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