Re: [PATCH 2/2] powerpc/kexec: Disable KASAN for VMX helpers used in MMU-off path

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

On 29/03/26 06:48, Ritesh Harjani (IBM) wrote:
> Sourabh Jain <sourabhjain@linux.ibm.com> writes:
>
>> The kexec sequence invokes enter_vmx_ops() and exit_vmx_ops() with the
>> MMU disabled. In this context, code must not rely on normal virtual
>> address translations or trigger page faults.
>> With KASAN enabled, these 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.
> Right, so with mmu off, kernel can't access KASAN shadow memory.
>
> So, let me trace down the path a bit... you skipped an important detail
> i.e. preempt_count() is always inline, and we play a few tricks in kexec
> path to tell enter_vmx_ops() that we are in HARDIRQ mode.
>
> default_machine_kexec(image)
>      current_thread_info()->preempt_count = HARDIRQ_OFFSET
>
>      kexec_sequence(..., copy_with_mmu_off = 1)
>          if (copy_with_mmu_off) bl real_mode
>
>          bl kexec_copy_flush(image)
>              memcpy(ranges, image->segment, ...)
>
>              copy_segments()
>                  copy_page(dest, addr)
>
>                      bl enter_vmx_ops()
>                          if (in_interrupt() == true) return 0 // preempt_count == HARDIRQ_OFFSET
>                      beq .Lnonvmx_copy

Yes since preempt_count for the current thread is set to HARDIRQ_OFFSET 
we return early from copy_page() -> copypage_power7 -> enter_vmx_ops() 
and call to exit_vmx_ops is skipped.
>
>> Mark enter_vmx_ops() and exit_vmx_ops() with __no_sanitize_address to
>> avoid KASAN instrumentation and ensure kexec boots fine with KASAN
>> enabled.
>>
> IIUC, preempt_count() is always inline, and since you are disabling kasan
> instrumentation on enter_vmx_ops(), hence it just works for this reason.
> But you missed adding that detail here.

Yeah it is worth adding that in commit message. I will add it in v2.

>
> enter_vmx_ops()
>      if (in_interrupt()) // return 0
>             preempt_count() & ... | HARDIRQ_OFFSET   // preempt_count() is this is __always_inline
>
> static __always_inline int preempt_count(void)
> {
> 	return READ_ONCE(current_thread_info()->preempt_count);
> }
>
>> 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>
>> ---
>>   arch/powerpc/lib/vmx-helper.c | 4 ++--
>>   1 file changed, 2 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/powerpc/lib/vmx-helper.c b/arch/powerpc/lib/vmx-helper.c
>> index 554b248002b4..c01b2d856650 100644
>> --- a/arch/powerpc/lib/vmx-helper.c
>> +++ b/arch/powerpc/lib/vmx-helper.c
>> @@ -52,7 +52,7 @@ int exit_vmx_usercopy(void)
>>   }
>>   EXPORT_SYMBOL(exit_vmx_usercopy);
>>   
>> -int enter_vmx_ops(void)
> In that case, should we should add a comment here saying:
>
> /*
>   * 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.
>   */

Thanks for the write up, I will add it in v2.

>> +int __no_sanitize_address enter_vmx_ops(void)
>>   {
>>   	if (in_interrupt())
>>   		return 0;
>> @@ -69,7 +69,7 @@ int enter_vmx_ops(void)
>>    * passed a pointer to the destination which we return as required by a
>>    * memcpy implementation.
>>    */
>> -void *exit_vmx_ops(void *dest)
>> +void __no_sanitize_address *exit_vmx_ops(void *dest)
> I am assuming since we never enter into VMX in kexec path, so kexec path
> must not be calling exit_vmx_ops anyways? So do we need __no_sanitize_address here?

Agree in copypage_power7() we jump to  .Lnonvmx_copy label and do
not call exit_vmx_ops. I will remove __no_sanitize_address from 
exit_vmx_ops().

Thanks for the detailed review Ritesh.

- Soruabh Jain