Re: [RFC PATCH 08/10] powerpc/fsl_booke/kaslr: clear the original kernel if randomized

[Jason Yan <yanaijie@huawei.com>]

On 2019/7/29 19:19, Christophe Leroy wrote:
> 
> 
> Le 17/07/2019 à 10:06, Jason Yan a écrit :
>> The original kernel still exists in the memory, clear it now.
>>
>> Signed-off-by: Jason Yan <yanaijie@huawei.com>
>> Cc: Diana Craciun <diana.craciun@nxp.com>
>> Cc: Michael Ellerman <mpe@ellerman.id.au>
>> Cc: Christophe Leroy <christophe.leroy@c-s.fr>
>> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
>> Cc: Paul Mackerras <paulus@samba.org>
>> Cc: Nicholas Piggin <npiggin@gmail.com>
>> Cc: Kees Cook <keescook@chromium.org>
>> ---
>>   arch/powerpc/kernel/kaslr_booke.c  | 11 +++++++++++
>>   arch/powerpc/mm/mmu_decl.h         |  2 ++
>>   arch/powerpc/mm/nohash/fsl_booke.c |  1 +
>>   3 files changed, 14 insertions(+)
>>
>> diff --git a/arch/powerpc/kernel/kaslr_booke.c 
>> b/arch/powerpc/kernel/kaslr_booke.c
>> index 90357f4bd313..00339c05879f 100644
>> --- a/arch/powerpc/kernel/kaslr_booke.c
>> +++ b/arch/powerpc/kernel/kaslr_booke.c
>> @@ -412,3 +412,14 @@ notrace void __init kaslr_early_init(void 
>> *dt_ptr, phys_addr_t size)
>>       reloc_kernel_entry(dt_ptr, kimage_vaddr);
>>   }
>> +
>> +void __init kaslr_second_init(void)
>> +{
>> +    /* If randomized, clear the original kernel */
>> +    if (kimage_vaddr != KERNELBASE) {
>> +        unsigned long kernel_sz;
>> +
>> +        kernel_sz = (unsigned long)_end - kimage_vaddr;
>> +        memset((void *)KERNELBASE, 0, kernel_sz);
> 
> Why are we clearing ? Is that just to tidy up or is it of security 
> importance ?
> 

If we leave it there, attackers can still find the kernel object very
easy, it's still dangerous especially if without 
CONFIG_STRICT_KERNEL_RWX enabled.

> If so, maybe memzero_explicit() should be used instead ?
> 

OK

>> +    }
>> +}
>> diff --git a/arch/powerpc/mm/mmu_decl.h b/arch/powerpc/mm/mmu_decl.h
>> index 754ae1e69f92..9912ee598f9b 100644
>> --- a/arch/powerpc/mm/mmu_decl.h
>> +++ b/arch/powerpc/mm/mmu_decl.h
>> @@ -150,8 +150,10 @@ extern void loadcam_multi(int first_idx, int num, 
>> int tmp_idx);
>>   #ifdef CONFIG_RANDOMIZE_BASE
>>   extern void kaslr_early_init(void *dt_ptr, phys_addr_t size);
>> +extern void kaslr_second_init(void);
> 
> No new 'extern' please.
> 
>>   #else
>>   static inline void kaslr_early_init(void *dt_ptr, phys_addr_t size) {}
>> +static inline void kaslr_second_init(void) {}
>>   #endif
>>   struct tlbcam {
>> diff --git a/arch/powerpc/mm/nohash/fsl_booke.c 
>> b/arch/powerpc/mm/nohash/fsl_booke.c
>> index 8d25a8dc965f..fa5a87f5c08e 100644
>> --- a/arch/powerpc/mm/nohash/fsl_booke.c
>> +++ b/arch/powerpc/mm/nohash/fsl_booke.c
>> @@ -269,6 +269,7 @@ notrace void __init relocate_init(u64 dt_ptr, 
>> phys_addr_t start)
>>       kernstart_addr = start;
>>       if (is_second_reloc) {
>>           virt_phys_offset = PAGE_OFFSET - memstart_addr;
>> +        kaslr_second_init();
>>           return;
>>       }
>>
> 
> Christophe
> 
> .
>