linux-kernel - Re: [PATCH] usercopy: Skip HIGHMEM page checking

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Message-ID: <20190917003209.GS29434@bombadil.infradead.org>
Date:   Mon, 16 Sep 2019 17:32:09 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org
Subject: Re: [PATCH] usercopy: Skip HIGHMEM page checking

On Mon, Sep 16, 2019 at 02:32:56PM -0700, Kees Cook wrote:
> When running on a system with >512MB RAM with a 32-bit kernel built with:
> 
> 	CONFIG_DEBUG_VIRTUAL=y
> 	CONFIG_HIGHMEM=y
> 	CONFIG_HARDENED_USERCOPY=y
> 
> all execve()s will fail due to argv copying into kmap()ed pages, and on
> usercopy checking the calls ultimately of virt_to_page() will be looking
> for "bad" kmap (highmem) pointers due to CONFIG_DEBUG_VIRTUAL=y:

I don't understand why you want to skip the check.  We must not cross a
page boundary of a kmapped page.