Re: [PATCH 5.4 v2] KVM: x86/mmu: do compare-and-exchange of gPTE via the user address

[Greg KH <gregkh@linuxfoundation.org>]

On Mon, Apr 04, 2022 at 11:49:13AM -0400, Paolo Bonzini wrote:
> commit 2a8859f373b0a86f0ece8ec8312607eacf12485d upstream.
> 
> FNAME(cmpxchg_gpte) is an inefficient mess.  It is at least decent if it
> can go through get_user_pages_fast(), but if it cannot then it tries to
> use memremap(); that is not just terribly slow, it is also wrong because
> it assumes that the VM_PFNMAP VMA is contiguous.
> 
> The right way to do it would be to do the same thing as
> hva_to_pfn_remapped() does since commit add6a0cd1c5b ("KVM: MMU: try to
> fix up page faults before giving up", 2016-07-05), using follow_pte()
> and fixup_user_fault() to determine the correct address to use for
> memremap().  To do this, one could for example extract hva_to_pfn()
> for use outside virt/kvm/kvm_main.c.  But really there is no reason to
> do that either, because there is already a perfectly valid address to
> do the cmpxchg() on, only it is a userspace address.  That means doing
> user_access_begin()/user_access_end() and writing the code in assembly
> to handle any exception correctly.  Worse, the guest PTE can be 8-byte
> even on i686 so there is the extra complication of using cmpxchg8b to
> account for.  But at least it is an efficient mess.
> 
> Reported-by: Qiuhao Li <qiuhao@sysec.org>
> Reported-by: Gaoning Pan <pgn@zju.edu.cn>
> Reported-by: Yongkang Jia <kangel@zju.edu.cn>
> Reported-by: syzbot+6cde2282daa792c49ab8@syzkaller.appspotmail.com
> Debugged-by: Tadeusz Struk <tadeusz.struk@linaro.org>
> Tested-by: Maxim Levitsky <mlevitsk@redhat.com>
> Cc: stable@vger.kernel.org
> Fixes: bd53cb35a3e9 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs")
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---
>  arch/x86/kvm/paging_tmpl.h | 77 ++++++++++++++++++--------------------
>  1 file changed, 37 insertions(+), 40 deletions(-)

Thanks for the fix, now queued up.

greg k-h