Re: [PATCH] slub: fix data loss and overflow in krealloc()

["Vlastimil Babka (SUSE)" <vbabka@kernel.org>]

On 4/16/26 15:25, Marco Elver wrote:
> Commit 2cd8231796b5 ("mm/slub: allow to set node and align in
> k[v]realloc") introduced the ability to force a reallocation if the
> original object does not satisfy new alignment or NUMA node, even when
> the object is being shrunk.
> 
> This introduced two bugs in the reallocation fallback path:
> 
> 1. Data loss during NUMA migration: The jump to 'alloc_new' happens
>    before 'ks' and 'orig_size' are initialized. As a result, the
>    memcpy() in the 'alloc_new' block would copy 0 bytes into the new
>    allocation.
> 
> 2. Buffer overflow during shrinking: When shrinking an object while
>    forcing a new alignment, 'new_size' is smaller than the old size.
>    However, the memcpy() used the old size ('orig_size ?: ks'), leading
>    to an out-of-bounds write.
> 
> The same overflow bug exists in the kvrealloc() fallback path, where the
> old bucket size ksize(p) is copied into the new buffer without being
> bounded by the new size.
> 
> A simple reproducer:
> 
> 	// e.g. add to lkdtm as KREALLOC_SHRINK_OVERFLOW
> 	while (1) {
> 		void *p = kmalloc(128, GFP_KERNEL);
> 		p = krealloc_node_align(p, 64, 256, GFP_KERNEL, NUMA_NO_NODE);
> 		kfree(p);
> 	}
> 
> demonstrates the issue:
> 
>   ==================================================================
>   BUG: KFENCE: out-of-bounds write in memcpy_orig+0x68/0x130
> 
>   Out-of-bounds write at 0xffff8883ad757038 (120B right of kfence-#47):
>    memcpy_orig+0x68/0x130
>    krealloc_node_align_noprof+0x1c8/0x340
>    lkdtm_KREALLOC_SHRINK_OVERFLOW+0x8c/0xc0 [lkdtm]
>    lkdtm_do_action+0x3a/0x60 [lkdtm]
>    ...
> 
>   kfence-#47: 0xffff8883ad756fc0-0xffff8883ad756fff, size=64, cache=kmalloc-64
> 
>   allocated by task 316 on cpu 7 at 97.680481s (0.021813s ago):
>    krealloc_node_align_noprof+0x19c/0x340
>    lkdtm_KREALLOC_SHRINK_OVERFLOW+0x8c/0xc0 [lkdtm]
>    lkdtm_do_action+0x3a/0x60 [lkdtm]
>    ...
>   ==================================================================
> 
> Fix it by moving the old size calculation to the top of __do_krealloc()
> and bounding all copy lengths by the new allocation size.
> 
> Fixes: 2cd8231796b5 ("mm/slub: allow to set node and align in k[v]realloc")
> Cc: <stable@vger.kernel.org>
> Reported-by: https://sashiko.dev/#/patchset/20260415143735.2974230-1-elver%40google.com
> Signed-off-by: Marco Elver <elver@google.com>

Ouch, thanks. Added to slab/for-next-fixes
Indeed the vrealloc would be separate patch with different Fixes: commit and
handled in the mm tree.

> ---
>  mm/slub.c | 24 ++++++++++++------------
>  1 file changed, 12 insertions(+), 12 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 92362eeb13e5..161079ac5ba1 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -6645,16 +6645,6 @@ __do_krealloc(const void *p, size_t new_size, unsigned long align, gfp_t flags,
>  	if (!kasan_check_byte(p))
>  		return NULL;
>  
> -	/*
> -	 * If reallocation is not necessary (e. g. the new size is less
> -	 * than the current allocated size), the current allocation will be
> -	 * preserved unless __GFP_THISNODE is set. In the latter case a new
> -	 * allocation on the requested node will be attempted.
> -	 */
> -	if (unlikely(flags & __GFP_THISNODE) && nid != NUMA_NO_NODE &&
> -		     nid != page_to_nid(virt_to_page(p)))
> -		goto alloc_new;
> -
>  	if (is_kfence_address(p)) {
>  		ks = orig_size = kfence_ksize(p);
>  	} else {
> @@ -6673,6 +6663,16 @@ __do_krealloc(const void *p, size_t new_size, unsigned long align, gfp_t flags,
>  		}
>  	}
>  
> +	/*
> +	 * If reallocation is not necessary (e. g. the new size is less
> +	 * than the current allocated size), the current allocation will be
> +	 * preserved unless __GFP_THISNODE is set. In the latter case a new
> +	 * allocation on the requested node will be attempted.
> +	 */
> +	if (unlikely(flags & __GFP_THISNODE) && nid != NUMA_NO_NODE &&
> +		     nid != page_to_nid(virt_to_page(p)))
> +		goto alloc_new;
> +
>  	/* If the old object doesn't fit, allocate a bigger one */
>  	if (new_size > ks)
>  		goto alloc_new;
> @@ -6707,7 +6707,7 @@ __do_krealloc(const void *p, size_t new_size, unsigned long align, gfp_t flags,
>  	if (ret && p) {
>  		/* Disable KASAN checks as the object's redzone is accessed. */
>  		kasan_disable_current();
> -		memcpy(ret, kasan_reset_tag(p), orig_size ?: ks);
> +		memcpy(ret, kasan_reset_tag(p), min(new_size, (size_t)(orig_size ?: ks)));
>  		kasan_enable_current();
>  	}
>  
> @@ -6941,7 +6941,7 @@ void *kvrealloc_node_align_noprof(const void *p, size_t size, unsigned long alig
>  		if (p) {
>  			/* We already know that `p` is not a vmalloc address. */
>  			kasan_disable_current();
> -			memcpy(n, kasan_reset_tag(p), ksize(p));
> +			memcpy(n, kasan_reset_tag(p), min(size, ksize(p)));
>  			kasan_enable_current();
>  
>  			kfree(p);