linux-hardening - Re: [PATCH v3] powerpc/mm: Support execute-only memory on the Radix MMU

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Date:   Tue, 9 Aug 2022 11:57:22 +0530
From:   Aneesh Kumar K V <aneesh.kumar@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Russell Currey <ruscur@...sell.cc>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Cc:     "mpe@...erman.id.au" <mpe@...erman.id.au>,
        "ajd@...ux.ibm.com" <ajd@...ux.ibm.com>,
        "npiggin@...il.com" <npiggin@...il.com>,
        "anshuman.khandual@....com" <anshuman.khandual@....com>,
        "linux-hardening@...r.kernel.org" <linux-hardening@...r.kernel.org>
Subject: Re: [PATCH v3] powerpc/mm: Support execute-only memory on the Radix
 MMU

On 8/9/22 11:21 AM, Christophe Leroy wrote:
> Le 09/08/2022 à 04:44, Russell Currey a écrit :
>> The Hash MMU already supports XOM (i.e. mmap with PROT_EXEC only)
>> through the execute-only pkey.  A PROT_EXEC-only mapping will actually
>> map to RX, and then the pkey will be applied on top of it.
> 
> I don't think XOM is a commonly understood accronym. Maybe the first 
> time you use it it'd be better to say something like:
> 
> The Hash MMU already supports execute-only memory (XOM)
> 
> When you say that Hash MMU supports it through the execute-only pkey, 
> does it mean that it is taken into account automatically at mmap time, 
> or does the userspace app has to do something special to use the key ? 
> If it is the second, it means that depending on whether you are radix or 
> not, you must do something different ? Is that expected ?
> 
>>
>> Radix doesn't have pkeys, but it does have execute permissions built-in
>> to the MMU, so all we have to do to support XOM is expose it.
>>
>> Signed-off-by: Russell Currey <ruscur@...sell.cc>
>> ---
>> v3: Incorporate Aneesh's suggestions, leave protection_map untouched
>> Basic test: https://github.com/ruscur/junkcode/blob/main/mmap_test.c
>>
>>   arch/powerpc/include/asm/book3s/64/pgtable.h |  2 ++
>>   arch/powerpc/mm/book3s64/pgtable.c           | 11 +++++++++--
>>   arch/powerpc/mm/fault.c                      |  6 +++++-
>>   3 files changed, 16 insertions(+), 3 deletions(-)
>>
>> diff --git a/arch/powerpc/include/asm/book3s/64/pgtable.h b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> index 392ff48f77df..486902aff040 100644
>> --- a/arch/powerpc/include/asm/book3s/64/pgtable.h
>> +++ b/arch/powerpc/include/asm/book3s/64/pgtable.h
>> @@ -151,6 +151,8 @@
>>   #define PAGE_COPY_X	__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
>>   #define PAGE_READONLY	__pgprot(_PAGE_BASE | _PAGE_READ)
>>   #define PAGE_READONLY_X	__pgprot(_PAGE_BASE | _PAGE_READ | _PAGE_EXEC)
>> +/* Radix only, Hash uses PAGE_READONLY_X + execute-only pkey instead */
>> +#define PAGE_EXECONLY	__pgprot(_PAGE_BASE | _PAGE_EXEC)
>>   
>>   /* Permission masks used for kernel mappings */
>>   #define PAGE_KERNEL	__pgprot(_PAGE_BASE | _PAGE_KERNEL_RW)
>> diff --git a/arch/powerpc/mm/book3s64/pgtable.c b/arch/powerpc/mm/book3s64/pgtable.c
>> index 7b9966402b25..62f63d344596 100644
>> --- a/arch/powerpc/mm/book3s64/pgtable.c
>> +++ b/arch/powerpc/mm/book3s64/pgtable.c
>> @@ -553,8 +553,15 @@ EXPORT_SYMBOL_GPL(memremap_compat_align);
>>   
>>   pgprot_t vm_get_page_prot(unsigned long vm_flags)
>>   {
>> -	unsigned long prot = pgprot_val(protection_map[vm_flags &
>> -					(VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]);
>> +	unsigned long prot;
>> +
>> +	/* Radix supports execute-only, but protection_map maps X -> RX */
>> +	if (radix_enabled() && ((vm_flags & (VM_READ|VM_WRITE|VM_EXEC)) == VM_EXEC)) {
> 
> Maybe use VM_ACCESS_FLAGS ?
> 
>> +		prot = pgprot_val(PAGE_EXECONLY);
>> +	} else {
>> +		prot = pgprot_val(protection_map[vm_flags &
>> +				  (VM_READ|VM_WRITE|VM_EXEC|VM_SHARED)]);
>> +	}
>>   
>>   	if (vm_flags & VM_SAO)
>>   		prot |= _PAGE_SAO;
>> diff --git a/arch/powerpc/mm/fault.c b/arch/powerpc/mm/fault.c
>> index 014005428687..59e4cbcf3109 100644
>> --- a/arch/powerpc/mm/fault.c
>> +++ b/arch/powerpc/mm/fault.c
>> @@ -270,7 +270,11 @@ static bool access_error(bool is_write, bool is_exec, struct vm_area_struct *vma
>>   		return false;
>>   	}
>>   
>> -	if (unlikely(!vma_is_accessible(vma)))
>> +	/* On Radix, a read fault could be from PROT_NONE or PROT_EXEC */
>> +	if (unlikely(radix_enabled() && !(vma->vm_flags & VM_READ)))
>> +		return true;
> 
> Why do you need the radix_enabled() here ?
> Even if it doesn't fault directly, reading a non readable area is still 
> an error and should be handled as such, even on hardware that will not 
> generate a fault for it at the first place. So I'd just do:
> 
> 	if (!(vma->vm_flags & VM_READ)))
> 		return true;
> 
>> +	/* Check for a PROT_NONE fault on other MMUs */
>> +	else if (unlikely(!vma_is_accessible(vma)))
>>   		return true;
>>   	/*
>>   	 * We should ideally do the vma pkey access check here. But in the
> 
> Don't use an if/else construct, there is no other 'else' in that 
> function, or in similar functions like bad_kernel_fault() for instance.
> 
> So leave the !vma_is_accessible(vma) untouched and add your check as a 
> standalone check before or after it.


What does vma_is_accessible() check bring if we have the VM_READ check unconditional ?

-aneesh