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Date:   Sat, 30 May 2020 20:42:32 +0100
From:   Al Viro <viro@zeniv.linux.org.uk>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        KVM list <kvm@vger.kernel.org>
Subject: Re: [PATCH 8/9] x86: kvm_hv_set_msr(): use __put_user() instead of
 32bit __clear_user()
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On Sat, May 30, 2020 at 12:20:54PM -0700, Linus Torvalds wrote:
> On Sat, May 30, 2020 at 12:14 PM Al Viro <viro@zeniv.linux.org.uk> wrote:
> >
> > > And none of that code verifies that the end result is a user address.
> >
> > kvm_is_error_hva() is
> >         return addr >= PAGE_OFFSET;
> 
> Ahh, that's what I missed. It won't work on other architectures, but
> within x86 it's fine.

FWIW, we use virt/kvm on x86, powerpc, mips, s390 and arm64.

For x86 and powerpc the check is, AFAICS, OK (ppc kernel might start
higher than PAGE_OFFSET, but not lower than it).  For arm64... not
sure - I'm not familiar with the virtual address space layout we use
there.  mips does *NOT* get that protection at all - there kvm_is_error_hva()
is IS_ERR_VALUE() (thus the "at least on non-mips" upthread).  And
for s390 it's also IS_ERR_VALUE(), but that's an separate can of worms -
there access_ok() is constant true; if we ever hit any of that code in
virt/kvm while under KERNEL_DS, we are well and truly fucked there.