Re: [PATCH v2 2/2] powerpc/vmx: avoid KASAN instrumentation in enter_vmx_ops() for kexec

[Sourabh Jain <sourabhjain@linux.ibm.com>]

On 07/04/26 01:23, Aboorva Devarajan wrote:
> On Sat, 2026-04-04 at 00:31 +0530, Sourabh Jain wrote:
>> The kexec sequence invokes enter_vmx_ops() via copy_page() with the MMU
>> disabled. In this context, code must not rely on normal virtual address
>> translations or trigger page faults.
>>
>> With KASAN enabled, functions get instrumented and may access shadow
>> memory using regular address translation. When executed with the MMU
>> off, this can lead to page faults (bad_page_fault) from which the
>> kernel cannot recover in the kexec path, resulting in a hang.
>>
>> The kexec path sets preempt_count to HARDIRQ_OFFSET before entering
>> the MMU-off copy sequence.
>>
>> current_thread_info()->preempt_count = HARDIRQ_OFFSET
>>    kexec_sequence(..., copy_with_mmu_off = 1)
>>      -> kexec_copy_flush(image)
>>           copy_segments()
>>             -> copy_page(dest, addr)
>> 	         bl enter_vmx_ops()
>>                     if (in_interrupt())
>>                       return 0
>> 	         beq .Lnonvmx_copy
>>
>> Since kexec sets preempt_count to HARDIRQ_OFFSET, in_interrupt()
>> evaluates to true and enter_vmx_ops() returns early.
>>
>> As in_interrupt() (and preempt_count()) are always inlined, mark
>> enter_vmx_ops() with __no_sanitize_address to avoid KASAN
>> instrumentation and shadow memory access with MMU disabled, helping
>> kexec boot fine with KASAN enabled.
>>
>> Cc: Aditya Gupta <adityag@linux.ibm.com>
>> Cc: Daniel Axtens <dja@axtens.net>
>> Cc: Hari Bathini <hbathini@linux.ibm.com>
>> Cc: Madhavan Srinivasan <maddy@linux.ibm.com>
>> Cc: Mahesh Salgaonkar <mahesh@linux.ibm.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Ritesh Harjani (IBM) <ritesh.list@gmail.com>
>> Cc: Shivang Upadhyay <shivangu@linux.ibm.com>
>> Cc: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
>> Reported-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
>> Signed-off-by: Sourabh Jain <sourabhjain@linux.ibm.com>
>> ---
>> Changelog:
>>
>> v2:
>> - Remove __no_sanitize_address from exit_vmx_ops
>> - Add a comment explaining that marking only enter_vmx_ops
>>    with __no_sanitize_address is sufficient for kexec to
>>    function properly with KASAN enabled
>>
>> v1:
>> https://lore.kernel.org/all/20260321053121.614022-1-sourabhjain@linux.ibm.com/
>> ---
>>   arch/powerpc/lib/vmx-helper.c | 9 ++++++++-
>>   1 file changed, 8 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
>> index 554b248002b4..57e897b60db8 100644
>> --- a/arch/powerpc/lib/vmx-helper.c
>> +++ b/arch/powerpc/lib/vmx-helper.c
>> @@ -52,7 +52,14 @@ int exit_vmx_usercopy(void)
>>   }
>>   EXPORT_SYMBOL(exit_vmx_usercopy);
>>   
>> -int enter_vmx_ops(void)
>> +/*
>> + * Can be called from kexec copy_page() path with MMU off. The kexec
>> + * code sets preempt_count to HARDIRQ_OFFSET so we return early here.
>> + * Since in_interrupt() is always inline, __no_sanitize_address on this
>> + * function is sufficient to avoid KASAN shadow memory accesses in real
>> + * mode.
>> + */
>> +int __no_sanitize_address enter_vmx_ops(void)
>>   {
>>   	if (in_interrupt())
>>   		return 0;
>
> Without these patches, when KASAN is enabled, I observe a hang during kexec boot on
> pseries (PowerVM):
>
> [ 3459.012617][ T4209] kexec_core: Starting new kernel
> [ 3459.012814][ T4209] kexec: waiting for cpu 1 (physical 1) to enter 2 state
> [ 3459.016236][ T4209] kexec: waiting for cpu 11 (physical 11) to enter 2 state
> [ 3459.016287][ T4209] kexec: waiting for cpu 12 (physical 12) to enter 2 state
> [ 3459.016380][ T4209] kexec: waiting for cpu 13 (physical 13) to enter 2 state
> [ 3459.016418][ T4209] kexec: waiting for cpu 14 (physical 14) to enter 2 state
> [ 3459.016444][ T4209] kexec: waiting for cpu 15 (physical 15) to enter 2 state
> [ 3459.016462][ T4209] kexec: waiting for cpu 18 (physical 18) to enter 2 state
> [ 3459.271929][ T4209] kexec: Starting switchover sequence.
> [system hangs here and no further progress]
>
> ==============
>
> With both the patches applied, kexec completes successfully with KASAN enabled.
>
> Reviewed-by: Aboorva Devarajan <aboorvad@linux.ibm.com>
> Tested-by: Aboorva Devarajan <aboorvad@linux.ibm.com>

Thanks for testing and the review.

- Sourabh Jain