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

["Arnd Bergmann" <arnd@arndb.de>]

On Wed, Nov 26, 2025, at 07:04, Jason Wang wrote:
> On Wed, Nov 26, 2025 at 3:45 AM Jon Kohler <jon@nutanix.com> wrote:
>> > On Nov 19, 2025, at 8:57 PM, Jason Wang <jasowang@redhat.com> wrote:
>> > On Tue, Nov 18, 2025 at 1:35 AM Jon Kohler <jon@nutanix.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").

Note that there is no KVM on 32-bit Arm any more, so we really don't
care about vhost performance there. The added access_ok() check in
arm32 __get_user() is probably avoidable, as embedded systems with
in-order cores could turn off the spectre workarounds, but as
Will explained in the arm64 commit, it's not that expensive either.

>> If so, that mean at least for a non-zero population of ARM’ers,
>> they wouldn’t notice anything from this patch, yea?
>
> Adding ARM maintainers for more thought.

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.

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.

     Arnd