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

Matthew Wilcox
2022-10-31T15:41:28Z

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.