Re: [PATCH v5 08/10] ARM: uaccess: add __{get,put}_kernel_nofault

From: Arnd Bergmann <arnd@kernel.org>
To: Daniel Thompson <daniel.thompson@linaro.org>
Cc: "Russell King (Oracle)" <linux@armlinux.org.uk>,
	Arnd Bergmann <arnd@arndb.de>,
	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
	Linux ARM <linux-arm-kernel@lists.infradead.org>,
	linux-arch <linux-arch@vger.kernel.org>,
	Linux-MM <linux-mm@kvack.org>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Linus Walleij <linus.walleij@linaro.org>
Subject: Re: [PATCH v5 08/10] ARM: uaccess: add __{get,put}_kernel_nofault
Date: Thu, 13 Jan 2022 12:14:50 +0100	[thread overview]
Message-ID: <CAK8P3a0=OkFcKbL+utDPTPf+RskFNdR8Vt-3BEWkO9g_FqSj5w@mail.gmail.com> (raw)
In-Reply-To: <20220113094754.6ei6ssiqbuw7tfj7@maple.lan>

On Thu, Jan 13, 2022 at 10:47 AM Daniel Thompson
<daniel.thompson@linaro.org> wrote:
> On Wed, Jan 12, 2022 at 06:08:17PM +0000, Russell King (Oracle) wrote:
>
> > The kernel attempted to access an address that is in the userspace
> > domain (NULL pointer) and took an exception.
> >
> > I suppose we should handle a domain fault more gracefully - what are
> > the required semantics if the kernel attempts a userspace access
> > using one of the _nofault() accessors?
>
> I think the best answer might well be that, if the arch provides
> implementations of hooks such as copy_from_kernel_nofault_allowed()
> then the kernel should never attempt a userspace access using the
> _nofault() accessors. That means they can do whatever they like!
>
> In other words something like the patch below looks like a promising
> approach.

Right, it seems this is the same as on x86.

> From f66a63b504ff582f261a506c54ceab8c0e77a98c Mon Sep 17 00:00:00 2001
> From: Daniel Thompson <daniel.thompson@linaro.org>
> Date: Thu, 13 Jan 2022 09:34:45 +0000
> Subject: [PATCH] arm: mm: Implement copy_from_kernel_nofault_allowed()
>
> Currently copy_from_kernel_nofault() can actually fault (due to software
> PAN) if we attempt userspace access. In any case, the documented
> behaviour for this function is to return -ERANGE if we attempt an access
> outside of kernel space.
>
> Implementing copy_from_kernel_nofault_allowed() solves both these
> problems.
>
> Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>

Reviewed-by: Arnd Bergmann <arnd@arndb.de>