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Paolo Bonzini authored
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 add6a0cd ("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 exceptions 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. (Thanks to Linus for suggesting improvement on the inline assembly). 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: bd53cb35 ("X86/KVM: Handle PFNs outside of kernel reach when touching GPTEs") Signed-off-by:
Paolo Bonzini <pbonzini@redhat.com>
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