[PATCH] arm64: mm: reset address tag set by kasan sw tagging

[PATCH] arm64: mm: reset address tag set by kasan sw tagging

[Shyam Thombre]

KASAN sw tagging sets a random tag of 8 bits in the top byte of the pointer
returned by the memory allocating functions. So for the functions unaware
of this change, the top 8 bits of the address must be reset which is done
by the function arch_kasan_reset_tag().

Signed-off-by: Shyam Thombre <sthombre@codeaurora.org>
---
 arch/arm64/mm/mmu.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
index e7fbc62..eae7655 100644
--- a/arch/arm64/mm/mmu.c
+++ b/arch/arm64/mm/mmu.c
@@ -723,6 +723,7 @@ int kern_addr_valid(unsigned long addr)
 	pmd_t *pmdp, pmd;
 	pte_t *ptep, pte;
 
+	addr = arch_kasan_reset_tag(addr);
 	if ((((long)addr) >> VA_BITS) != -1UL)
 		return 0;
 
-- 
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a Linux Foundation Collaborative Project


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Re: [PATCH] arm64: mm: reset address tag set by kasan sw tagging

[Catalin Marinas]

On Wed, Jun 10, 2020 at 04:39:44PM +0530, Shyam Thombre wrote:
> KASAN sw tagging sets a random tag of 8 bits in the top byte of the pointer
> returned by the memory allocating functions. So for the functions unaware
> of this change, the top 8 bits of the address must be reset which is done
> by the function arch_kasan_reset_tag().
> 
> Signed-off-by: Shyam Thombre <sthombre@codeaurora.org>
> ---
>  arch/arm64/mm/mmu.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index e7fbc62..eae7655 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -723,6 +723,7 @@ int kern_addr_valid(unsigned long addr)
>  	pmd_t *pmdp, pmd;
>  	pte_t *ptep, pte;
>  
> +	addr = arch_kasan_reset_tag(addr);
>  	if ((((long)addr) >> VA_BITS) != -1UL)
>  		return 0;

It would be interesting to know what fails without this patch. The only
user seems to be read_kcore() and, at a quick look, I don't see how it
can generate tagged addresses.

-- 
Catalin

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Re: [PATCH] arm64: mm: reset address tag set by kasan sw tagging

[Shyam Thombre]

Hi Catalin,


On 6/10/2020 5:06 PM, Catalin Marinas wrote:
> On Wed, Jun 10, 2020 at 04:39:44PM +0530, Shyam Thombre wrote:
>> KASAN sw tagging sets a random tag of 8 bits in the top byte of the pointer
>> returned by the memory allocating functions. So for the functions unaware
>> of this change, the top 8 bits of the address must be reset which is done
>> by the function arch_kasan_reset_tag().
>>
>> Signed-off-by: Shyam Thombre <sthombre@codeaurora.org>
>> ---
>>   arch/arm64/mm/mmu.c | 1 +
>>   1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
>> index e7fbc62..eae7655 100644
>> --- a/arch/arm64/mm/mmu.c
>> +++ b/arch/arm64/mm/mmu.c
>> @@ -723,6 +723,7 @@ int kern_addr_valid(unsigned long addr)
>>   	pmd_t *pmdp, pmd;
>>   	pte_t *ptep, pte;
>>   
>> +	addr = arch_kasan_reset_tag(addr);
>>   	if ((((long)addr) >> VA_BITS) != -1UL)
>>   		return 0;
> 
> It would be interesting to know what fails without this patch. The only
> user seems to be read_kcore() and, at a quick look, I don't see how it
> can generate tagged addresses.
> 

This issue is seen in downstream GPU drivers. It currently doesn't look 
to impact any upstream users.


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Re: [PATCH] arm64: mm: reset address tag set by kasan sw tagging

[Will Deacon]

On Wed, 10 Jun 2020 16:39:44 +0530, Shyam Thombre wrote:
> KASAN sw tagging sets a random tag of 8 bits in the top byte of the pointer
> returned by the memory allocating functions. So for the functions unaware
> of this change, the top 8 bits of the address must be reset which is done
> by the function arch_kasan_reset_tag().

Applied to arm64 (for-next/fixes), thanks!

[1/1] arm64: mm: reset address tag set by kasan sw tagging
      https://git.kernel.org/arm64/c/8dd4daa04278

Cheers,
-- 
Will

https://fixes.arm64.dev
https://next.arm64.dev
https://will.arm64.dev

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