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

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Date:   Mon, 16 Sep 2019 20:05:00 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Matthew Wilcox <willy@...radead.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 05:32:09PM -0700, Matthew Wilcox wrote:
> 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.

That requires a new test which hasn't existed before. First I need to
fix the bug, and then we can add a new test and get that into -next,
etc.

-- 
Kees Cook