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From:   Marco Elver <elver@google.com>
Date:   Fri, 4 Mar 2022 17:45:05 +0100
Message-ID: <CANpmjNP3EJWpt3qHEMGmi3uVBMYthp66j9jvYggdbr1kTfpKBA@mail.gmail.com>
Subject: Re: [PATCH v2 0/5] slab cleanups
To:     Vlastimil Babka <vbabka@suse.cz>
Cc:     Hyeonggon Yoo <42.hyeyoo@gmail.com>, linux-mm@kvack.org,
        Christoph Lameter <cl@linux.com>,
        Pekka Enberg <penberg@kernel.org>,
        David Rientjes <rientjes@google.com>,
        Joonsoo Kim <iamjoonsoo.kim@lge.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Matthew WilCox <willy@infradead.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        linux-kernel@vger.kernel.org
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On Fri, 4 Mar 2022 at 17:42, Vlastimil Babka <vbabka@suse.cz> wrote:
>
> On 3/4/22 14:11, Marco Elver wrote:
> > On Fri, 4 Mar 2022 at 13:02, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> >>
> >> On Fri, Mar 04, 2022 at 12:50:21PM +0100, Marco Elver wrote:
> >> > On Fri, 4 Mar 2022 at 07:34, Hyeonggon Yoo <42.hyeyoo@gmail.com> wrote:
> >> > >
> >> > > Changes from v1:
> >> > >         Now SLAB passes requests larger than order-1 page
> >> > >         to page allocator.
> >> > >
> >> > >         Adjusted comments from Matthew, Vlastimil, Rientjes.
> >> > >         Thank you for feedback!
> >> > >
> >> > >         BTW, I have no idea what __ksize() should return when an object that
> >> > >         is not allocated from slab is passed. both 0 and folio_size()
> >> > >         seems wrong to me.
> >> >
> >> > Didn't we say 0 would be the safer of the two options?
> >> > https://lkml.kernel.org/r/0e02416f-ef43-dc8a-9e8e-50ff63dd3c61@suse.cz
> >> >
> >>
> >> Oh sorry, I didn't understand why 0 was safer when I was reading it.
> >>
> >> Reading again, 0 is safer because kasan does not unpoison for
> >> wrongly passed object, right?
> >
> > Not quite. KASAN can tell if something is wrong, i.e. invalid object.
> > Similarly, if you are able to tell if the passed pointer is not a
> > valid object some other way, you can do something better - namely,
> > return 0.
>
> Hmm, but how paranoid do we have to be? Patch 1 converts SLAB to use
> kmalloc_large(). So it's now legitimate to have objects allocated by SLAB's
> kmalloc() that don't have a slab folio flag set, and their size is
> folio_size(). It would be more common than getting a bogus pointer, so
> should we return 0 just because a bogus pointer is possible?

No of course not, which is why I asked in the earlier email if it's a
"definitive failure case".

> If we do that,
> then KASAN will fail to unpoison legitimate kmalloc_large() objects, no?
> What I suggested earlier is we could make the checks more precise - if
> folio_size() is smaller or equal order-1 page, then it's bogus because we
> only do kmalloc_large() for >order-1. If the object pointer is not to the
> beginning of the folio, then it's bogus, because kmalloc_large() returns the
> beginning of the folio. Then in these case we return 0, but otherwise we
> should return folio_size()?
>
> > The intuition here is that the caller has a pointer to an
> > invalid object, and wants to use ksize() to determine its size, and
> > most likely access all those bytes. Arguably, at that point the kernel
> > is already in a degrading state. But we can try to not let things get
> > worse by having ksize() return 0, in the hopes that it will stop
> > corrupting more memory. It won't work in all cases, but should avoid
> > things like "s = ksize(obj); touch_all_bytes(obj, s)" where the size
> > bounds the memory accessed corrupting random memory.
> >
> > The other reason is that a caller could actually check the size, and
> > if 0, do something else. Few callers will do so, because nobody
> > expects that their code has a bug. :-)
>