linux-kernel - Re: [PATCH 1/6] syscalls: use uaccess_kernel in addr_limit_user

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Date:   Tue, 21 Jul 2020 07:35:42 +0200
From:   Christoph Hellwig <hch@....de>
To:     Guenter Roeck <linux@...ck-us.net>
Cc:     Christoph Hellwig <hch@....de>, Nick Hu <nickhu@...estech.com>,
        Greentime Hu <green.hu@...il.com>,
        Vincent Chen <deanbo422@...il.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-riscv@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] syscalls: use uaccess_kernel in
 addr_limit_user_check

On Mon, Jul 20, 2020 at 10:30:30PM -0700, Guenter Roeck wrote:
> Guess I lost it somewhere. Are you saying the check was wrong all along
> and your patch fixed it ?

Oh, it is a little complicated.

Normally we have two address space limits, KERNEL_DS and USER_DS,
and they are supposed to be different.  armnommu and m68k define them
to the same value for no good reason.  That leads to:

uaccess_kernel always returning true as it does a positive check
agains KERNEL_DS, which disables a bunch of drivers like sg and
rdma, and could also lead to really strange and probably broken
results in a few places.

It also leads to the SIGKILL in addr_limit_user_check never
triggering due to the negatіve check, which is ok as the limits
never are different.