Re: [PATCH] powerpc/pseries: Fix scv instruction crash with kexec

[Michael Ellerman <mpe@ellerman.id.au>]

Michal Suchánek <msuchanek@suse.de> writes:
> Hello,
>
> On Tue, Jun 25, 2024 at 11:40:47PM +1000, Nicholas Piggin wrote:
>> kexec on pseries disables AIL (reloc_on_exc), required for scv
>> instruction support, before other CPUs have been shut down. This means
>> they can execute scv instructions after AIL is disabled, which causes an
>> interrupt at an unexpected entry location that crashes the kernel.
>> 
>> Change the kexec sequence to disable AIL after other CPUs have been
>> brought down.
>> 
>> As a refresher, the real-mode scv interrupt vector is 0x17000, and the
>> fixed-location head code probably couldn't easily deal with implementing
>> such high addresses so it was just decided not to support that interrupt
>> at all.
>> 
>> Reported-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>> Fixes: 7fa95f9adaee7 ("powerpc/64s: system call support for scv/rfscv instructions")
>
> looks like this is only broken by
> commit 2ab2d5794f14 ("powerpc/kasan: Disable address sanitization in kexec paths")
>
> This change reverts the kexec parts done in that commit.
>
> That is the fix is 5.19+, not 5.9+

Commit 2ab2d5794f14 moved the kexec code from one file to another, but
didn't change when the key function (pseries_disable_reloc_on_exc()) was
called.

The old code was:

diff --git a/arch/powerpc/platforms/pseries/setup.c b/arch/powerpc/platforms/pseries/setup.c
index a3dab15b0a2f..c9fcc30a0365 100644
--- a/arch/powerpc/platforms/pseries/setup.c
+++ b/arch/powerpc/platforms/pseries/setup.c
@@ -421,16 +421,6 @@ void pseries_disable_reloc_on_exc(void)
 }
 EXPORT_SYMBOL(pseries_disable_reloc_on_exc);

-#ifdef CONFIG_KEXEC_CORE
-static void pSeries_machine_kexec(struct kimage *image)
-{
-       if (firmware_has_feature(FW_FEATURE_SET_MODE))
-               pseries_disable_reloc_on_exc();
-
-       default_machine_kexec(image);
-}
-#endif
-

ie. pseries_disable_reloc_on_exc() (which disables AIL) is called before
default_machine_kexec() where secondary CPUs are collected.

So AFAICS the bug would still have been there prior to 2ab2d5794f14. But
it's late here so I could be reading it wrong.

cheers