KVM: PPC: Book3S HV: H_ENTER filter out reserved HPTE[B] value

The HPTE B field is a 2-bit field with values 0b10 and 0b11 reserved.
This field is also taken from the HPTE and used when KVM executes
TLBIEs to set the B field of those instructions.

Disallow the guest setting B to a reserved value with H_ENTER by
rejecting it. This is the same approach already taken for rejecting
reserved (unsupported) LLP values. This prevents the guest from being
able to induce the host to execute TLBIE with reserved values, which
is not known to be a problem with current processors but in theory it
could prevent the TLBIE from working correctly in a future processor.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211004145749.1331331-1-npiggin@gmail.com

arch/powerpc/include/asm/kvm_book3s_64.h

@@ -378,6 +378,10 @@ static inline unsigned long compute_tlbie_rb(unsigned long v, unsigned long r,
 		rb |= 1;		/* L field */
 		rb |= r & 0xff000 & ((1ul << a_pgshift) - 1); /* LP field */
 	}
+	/*
+	 * This sets both bits of the B field in the PTE. 0b1x values are
+	 * reserved, but those will have been filtered by kvmppc_do_h_enter.
+	 */
 	rb |= (v >> HPTE_V_SSIZE_SHIFT) << 8;	/* B field */
 	return rb;
 }

arch/powerpc/kvm/book3s_hv_rm_mmu.c

@@ -207,6 +207,15 @@ long kvmppc_do_h_enter(struct kvm *kvm, unsigned long flags,
 
 	if (kvm_is_radix(kvm))
 		return H_FUNCTION;
+	/*
+	 * The HPTE gets used by compute_tlbie_rb() to set TLBIE bits, so
+	 * these functions should work together -- must ensure a guest can not
+	 * cause problems with the TLBIE that KVM executes.
+	 */
+	if ((pteh >> HPTE_V_SSIZE_SHIFT) & 0x2) {
+		/* B=0b1x is a reserved value, disallow it. */
+		return H_PARAMETER;
+	}
 	psize = kvmppc_actual_pgsz(pteh, ptel);
 	if (!psize)
 		return H_PARAMETER;