linux-kernel - Re: [PATCH 8/9] x86: kvm_hv_set_msr(): use __put_user() instead of 32bit __clear

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Date:   Sat, 30 May 2020 18:20:04 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Al Viro <viro@...iv.linux.org.uk>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        KVM list <kvm@...r.kernel.org>
Subject: Re: [PATCH 8/9] x86: kvm_hv_set_msr(): use __put_user() instead of
 32bit __clear_user()

> On Fri, May 29, 2020 at 04:52:59PM -0700, Linus Torvalds wrote:
>> It looks like the argument for the address being validated is that it
>> comes from "gfn_to_hva()", which should only return
>> host-virtual-addresses. That may be true.

Yes, the access_ok is done in __kvm_set_memory_region and gfn_to_hva()
returns a page-aligned address so it's obviously ok for a u32.

But I have no objections to removing the __ because if a read or write
is in the hot path it will use kvm_write_guest_cached and similar.

Paolo

>> But "should" is not "does", and honestly, the cost of gfn_to_hva() is
>> high enough that then using that as an argument for removing
>> "access_ok()" smells.