arm/unwind: Check stack pointer boundaries before dereferencing

If the unwinder somehow ends up with a stack pointer that lies outside
the stack, then an attempt to dereference can lead to a fault, which
causes the kernel to panic again and unwind the stack, which leads to a
fault...

Add kstack_contains() checks at points where we dereference the stack
pointer.  This avoids the aforementioned infinite loop in one case I hit
where some OpenSSL assembly code apparently confuses the unwinder.

Reviewed by:	jhb
MFC after:	2 weeks
Sponsored by:	Klara, Inc.
Sponsored by:	Stormshield
Differential Revision:	https://reviews.freebsd.org/D41210

sys/arm/arm/unwind.c

@@ -35,6 +35,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/kernel.h>
 #include <sys/linker.h>
 #include <sys/malloc.h>
+#include <sys/proc.h>
 #include <sys/queue.h>
 #include <sys/systm.h>
 
@@ -370,6 +371,7 @@ unwind_exec_read_byte(struct unwind_state *state)
 static int
 unwind_exec_insn(struct unwind_state *state)
 {
+	struct thread *td = curthread;
 	unsigned int insn;
 	uint32_t *vsp = (uint32_t *)state->registers[SP];
 	int update_vsp = 0;
@@ -404,6 +406,10 @@ unwind_exec_insn(struct unwind_state *state)
 		/* Load the registers */
 		for (reg = 4; mask && reg < 16; mask >>= 1, reg++) {
 			if (mask & 1) {
+				if (!kstack_contains(td, (uintptr_t)vsp,
+				    sizeof(*vsp)))
+					return 1;
+
 				state->registers[reg] = *vsp++;
 				state->update_mask |= 1 << reg;
 
@@ -430,6 +436,9 @@ unwind_exec_insn(struct unwind_state *state)
 		update_vsp = 1;
 
 		/* Pop the registers */
+		if (!kstack_contains(td, (uintptr_t)vsp,
+		    sizeof(*vsp) * (4 + count)))
+			return 1;
 		for (reg = 4; reg <= 4 + count; reg++) {
 			state->registers[reg] = *vsp++;
 			state->update_mask |= 1 << reg;
@@ -437,6 +446,8 @@ unwind_exec_insn(struct unwind_state *state)
 
 		/* Check if we are in the pop r14 version */
 		if ((insn & INSN_POP_TYPE_MASK) != 0) {
+			if (!kstack_contains(td, (uintptr_t)vsp, sizeof(*vsp)))
+				return 1;
 			state->registers[14] = *vsp++;
 		}
 
@@ -457,6 +468,9 @@ unwind_exec_insn(struct unwind_state *state)
 		/* Load the registers */
 		for (reg = 0; mask && reg < 4; mask >>= 1, reg++) {
 			if (mask & 1) {
+				if (!kstack_contains(td, (uintptr_t)vsp,
+				    sizeof(*vsp)))
+					return 1;
 				state->registers[reg] = *vsp++;
 				state->update_mask |= 1 << reg;
 			}