Re: [PATCH] kasan: Fix tag for large allocations when using CONFIG_SLAB

[Matthew Wilcox <willy@infradead.org>]

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?