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

[Mike Rapoport <rppt@kernel.org>]

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?

Also this can be done when addr is initialized 

> +
>  	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
  
>  	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.