Re: [PATCH] slab: Mark large folios for debugging purposes

[Harry Yoo <harry.yoo@oracle.com>]

On Mon, Mar 03, 2025 at 05:28:05PM +0000, Matthew Wilcox (Oracle) wrote:
> If a user calls p = kmalloc(1024); kfree(p); kfree(p); and 'p' was the
> only object in the slab, we may free the slab after the first call to
> kfree().  If we do, we clear PGTY_slab and the second call to kfree()
> will call free_large_kmalloc().  That will leave a trace in the logs
> ("object pointer: 0x%p"), but otherwise proceed to free the memory,
> which is likely to corrupt the page allocator's metadata.
> 
> Allocate a new page type for large kmalloc and mark the memory with it
> while it's allocated.  That lets us detect this double-free and return
> without harming any data structures.
> 
> Reported-by: Hannes Reinecke <hare@suse.com>
> Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
> ---

Looks good to me,
Reviewed-by: Harry Yoo <harry.yoo@oracle.com>

-- 
Cheers,
Harry

>  include/linux/page-flags.h | 18 ++++++++++--------
>  mm/slub.c                  |  7 +++++++
>  2 files changed, 17 insertions(+), 8 deletions(-)
> 
> diff --git a/include/linux/page-flags.h b/include/linux/page-flags.h
> index 36d283552f80..df9234e5f478 100644
> --- a/include/linux/page-flags.h
> +++ b/include/linux/page-flags.h
> @@ -925,14 +925,15 @@ FOLIO_FLAG_FALSE(has_hwpoisoned)
>  enum pagetype {
>  	/* 0x00-0x7f are positive numbers, ie mapcount */
>  	/* Reserve 0x80-0xef for mapcount overflow. */
> -	PGTY_buddy	= 0xf0,
> -	PGTY_offline	= 0xf1,
> -	PGTY_table	= 0xf2,
> -	PGTY_guard	= 0xf3,
> -	PGTY_hugetlb	= 0xf4,
> -	PGTY_slab	= 0xf5,
> -	PGTY_zsmalloc	= 0xf6,
> -	PGTY_unaccepted	= 0xf7,
> +	PGTY_buddy		= 0xf0,
> +	PGTY_offline		= 0xf1,
> +	PGTY_table		= 0xf2,
> +	PGTY_guard		= 0xf3,
> +	PGTY_hugetlb		= 0xf4,
> +	PGTY_slab		= 0xf5,
> +	PGTY_zsmalloc		= 0xf6,
> +	PGTY_unaccepted		= 0xf7,
> +	PGTY_large_kmalloc	= 0xf8,
>  
>  	PGTY_mapcount_underflow = 0xff
>  };
> @@ -1075,6 +1076,7 @@ PAGE_TYPE_OPS(Zsmalloc, zsmalloc, zsmalloc)
>   * Serialized with zone lock.
>   */
>  PAGE_TYPE_OPS(Unaccepted, unaccepted, unaccepted)
> +FOLIO_TYPE_OPS(large_kmalloc, large_kmalloc)
>  
>  /**
>   * PageHuge - Determine if the page belongs to hugetlbfs
> diff --git a/mm/slub.c b/mm/slub.c
> index 1f50129dcfb3..872e1bab3bd1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -4241,6 +4241,7 @@ static void *___kmalloc_large_node(size_t size, gfp_t flags, int node)
>  		ptr = folio_address(folio);
>  		lruvec_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B,
>  				      PAGE_SIZE << order);
> +		__folio_set_large_kmalloc(folio);
>  	}
>  
>  	ptr = kasan_kmalloc_large(ptr, size, flags);
> @@ -4716,6 +4717,11 @@ static void free_large_kmalloc(struct folio *folio, void *object)
>  {
>  	unsigned int order = folio_order(folio);
>  
> +	if (WARN_ON_ONCE(!folio_test_large_kmalloc(folio))) {
> +		dump_page(&folio->page, "Not a kmalloc allocation");
> +		return;
> +	}
> +
>  	if (WARN_ON_ONCE(order == 0))
>  		pr_warn_once("object pointer: 0x%p\n", object);
>  
> @@ -4725,6 +4731,7 @@ static void free_large_kmalloc(struct folio *folio, void *object)
>  
>  	lruvec_stat_mod_folio(folio, NR_SLAB_UNRECLAIMABLE_B,
>  			      -(PAGE_SIZE << order));
> +	__folio_clear_large_kmalloc(folio);
>  	folio_put(folio);
>  }
>  
> -- 
> 2.47.2
>