linux-kernel - Re: [PATCH 16/43] kmsan: mm: call KMSAN hooks from SLUB code

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Date:   Fri, 7 Jan 2022 18:22:10 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Alexander Potapenko <glider@...gle.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andrey Konovalov <andreyknvl@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        Christoph Hellwig <hch@....de>,
        Christoph Lameter <cl@...ux.com>,
        David Rientjes <rientjes@...gle.com>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Eric Dumazet <edumazet@...gle.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Herbert Xu <herbert@...dor.apana.org.au>,
        Ilya Leoshkevich <iii@...ux.ibm.com>,
        Ingo Molnar <mingo@...hat.com>, Jens Axboe <axboe@...nel.dk>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Pekka Enberg <penberg@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Vegard Nossum <vegard.nossum@...cle.com>, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 16/43] kmsan: mm: call KMSAN hooks from SLUB code

On 12/14/21 17:20, Alexander Potapenko wrote:
> In order to report uninitialized memory coming from heap allocations
> KMSAN has to poison them unless they're created with __GFP_ZERO.
> 
> It's handy that we need KMSAN hooks in the places where
> init_on_alloc/init_on_free initialization is performed.
> 
> Signed-off-by: Alexander Potapenko <glider@...gle.com>
> ---
> Link: https://linux-review.googlesource.com/id/I6954b386c5c5d7f99f48bb6cbcc74b75136ce86e
> ---
>  mm/slab.h |  1 +
>  mm/slub.c | 26 +++++++++++++++++++++++---
>  2 files changed, 24 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/slab.h b/mm/slab.h
> index 56ad7eea3ddfb..6175a74047b47 100644
> --- a/mm/slab.h
> +++ b/mm/slab.h
> @@ -521,6 +521,7 @@ static inline void slab_post_alloc_hook(struct kmem_cache *s,
>  			memset(p[i], 0, s->object_size);
>  		kmemleak_alloc_recursive(p[i], s->object_size, 1,
>  					 s->flags, flags);
> +		kmsan_slab_alloc(s, p[i], flags);
>  	}
>  
>  	memcg_slab_post_alloc_hook(s, objcg, flags, size, p);
> diff --git a/mm/slub.c b/mm/slub.c
> index abe7db581d686..5a63486e52531 100644
> --- a/mm/slub.c
> +++ b/mm/slub.c
> @@ -22,6 +22,7 @@
>  #include <linux/proc_fs.h>
>  #include <linux/seq_file.h>
>  #include <linux/kasan.h>
> +#include <linux/kmsan.h>
>  #include <linux/cpu.h>
>  #include <linux/cpuset.h>
>  #include <linux/mempolicy.h>
> @@ -346,10 +347,13 @@ static inline void *freelist_dereference(const struct kmem_cache *s,
>  			    (unsigned long)ptr_addr);
>  }
>  
> +/*
> + * See the comment to get_freepointer_safe().
> + */

I did...

>  static inline void *get_freepointer(struct kmem_cache *s, void *object)
>  {
>  	object = kasan_reset_tag(object);
> -	return freelist_dereference(s, object + s->offset);
> +	return kmsan_init(freelist_dereference(s, object + s->offset));

... but I don't see why it applies to get_freepointer() too? What am I missing?

>  }
>  
>  static void prefetch_freepointer(const struct kmem_cache *s, void *object)
> @@ -357,18 +361,28 @@ static void prefetch_freepointer(const struct kmem_cache *s, void *object)
>  	prefetchw(object + s->offset);
>  }
>  
> +/*
> + * When running under KMSAN, get_freepointer_safe() may return an uninitialized
> + * pointer value in the case the current thread loses the race for the next
> + * memory chunk in the freelist. In that case this_cpu_cmpxchg_double() in
> + * slab_alloc_node() will fail, so the uninitialized value won't be used, but
> + * KMSAN will still check all arguments of cmpxchg because of imperfect
> + * handling of inline assembly.
> + * To work around this problem, use kmsan_init() to force initialize the
> + * return value of get_freepointer_safe().
> + */
>  static inline void *get_freepointer_safe(struct kmem_cache *s, void *object)
>  {
>  	unsigned long freepointer_addr;
>  	void *p;
>  
>  	if (!debug_pagealloc_enabled_static())
> -		return get_freepointer(s, object);
> +		return kmsan_init(get_freepointer(s, object));

So here kmsan_init() is done twice?

>  
>  	object = kasan_reset_tag(object);
>  	freepointer_addr = (unsigned long)object + s->offset;
>  	copy_from_kernel_nofault(&p, (void **)freepointer_addr, sizeof(p));
> -	return freelist_ptr(s, p, freepointer_addr);
> +	return kmsan_init(freelist_ptr(s, p, freepointer_addr));
>  }
>  
>  static inline void set_freepointer(struct kmem_cache *s, void *object, void *fp)