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Date:   Fri, 1 Oct 2021 15:05:40 +0100
From:   Matthew Wilcox <willy@infradead.org>
To:     Andrey Konovalov <andreyknvl@gmail.com>
Cc:     Andrey Ryabinin <ryabinin.a.a@gmail.com>,
        Alexander Potapenko <glider@google.com>,
        Dmitry Vyukov <dvyukov@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        kasan-dev <kasan-dev@googlegroups.com>,
        Linux Memory Management List <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] kasan: Fix tag for large allocations when using
 CONFIG_SLAB
Message-ID: <YVcVtNLnyJModOhn@casper.infradead.org>
References: <20211001024105.3217339-1-willy@infradead.org>
 <CA+fCnZfSUxToYKUfHwQT0r3bC9NYZNc2iC3PXv+GciuW0Fm79A@mail.gmail.com>
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On Fri, Oct 01, 2021 at 03:29:29PM +0200, Andrey Konovalov wrote:
> On Fri, Oct 1, 2021 at 4:42 AM Matthew Wilcox (Oracle)
> <willy@infradead.org> wrote:
> >
> > If an object is allocated on a tail page of a multi-page slab, kasan
> > will get the wrong tagbecause page->s_mem is NULL for tail pages.
> 
> Interesting. Is this a known property of tail pages? Why does this
> happen? I failed to find this exception in the code.

Yes, it's a known property of tail pages.  kmem_getpages() calls
__alloc_pages_node() which returns a pointer to the head page.
All the tail pages are initialised to point to the head page.
Then in alloc_slabmgmt(), we set ->s_mem of the head page, but
we never set ->s_mem of the tail pages.  Instead, we rely on
people always passing in the head page.  I have a patch in the works
to change the type from struct page to struct slab so you can't
make this mistake.  That was how I noticed this problem.

> The tag value won't really be "wrong", just unexpected. But if s_mem
> is indeed NULL for tail pages, your fix makes sense.
> 
> > I'm not quite sure what the user-visible effect of this might be.
> 
> Everything should work, as long as tag values are assigned
> consistently based on the object address.

OK, maybe this doesn't need to be backported then?  Actually, why
subtract s_mem in the first place?  Can we just avoid that for all
tag calculations?