KVM: arm64: Fix masks in stage2_pte_cacheable()

stage2_pte_cacheable() tries to figure out whether the mapping installed
in its 'pte' parameter is cacheable or not. Unfortunately, it fails
miserably because it extracts the memory attributes from the entry using
FIELD_GET(), which returns the attributes shifted down to bit 0, but then
compares this with the unshifted value generated by the PAGE_S2_MEMATTR()
macro.

A direct consequence of this bug is that cache maintenance is silently
skipped, which in turn causes 32-bit guests to crash early on when their
set/way maintenance is trapped but not emulated correctly.

Fix the broken masks by avoiding the use of FIELD_GET() altogether.

Fixes: 6d9d2115 ("KVM: arm64: Add support for stage-2 map()/unmap() in generic page-table")
Reported-by: NMarc Zyngier <maz@kernel.org>
Signed-off-by: NWill Deacon <will@kernel.org>
Signed-off-by: NMarc Zyngier <maz@kernel.org>
Cc: Quentin Perret <qperret@google.com>
Link: https://lore.kernel.org/r/20201029144716.30476-1-will@kernel.org

arch/arm64/kvm/hyp/pgtable.c

@@ -635,7 +635,7 @@ static void stage2_flush_dcache(void *addr, u64 size)
 
 static bool stage2_pte_cacheable(kvm_pte_t pte)
 {
-	u64 memattr = FIELD_GET(KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR, pte);
+	u64 memattr = pte & KVM_PTE_LEAF_ATTR_LO_S2_MEMATTR;
 	return memattr == PAGE_S2_MEMATTR(NORMAL);
 }