linux-kernel - Re: [PATCH net-next] vhost: use "checked" versions of get_user() and put

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Message-ID: <02B0FDF1-41D4-4A7D-A57E-089D2B69CEF2@nutanix.com>
Date: Wed, 26 Nov 2025 19:47:27 +0000
From: Jon Kohler <jon@...anix.com>
To: Arnd Bergmann <arnd@...db.de>
CC: Jason Wang <jasowang@...hat.com>, "Michael S. Tsirkin" <mst@...hat.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        "kvm@...r.kernel.org"
	<kvm@...r.kernel.org>,
        "virtualization@...ts.linux.dev"
	<virtualization@...ts.linux.dev>,
        Netdev <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Linus Torvalds
	<torvalds@...ux-foundation.org>,
        Borislav Petkov <bp@...en8.de>,
        Sean
 Christopherson <seanjc@...gle.com>,
        "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>,
        Russell King <linux@...linux.org.uk>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>, Krzysztof Kozlowski <krzk@...nel.org>,
        Alexandre Belloni
	<alexandre.belloni@...tlin.com>,
        Linus Walleij <linus.walleij@...aro.org>,
        Drew Fustini <fustini@...nel.org>
Subject: Re: [PATCH net-next] vhost: use "checked" versions of get_user() and
 put_user()



> On Nov 26, 2025, at 5:25 AM, Arnd Bergmann <arnd@...db.de> wrote:
> 
> On Wed, Nov 26, 2025, at 07:04, Jason Wang wrote:
>> On Wed, Nov 26, 2025 at 3:45 AM Jon Kohler <jon@...anix.com> wrote:
>>>> On Nov 19, 2025, at 8:57 PM, Jason Wang <jasowang@...hat.com> wrote:
>>>> On Tue, Nov 18, 2025 at 1:35 AM Jon Kohler <jon@...anix.com> wrote:
>>> Same deal goes for __put_user() vs put_user by way of commit
>>> e3aa6243434f ("ARM: 8795/1: spectre-v1.1: use put_user() for __put_user()”)
>>> 
>>> Looking at arch/arm/mm/Kconfig, there are a variety of scenarios
>>> where CONFIG_CPU_SPECTRE will be enabled automagically. Looking at
>>> commit 252309adc81f ("ARM: Make CONFIG_CPU_V7 valid for 32bit ARMv8 implementations")
>>> it says that "ARMv8 is a superset of ARMv7", so I’d guess that just
>>> about everything ARM would include this by default?
> 
> I think the more relevant commit is for 64-bit Arm here, but this does
> the same thing, see 84624087dd7e ("arm64: uaccess: Don't bother
> eliding access_ok checks in __{get, put}_user").

Ah! Right, this is definitely the important bit, as it makes it
crystal clear that these are exactly the same thing. The current
code is:
#define get_user	__get_user
#define put_user	__put_user

So, this patch changing from __* to regular versions is a no-op
on arm side of the house, yea?

> I would think that if we change the __get_user() to get_user()
> in this driver, the same should be done for the
> __copy_{from,to}_user(), which similarly skips the access_ok()
> check but not the PAN/SMAP handling.

Perhaps, thats a good call out. I’d file that under one battle
at a time. Let’s get get/put user dusted first, then go down
that road?

> In general, the access_ok()/__get_user()/__copy_from_user()
> pattern isn't really helpful any more, as Linus already
> explained. I can't tell from the vhost driver code whether
> we can just drop the access_ok() here and use the plain
> get_user()/copy_from_user(), or if it makes sense to move
> to the newer user_access_begin()/unsafe_get_user()/
> unsafe_copy_from_user()/user_access_end() and try optimize
> out a few PAN/SMAP flips in the process.

In general, I think there are a few spots where we might be
able to optimize (vhost_get_vq_desc perhaps?) as that gets
called quite a bit and IIRC there are at least two flips
in there that perhaps we could elide to one? An investigation
for another day I think.

Anyhow, with this info - Jason - is there anything else you
can think of that we want to double click on?

Jon