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Date:   Tue, 10 May 2022 15:01:22 -0700
From:   Kees Cook <>
To:     Andrew Morton <>
Cc:     "Matthew Wilcox (Oracle)" <>,,
Subject: Re: [PATCH 1/4] mm/usercopy: Check kmap addresses properly

On Mon, May 09, 2022 at 08:37:42PM -0700, Andrew Morton wrote:
> On Mon, 10 Jan 2022 23:15:27 +0000 "Matthew Wilcox (Oracle)" <> 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?

This depends on direction. For copying to userspace, the source (kmap).
For copying from userspace, the destination (kmap).

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

As in it'll be processed as a slab page instead of kmap by the usercopy

Kees Cook

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