Re: [PATCH] usercopy: Skip HIGHMEM page checking

[Matthew Wilcox]

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.