Re: [PATCH v4 07/18] mm: x86: Untag addresses in EXECMEM_ROX related pointer arithmetic

[Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>]

On 2025-08-14 at 10:26:19 +0300, Mike Rapoport wrote:
>On Tue, Aug 12, 2025 at 03:23:43PM +0200, Maciej Wieczor-Retman wrote:
>> ARCH_HAS_EXECMEM_ROX was re-enabled in x86 at Linux 6.14 release.
>> Related code has multiple spots where page virtual addresses end up used
>> as arguments in arithmetic operations. Combined with enabled tag-based
>> KASAN it can result in pointers that don't point where they should or
>> logical operations not giving expected results.
>> 
>> vm_reset_perms() calculates range's start and end addresses using min()
>> and max() functions. To do that it compares pointers but some are not
>> tagged - addr variable is, start and end variables aren't.
>> 
>> within() and within_range() can receive tagged addresses which get
>> compared to untagged start and end variables.
>> 
>> Reset tags in addresses used as function arguments in min(), max(),
>> within() and within_range().
>> 
>> execmem_cache_add() adds tagged pointers to a maple tree structure,
>> which then are incorrectly compared when walking the tree. That results
>> in different pointers being returned later and page permission violation
>> errors panicking the kernel.
>> 
>> Reset tag of the address range inserted into the maple tree inside
>> execmem_cache_add().
>> 
>> Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
>> ---
>> Changelog v4:
>> - Add patch to the series.
>> 
>>  arch/x86/mm/pat/set_memory.c | 1 +
>>  mm/execmem.c                 | 4 +++-
>>  mm/vmalloc.c                 | 4 ++--
>>  3 files changed, 6 insertions(+), 3 deletions(-)
>> 
>> diff --git a/arch/x86/mm/pat/set_memory.c b/arch/x86/mm/pat/set_memory.c
>> index 8834c76f91c9..1f14a1297db0 100644
>> --- a/arch/x86/mm/pat/set_memory.c
>> +++ b/arch/x86/mm/pat/set_memory.c
>> @@ -222,6 +222,7 @@ static inline void cpa_inc_lp_preserved(int level) { }
>>  static inline int
>>  within(unsigned long addr, unsigned long start, unsigned long end)
>>  {
>> +	addr = (unsigned long)kasan_reset_tag((void *)addr);
>>  	return addr >= start && addr < end;
>>  }
>>  
>> diff --git a/mm/execmem.c b/mm/execmem.c
>> index 0822305413ec..743fa4a8c069 100644
>> --- a/mm/execmem.c
>> +++ b/mm/execmem.c
>> @@ -191,6 +191,8 @@ static int execmem_cache_add_locked(void *ptr, size_t size, gfp_t gfp_mask)
>>  	unsigned long lower, upper;
>>  	void *area = NULL;
>>  
>> +	addr = arch_kasan_reset_tag(addr);
>
>Shouldn't this use kasan_reset_tag()?
>And the calls below as well?

Yes, my mistake, the kernel bot pointed that out for me too :b.

>
>Also this can be done when addr is initialized 

Sure, I'll do that there.

>
>> +
>>  	lower = addr;
>>  	upper = addr + size - 1;
>>  
>> @@ -216,7 +218,7 @@ static int execmem_cache_add(void *ptr, size_t size, gfp_t gfp_mask)
>>  static bool within_range(struct execmem_range *range, struct ma_state *mas,
>>  			 size_t size)
>>  {
>> -	unsigned long addr = mas->index;
>> +	unsigned long addr = arch_kasan_reset_tag(mas->index);
>
>AFAIU, we use plain address without the tag as an index in
>execmem_cache_add(), so here mas->index will be a plain address as well

I'll recheck to make sure but I had some unspecific errors such as "page
permission violation". So I thought a page address must be picked incorrectly
somewhere due to tagging. After revising most places where there is pointer
arithmetic / comparisons and printing these addresses I found some were tagged
in within_range().

But I'll recheck if my other changes didn't make this line redundant. I added
this first which fixed some issues but then I found more which were fixed by
resetting addr in execmem_cache_add_locked().

>  
>>  	if (addr >= range->start && addr + size < range->end)
>>  		return true;
>> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
>> index 6dbcdceecae1..83d666e4837a 100644
>> --- a/mm/vmalloc.c
>> +++ b/mm/vmalloc.c
>> @@ -3328,8 +3328,8 @@ static void vm_reset_perms(struct vm_struct *area)
>>  			unsigned long page_size;
>>  
>>  			page_size = PAGE_SIZE << page_order;
>> -			start = min(addr, start);
>> -			end = max(addr + page_size, end);
>> +			start = min((unsigned long)arch_kasan_reset_tag(addr), start);
>> +			end = max((unsigned long)arch_kasan_reset_tag(addr) + page_size, end);
>>  			flush_dmap = 1;
>>  		}
>>  	}
>> -- 
>> 2.50.1
>> 
>
>-- 
>Sincerely yours,
>Mike.

-- 
Kind regards
Maciej Wieczór-Retman