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

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Message-ID: <CAG_fn=UpzHLvmdsfTTet=j54N=HwYNn_z-n4GdqgBGhYb8eNEQ@mail.gmail.com>
Date:   Fri, 25 Mar 2022 14:15:13 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     Vlastimil Babka <vbabka@...e.cz>
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 Memory Management List <linux-mm@...ck.org>,
        Linux-Arch <linux-arch@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/43] kmsan: mm: call KMSAN hooks from SLUB code

> >  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?

Agreed, kmsan_init() is not needed here.

> >  }
> >
> >  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?

Yeah, removing it from get_freepointer() does not introduce new
errors. I'll fix this in v2.