Re: [PATCH v4] mempool: Do not use ksize() for poisoning

[Matthew Wilcox]

On Mon, Oct 31, 2022 at 08:22:50AM -0700, Kees Cook wrote:
> On Mon, Oct 31, 2022 at 03:12:33PM +0000, Matthew Wilcox wrote:
> > On Mon, Oct 31, 2022 at 04:00:25PM +0100, Vlastimil Babka wrote:
> > > +++ b/mm/mempool.c
> > > @@ -57,8 +57,10 @@ static void __check_element(mempool_t *pool, void *element, size_t size)
> > >  static void check_element(mempool_t *pool, void *element)
> > >  {
> > >  	/* Mempools backed by slab allocator */
> > > -	if (pool->free == mempool_free_slab || pool->free == mempool_kfree) {
> > > +	if (pool->free == mempool_kfree) {
> > >  		__check_element(pool, element, (size_t)pool->pool_data);
> > > +	} else if (pool->free == mempool_free_slab) {
> > > +		__check_element(pool, element, kmem_cache_size(pool->pool_data));
> > >  	} else if (pool->free == mempool_free_pages) {
> > >  		/* Mempools backed by page allocator */
> > >  		int order = (int)(long)pool->pool_data;
> > 
> > I had a quick look at this to be sure I understood what was going on,
> > and I found a grotesque bug that has been with us since the introduction
> > of check_element() in 2015.
> > 
> > +       if (pool->free == mempool_free_pages) {
> > +               int order = (int)(long)pool->pool_data;
> > +               void *addr = kmap_atomic((struct page *)element);
> > +
> > +               __check_element(pool, addr, 1UL << (PAGE_SHIFT + order));
> > +               kunmap_atomic(addr);
> > 
> > kmap_atomic() and friends only map a single page.  So this is all
> > nonsense for HIGHMEM kernels, GFP_HIGHMEM allocations and order > 0.
> > The consequence of doing that will be calling memset(POISON_INUSE)
> > on random pages that we don't own.
> 
> Ah-ha! Thank you both! Seems like the first fix should be squashed and
> the latter one is separate? Or just put it all together?

Yes, I have no objection to Vlastimil's patch as-is.  I haven't really
reviewed it, just used it as an excuse to look at this code.  A fix for
the kmap_atomic() problem will necessarily be separate and should be
backported separately.