ed815bd3fe
_PAGE_USER is used to select the zone. Today zone 0 is kernel and zone 1 is user. To implement _PAGE_NONE, _PAGE_USER is cleared, leading to no access for user but kernel still has access to the page so it's possible for a user application to write in that page by using a kernel function as trampoline. What is really wanted is to have user rights on pages below TASK_SIZE and no user rights on pages above TASK_SIZE. Use zones for that. There are 16 zones so lets use the 4 upper address bits to set the zone and declare zone rights based on TASK_SIZE. Then drop _PAGE_USER and reuse it as _PAGE_READ that will be checked in Data TLB miss handler. That will properly handle PAGE_NONE for both kernel and user. In addition, it partially implements execute-only right. The implementation won't be complete because once a TLB has been loaded via the Instruction TLB miss handler, it will be possible to read the page. But at least it can't be read unless it is executed first. Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu> Signed-off-by: Michael Ellerman <mpe@ellerman.id.au> Link: https://msgid.link/2a13e3ba8a5dec43143cc1f9a91ec71ea1529f3c.1695659959.git.christophe.leroy@csgroup.eu
Linux kernel
============
There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.
In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``. The formatted documentation can also be read online at:
https://www.kernel.org/doc/html/latest/
There are various text files in the Documentation/ subdirectory,
several of them using the Restructured Text markup notation.
Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.