x86/unwind: Ensure stack grows down

Add a sanity check to ensure the stack only grows down, and print a
warning if the check fails.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20161027131058.tpdffwlqipv7pcd6@treble
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/unwind_frame.c

@@ -32,6 +32,15 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
 }
 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
+static size_t regs_size(struct pt_regs *regs)
+{
+	/* x86_32 regs from kernel mode are two words shorter: */
+	if (IS_ENABLED(CONFIG_X86_32) && !user_mode(regs))
+		return sizeof(*regs) - 2*sizeof(long);
+
+	return sizeof(*regs);
+}
+
 static bool is_last_task_frame(struct unwind_state *state)
 {
 	unsigned long bp = (unsigned long)state->bp;
@@ -79,6 +88,7 @@ bool unwind_next_frame(struct unwind_state *state)
 	struct pt_regs *regs;
 	unsigned long *next_bp, *next_frame;
 	size_t next_len;
+	enum stack_type prev_type = state->stack_info.type;
 
 	if (unwind_done(state))
 		return false;
@@ -142,6 +152,15 @@ bool unwind_next_frame(struct unwind_state *state)
 		goto bad_address;
 	}
 
+	/* Make sure it only unwinds up and doesn't overlap the last frame: */
+	if (state->stack_info.type == prev_type) {
+		if (state->regs && (void *)next_frame < (void *)state->regs + regs_size(state->regs))
+			goto bad_address;
+
+		if (state->bp && (void *)next_frame < (void *)state->bp + FRAME_HEADER_SIZE)
+			goto bad_address;
+	}
+
 	/* move to the next frame */
 	if (regs) {
 		state->regs = regs;
@@ -154,10 +173,17 @@ bool unwind_next_frame(struct unwind_state *state)
 	return true;
 
 bad_address:
-	printk_deferred_once(KERN_WARNING
-		"WARNING: kernel stack frame pointer at %p in %s:%d has bad value %p\n",
-		state->bp, state->task->comm,
-		state->task->pid, next_bp);
+	if (state->regs) {
+		printk_deferred_once(KERN_WARNING
+			"WARNING: kernel stack regs at %p in %s:%d has bad 'bp' value %p\n",
+			state->regs, state->task->comm,
+			state->task->pid, next_frame);
+	} else {
+		printk_deferred_once(KERN_WARNING
+			"WARNING: kernel stack frame pointer at %p in %s:%d has bad value %p\n",
+			state->bp, state->task->comm,
+			state->task->pid, next_frame);
+	}
 the_end:
 	state->stack_info.type = STACK_TYPE_UNKNOWN;
 	return false;