linux-kernel - Re: [PATCH linux-next] alloc_tag: Fix boot failure due to NULL pointer dereference

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Message-ID: <96d00a94-1a4b-4378-8d89-0554f89778e1@suse.cz>
Date: Fri, 26 Sep 2025 14:48:13 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: ranxiaokai627@....com, akpm@...ux-foundation.org, cl@...two.org,
 rientjes@...gle.com, roman.gushchin@...ux.dev, harry.yoo@...cle.com,
 usamaarif642@...il.com, surenb@...gle.com, shakeel.butt@...ux.dev,
 hannes@...xchg.org
Cc: linux-kernel@...r.kernel.org, linux-mm@...ck.org, ran.xiaokai@....com.cn
Subject: Re: [PATCH linux-next] alloc_tag: Fix boot failure due to NULL
 pointer dereference

On 9/26/25 10:06, ranxiaokai627@....com wrote:
> From: Ran Xiaokai <ran.xiaokai@....com.cn>
> 
> There is a boot failure when both CONFIG_DEBUG_KMEMLEAK and
> CONFIG_MEM_ALLOC_PROFILING are enabled.
> 
> BUG: kernel NULL pointer dereference, address: 0000000000000000
> RIP: 0010:__alloc_tagging_slab_alloc_hook+0x181/0x2f0
> Call Trace:
>  kmem_cache_alloc_noprof+0x1c8/0x5c0
>  __alloc_object+0x2f/0x290
>  __create_object+0x22/0x80
>  kmemleak_init+0x122/0x190
>  mm_core_init+0xb6/0x160
>  start_kernel+0x39f/0x920
>  x86_64_start_reservations+0x18/0x30
>  x86_64_start_kernel+0x104/0x120
>  common_startup_64+0x12c/0x138
> 
> In kmemleak, mem_pool_alloc() directly calls kmem_cache_alloc_noprof(),
> as a result, the alloc_tag structure associated with object_cache is not
> defined neither initialized. So current->alloc_tag is NULL,
> leading to a null pointer dereference.

Agree with Harry. This should be enough:

"as a result, current->alloc_tag is NULL, leading to a null pointer
dereference."

> Move the checks for SLAB_NO_OBJ_EXT, SLAB_NOLEAKTRACE, and
> __GFP_NO_OBJ_EXT to the parent function __alloc_tagging_slab_alloc_hook()
> to fix this.
> 
> Also this distinguishes the SLAB_NOLEAKTRACE case between the actual memory
> allocation failures case, make CODETAG_FLAG_INACCURATE more accurate.

Good point.

> Fixes: b9e2f58ffb84 ("alloc_tag: mark inaccurate allocation counters in /proc/allocinfo output")

That's in mm-stable so the fix should go there (probably too late to fold
now) if it's to be in the merge window PR.

> Signed-off-by: Ran Xiaokai <ran.xiaokai@....com.cn>

Acked-by: Vlastimil Babka <vbabka@...e.cz>

> ---
>  mm/slub.c | 18 +++++++++---------
>  1 file changed, 9 insertions(+), 9 deletions(-)
> 
> diff --git a/mm/slub.c b/mm/slub.c
> index 867a07260acf..09cbe580842c 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -2197,15 +2197,6 @@ prepare_slab_obj_exts_hook(struct kmem_cache *s, gfp_t flags, void *p)
>  {
>  	struct slab *slab;
>  
> -	if (!p)
> -		return NULL;
> -
> -	if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
> -		return NULL;
> -
> -	if (flags & __GFP_NO_OBJ_EXT)
> -		return NULL;
> -
>  	slab = virt_to_slab(p);
>  	if (!slab_obj_exts(slab) &&
>  	    alloc_slab_obj_exts(slab, s, flags, false)) {
> @@ -2223,6 +2214,15 @@ __alloc_tagging_slab_alloc_hook(struct kmem_cache *s, void *object, gfp_t flags)
>  {
>  	struct slabobj_ext *obj_exts;
>  
> +	if (!object)
> +		return;
> +
> +	if (s->flags & (SLAB_NO_OBJ_EXT | SLAB_NOLEAKTRACE))
> +		return;
> +
> +	if (flags & __GFP_NO_OBJ_EXT)
> +		return;
> +
>  	obj_exts = prepare_slab_obj_exts_hook(s, flags, object);
>  	/*
>  	 * Currently obj_exts is used only for allocation profiling.