Re: [PATCH 1/4] mm/usercopy: Check kmap addresses properly

[Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>]

On Mon, 10 Jan 2022 23:15:27 +0000 "Matthew Wilcox (Oracle)" <willy@xxxxxxxxxxxxx> wrote:

> If you are copying to an address in the kmap region, you may not copy
> across a page boundary,

In the source, the destination or in both, and why may we not?

> no matter what the size of the underlying
> allocation.  You can't kmap() a slab page because slab pages always
> come from low memory.

Why not?  kmap() does

	if (!PageHighMem(page))
		addr = page_address(page);
	else
		addr = kmap_high(page);