Re: [PATCH] mm/slub: initialize the free pointer of the bootstrap kmem_cache_node

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

On 10/10/26 22:39, Karl Mehltretter wrote:
> With slub_debug=FZPU, validating kmem_cache_node reports
> 
>   BUG kmem_cache_node (Not tainted): Freepointer corrupt
>   Object 0xffff888000140040 @offset=64 fp=0x5a5a5a5a5a5a5a5a
> 
> and taints the kernel. Poisoning places the bootstrap object's free
> pointer outside the object, where check_object() checks it even while
> allocated.
> 
> early_kmem_cache_node_alloc() previously took this object from a
> freelist with an initialized pointer. It now takes it from the slab
> iterator and builds the freelist only for the remaining objects,
> leaving its pointer filled with POISON_INUSE.
> 
> Initialize the bootstrap object's free pointer to NULL.
> 
> Fixes: dc795d4c0282 ("mm/slub: defer freelist construction until after bulk allocation from a new slab")
> Assisted-by: LLM
> Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>

Reviewed-by: Vlastimil Babka (SUSE) <vbabka@kernel.org>

> ---
> QEMU x86_64 and custom QEMU SH7785LCR model, slub_debug=FZPU:
> validation reports "Freepointer corrupt" before the fix, clean after.
> 
>  mm/slub.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 54ec125033571..9a20aa820bb0f 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -7777,6 +7777,7 @@ static void early_kmem_cache_node_alloc(int node)
>  
>  	n = next_slab_obj(kmem_cache_node, &iter);
>  	BUG_ON(!n);
> +	set_freepointer(kmem_cache_node, n, NULL);
>  
>  	slab->inuse = 1;
>  	build_slab_freelist(kmem_cache_node, slab, &iter);