KVM: MMU: fix detecting misaligned accessed

Sometimes, we only modify the last one byte of a pte to update status bit,
for example, clear_bit is used to clear r/w bit in linux kernel and 'andb'
instruction is used in this function, in this case, kvm_mmu_pte_write will
treat it as misaligned access, and the shadow page table is zapped

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Avi Kivity <avi@redhat.com>

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 986aea55366b496598b8f4479c5617df44b6a97a..ca6f72ab4c3bb7817fbc84b3e1e0f8bae48a0421 100644 (file)
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -3602,6 +3602,14 @@ static bool detect_write_misaligned(struct kvm_mmu_page *sp, gpa_t gpa,
 
        offset = offset_in_page(gpa);
        pte_size = sp->role.cr4_pae ? 8 : 4;
+
+       /*
+        * Sometimes, the OS only writes the last one bytes to update status
+        * bits, for example, in linux, andb instruction is used in clear_bit().
+        */
+       if (!(offset & (pte_size - 1)) && bytes == 1)
+               return false;
+
        misaligned = (offset ^ (offset + bytes - 1)) & ~(pte_size - 1);
        misaligned |= bytes < 4;