linux-kernel - RE: [PATCH 12/14] x86: remove address space overrides using set

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Date:   Thu, 3 Sep 2020 21:30:14 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christoph Hellwig' <hch@....de>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Michael Ellerman" <mpe@...erman.id.au>,
        "x86@...nel.org" <x86@...nel.org>
CC:     Alexey Dobriyan <adobriyan@...il.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: RE: [PATCH 12/14] x86: remove address space overrides using set_fs()

From: Christoph Hellwig
> Sent: 03 September 2020 15:23
> 
> Stop providing the possibility to override the address space using
> set_fs() now that there is no need for that any more.  To properly
> handle the TASK_SIZE_MAX checking for 4 vs 5-level page tables on
> x86 a new alternative is introduced, which just like the one in
> entry_64.S has to use the hardcoded virtual address bits to escape
> the fact that TASK_SIZE_MAX isn't actually a constant when 5-level
> page tables are enabled.

Why does it matter whether 4 or 5 level page tables are in use?
Surely all access_ok() needs to do is ensure that a valid kernel
address isn't supplied.
A non-canonical (is that the right term) address between the highest
valid user address and the lowest valid kernel address (7ffe to fffe?)
will fault anyway.
So any limit between the valid user and kernel addresses should
work?
So a limit of 1<<63 would seem appropriate.

	David

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