KVM: arm64: Survive synchronous exceptions caused by AT instructions

commit 88a84ccccb3966bcc3f309cdb76092a9892c0260 upstream.

KVM doesn't expect any synchronous exceptions when executing, any such
exception leads to a panic(). AT instructions access the guest page
tables, and can cause a synchronous external abort to be taken.

The arm-arm is unclear on what should happen if the guest has configured
the hardware update of the access-flag, and a memory type in TCR_EL1 that
does not support atomic operations. B2.2.6 "Possible implementation
restrictions on using atomic instructions" from DDI0487F.a lists
synchronous external abort as a possible behaviour of atomic instructions
that target memory that isn't writeback cacheable, but the page table
walker may behave differently.

Make KVM robust to synchronous exceptions caused by AT instructions.
Add a get_user() style helper for AT instructions that returns -EFAULT
if an exception was generated.

While KVM's version of the exception table mixes synchronous and
asynchronous exceptions, only one of these can occur at each location.

Re-enter the guest when the AT instructions take an exception on the
assumption the guest will take the same exception. This isn't guaranteed
to make forward progress, as the AT instructions may always walk the page
tables, but guest execution may use the translation cached in the TLB.

This isn't a problem, as since commit 5dcd0fdbb492 ("KVM: arm64: Defer guest
entry when an asynchronous exception is pending"), KVM will return to the
host to process IRQs allowing the rest of the system to keep running.

Cc: stable@vger.kernel.org # v4.9
Signed-off-by: James Morse <james.morse@arm.com>
Reviewed-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/arm64/include/asm/kvm_asm.h

@@ -82,6 +82,34 @@ extern u32 __init_stage2_translation(void);
 		*__hyp_this_cpu_ptr(sym);				\
 	 })
 
+#define __KVM_EXTABLE(from, to)						\
+	"	.pushsection	__kvm_ex_table, \"a\"\n"		\
+	"	.align		3\n"					\
+	"	.long		(" #from " - .), (" #to " - .)\n"	\
+	"	.popsection\n"
+
+
+#define __kvm_at(at_op, addr)						\
+( { 									\
+	int __kvm_at_err = 0;						\
+	u64 spsr, elr;							\
+	asm volatile(							\
+	"	mrs	%1, spsr_el2\n"					\
+	"	mrs	%2, elr_el2\n"					\
+	"1:	at	"at_op", %3\n"					\
+	"	isb\n"							\
+	"	b	9f\n"						\
+	"2:	msr	spsr_el2, %1\n"					\
+	"	msr	elr_el2, %2\n"					\
+	"	mov	%w0, %4\n"					\
+	"9:\n"								\
+	__KVM_EXTABLE(1b, 2b)						\
+	: "+r" (__kvm_at_err), "=&r" (spsr), "=&r" (elr)		\
+	: "r" (addr), "i" (-EFAULT));					\
+	__kvm_at_err;							\
+} )
+
+
 #else /* __ASSEMBLY__ */
 
 .macro hyp_adr_this_cpu reg, sym, tmp

arch/arm64/kvm/hyp/hyp-entry.S

@@ -201,6 +201,15 @@ el1_error:
 	mov	x0, #ARM_EXCEPTION_EL1_SERROR
 	b	__guest_exit
 
+el2_sync:
+	save_caller_saved_regs_vect
+	stp     x29, x30, [sp, #-16]!
+	bl	kvm_unexpected_el2_exception
+	ldp     x29, x30, [sp], #16
+	restore_caller_saved_regs_vect
+
+	eret
+
 el2_error:
 	save_caller_saved_regs_vect
 	stp     x29, x30, [sp, #-16]!
@@ -238,7 +247,6 @@ ENDPROC(\label)
 	invalid_vector	el2t_irq_invalid
 	invalid_vector	el2t_fiq_invalid
 	invalid_vector	el2t_error_invalid
-	invalid_vector	el2h_sync_invalid
 	invalid_vector	el2h_irq_invalid
 	invalid_vector	el2h_fiq_invalid
 	invalid_vector	el1_sync_invalid
@@ -255,7 +263,7 @@ ENTRY(__kvm_hyp_vector)
 	ventry	el2t_fiq_invalid		// FIQ EL2t
 	ventry	el2t_error_invalid		// Error EL2t
 
-	ventry	el2h_sync_invalid		// Synchronous EL2h
+	ventry	el2_sync			// Synchronous EL2h
 	ventry	el2h_irq_invalid		// IRQ EL2h
 	ventry	el2h_fiq_invalid		// FIQ EL2h
 	ventry	el2_error			// Error EL2h

arch/arm64/kvm/hyp/switch.c

@@ -206,10 +206,10 @@ static bool __hyp_text __translate_far_to_hpfar(u64 far, u64 *hpfar)
 	 * saved the guest context yet, and we may return early...
 	 */
 	par = read_sysreg(par_el1);
-	asm volatile("at s1e1r, %0" : : "r" (far));
-	isb();
-
-	tmp = read_sysreg(par_el1);
+	if (!__kvm_at("s1e1r", far))
+		tmp = read_sysreg(par_el1);
+	else
+		tmp = 1; /* back to the guest */
 	write_sysreg(par, par_el1);
 
 	if (unlikely(tmp & 1))