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

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Message-Id: <20220509203742.0f4adfa4004e51e66c1c9a20@linux-foundation.org>
Date:   Mon, 9 May 2022 20:37:42 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        linux-hardening@...r.kernel.org
Subject: Re: [PATCH 1/4] mm/usercopy: Check kmap addresses properly

On Mon, 10 Jan 2022 23:15:27 +0000 "Matthew Wilcox (Oracle)" <willy@...radead.org> 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);