arm64: add fault address to trapframe

It was previously possible for the fault address register to get
clobbered before it was saved. This small window occurred when an
additional exception was encountered inside the exception handler,
overwriting the previous value.

Commit f29942229d24 ("Read the arm64 far early in el0 exceptions")
patched this issue, but avoided changing the trapframe since this could
be considered a KBI change in FreeBSD 13.

Revert the above fix and save the fault address in the trapframe
instead. This saves the fault address even earlier in the exception
handling process, and is a more robust and simple fix.

Reviewed by:	andrew, jhb, jrtc27
Sponsored by: Arm Ltd
Differential Revision:	https://reviews.freebsd.org/D38984

4 files changed, 14 insertions, 26 deletions

diff --git a/sys/arm64/arm64/exception.S b/sys/arm64/arm64/exception.S
index e23a7868b56f..c82b8307ab17 100644
--- a/sys/arm64/arm64/exception.S
+++ b/sys/arm64/arm64/exception.S
@@ -65,9 +65,10 @@ __FBSDID("$FreeBSD$");
 	mrs	x10, elr_el1
 	mrs	x11, spsr_el1
 	mrs	x12, esr_el1
+	mrs	x13, far_el1
 	stp	x18,  lr, [sp, #(TF_SP - TF_X)]!
-	str	x10, [sp, #(TF_ELR)]
-	stp	x11, x12, [sp, #(TF_SPSR)]
+	stp	x10, x11, [sp, #(TF_ELR)]
+	stp	x12, x13, [sp, #(TF_ESR)]
 	mrs	x18, tpidr_el1
 .endm
 
@@ -211,25 +212,10 @@ ENTRY(handle_el1h_irq)
 END(handle_el1h_irq)
 
 ENTRY(handle_el0_sync)
-	/*
-	 * Read the fault address early. The current thread structure may
-	 * be transiently unmapped if it is part of a memory range being
-	 * promoted or demoted to/from a superpage. As this involves a
-	 * break-before-make sequence there is a short period of time where
-	 * an access will raise an exception. If this happens the fault
-	 * address will be changed to the kernel address so a later read of
-	 * far_el1 will give the wrong value.
-	 *
-	 * The earliest memory access that could trigger a fault is in a
-	 * function called by the save_registers macro so this is the latest
-	 * we can read the userspace value.
-	 */
-	mrs	x19, far_el1
 	save_registers 0
 	ldr	x0, [x18, #PC_CURTHREAD]
 	mov	x1, sp
 	str	x1, [x0, #TD_FRAME]
-	mov	x2, x19
 	bl	do_el0_sync
 	do_ast
 	restore_registers 0
diff --git a/sys/arm64/arm64/genassym.c b/sys/arm64/arm64/genassym.c
index 3d88835c0f8e..db07f1714f5a 100644
--- a/sys/arm64/arm64/genassym.c
+++ b/sys/arm64/arm64/genassym.c
@@ -77,4 +77,5 @@ ASSYM(TF_SIZE, sizeof(struct trapframe));
 ASSYM(TF_SP, offsetof(struct trapframe, tf_sp));
 ASSYM(TF_ELR, offsetof(struct trapframe, tf_elr));
 ASSYM(TF_SPSR, offsetof(struct trapframe, tf_spsr));
+ASSYM(TF_ESR, offsetof(struct trapframe, tf_esr));
 ASSYM(TF_X, offsetof(struct trapframe, tf_x));
diff --git a/sys/arm64/arm64/trap.c b/sys/arm64/arm64/trap.c
index 50f6e9de874b..1b7d945ff115 100644
--- a/sys/arm64/arm64/trap.c
+++ b/sys/arm64/arm64/trap.c
@@ -76,7 +76,7 @@ __FBSDID("$FreeBSD$");
 
 /* Called from exception.S */
 void do_el1h_sync(struct thread *, struct trapframe *);
-void do_el0_sync(struct thread *, struct trapframe *, uint64_t far);
+void do_el0_sync(struct thread *, struct trapframe *);
 void do_el0_error(struct trapframe *);
 void do_serror(struct trapframe *);
 void unhandled_exception(struct trapframe *);
@@ -465,6 +465,7 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 	uint64_t esr, far;
 	int dfsc;
 
+	far = frame->tf_far;
 	/* Read the esr register to get the exception details */
 	esr = frame->tf_esr;
 	exception = ESR_ELx_EXCEPTION(esr);
@@ -502,7 +503,6 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 		break;
 	case EXCP_INSN_ABORT:
 	case EXCP_DATA_ABORT:
-		far = READ_SPECIALREG(far_el1);
 		dfsc = esr & ISS_DATA_DFSC_MASK;
 		if (dfsc < nitems(abort_handlers) &&
 		    abort_handlers[dfsc] != NULL) {
@@ -541,7 +541,7 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 	case EXCP_FPAC:
 		/* We can see this if the authentication on PAC fails */
 		print_registers(frame);
-		printf(" far: %16lx\n", READ_SPECIALREG(far_el1));
+		print_gp_register("far", far);
 		panic("FPAC kernel exception");
 		break;
 	case EXCP_UNKNOWN:
@@ -552,18 +552,18 @@ do_el1h_sync(struct thread *td, struct trapframe *frame)
 		/* FALLTHROUGH */
 	default:
 		print_registers(frame);
-		print_gp_register("far", READ_SPECIALREG(far_el1));
+		print_gp_register("far", far);
 		panic("Unknown kernel exception %x esr_el1 %lx", exception,
 		    esr);
 	}
 }
 
 void
-do_el0_sync(struct thread *td, struct trapframe *frame, uint64_t far)
+do_el0_sync(struct thread *td, struct trapframe *frame)
 {
 	pcpu_bp_harden bp_harden;
 	uint32_t exception;
-	uint64_t esr;
+	uint64_t esr, far;
 	int dfsc;
 
 	/* Check we have a sane environment when entering from userland */
@@ -571,6 +571,7 @@ do_el0_sync(struct thread *td, struct trapframe *frame, uint64_t far)
 	    ("Invalid pcpu address from userland: %p (tpidr %lx)",
 	     get_pcpu(), READ_SPECIALREG(tpidr_el1)));
 
+	far = frame->tf_far;
 	esr = frame->tf_esr;
 	exception = ESR_ELx_EXCEPTION(esr);
 	if (exception == EXCP_INSN_ABORT_L && far > VM_MAXUSER_ADDRESS) {
@@ -711,7 +712,7 @@ do_serror(struct trapframe *frame)
 {
 	uint64_t esr, far;
 
-	far = READ_SPECIALREG(far_el1);
+	far = frame->tf_far;
 	esr = frame->tf_esr;
 
 	print_registers(frame);
@@ -725,7 +726,7 @@ unhandled_exception(struct trapframe *frame)
 {
 	uint64_t esr, far;
 
-	far = READ_SPECIALREG(far_el1);
+	far = frame->tf_far;
 	esr = frame->tf_esr;
 
 	print_registers(frame);
diff --git a/sys/arm64/include/frame.h b/sys/arm64/include/frame.h
index 91ed6dbce920..fc10f8574161 100644
--- a/sys/arm64/include/frame.h
+++ b/sys/arm64/include/frame.h
@@ -47,7 +47,7 @@ struct trapframe {
 	uint64_t tf_elr;
 	uint64_t tf_spsr;
 	uint64_t tf_esr;
-	uint64_t pad; /* struct must be 16B aligned */
+	uint64_t tf_far;
 	uint64_t tf_x[30];
 };