Hangs at boot (Mac OS screen), no serial console output
> 
> 2) Wallstreet
> X fails, errors ("Kernel attempted to write user page", "BUG: Unable to
> handle kernel instruction fetch"), see Wallstreet_console-1.txt.
> 

The log shows that the error "Kernel attempted to write user page 
(b3399774) - exploit attempt?" happens after commit c16728835eec 
("powerpc/32: Manage KUAP in C").

> >>
> >> 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").

> 
> $ git checkout 0b45359aa2df
> ...
> 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-ge06b29ce146
> 
> 5) PB 3400c
> Hangs at boot (Mac OS screen)
> 
> 6) Wallstreet
> X failed (X login succeeded, but setting up desktop failed), errors in
> console log, see Wallstreet_console-3.txt.
> 

(No need for those two tests: it's exactly the same code and almost the 
same failure modes: "kernel BUG at arch/powerpc/kernel/interrupt.c:50".)

On Thu, 5 Aug 2021, Stan Johnson wrote:

> On 8/5/21 12:47 AM, Finn Thain wrote:
> 
> > On Wed, 4 Aug 2021, Christophe Leroy wrote:
> >
> >> Could you test without CONFIG_PPC_KUAP
> ...
> 
> $ git checkout c16728835eec
> ...
> HEAD is now at c16728835eec powerpc/32: Manage KUAP in C
> $ 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
> $ scripts/config -d CONFIG_PPC_KUAP
> $ make ARCH=powerpc CROSS_COMPILE=powerpc-linux-gnu- -j4 clean olddefconfig vmlinux
> $ grep CONFIG_PPC_KUAP .config
> # CONFIG_PPC_KUAP is not set
> 
> Linux version 5.12.0-rc3-pmac-00078-g5cac2bc3752
> 
> 7) PB 3400c
> Hangs at boot (Mac OS screen)
> 
> 8) Wallstreet
> Everything works, no errors (see Wallstreet_console-4.txt).
> 

That would seem to implicate CONFIG_PPC_KUAP itself. (Note that all builds 
up until this one have CONFIG_PPC_KUAP=y.)

> 
> >
> >> 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".

BTW, this procedure could be made simpler and easier if I pushed git 
branches to a public repo for Stan to build, which included Christophe's 
fix plus hard-wired Kconfig changes. That way, the .config file could be 
held constant and the commit hash in the serial console log would be more 
meaningful.
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