x86/entry/unwind: Create stack frames for saved interrupt registers

With frame pointers, when a task is interrupted, its stack is no longer
completely reliable because the function could have been interrupted
before it had a chance to save the previous frame pointer on the stack.
So the caller of the interrupted function could get skipped by a stack
trace.

This is problematic for live patching, which needs to know whether a
stack trace of a sleeping task can be relied upon.  There's currently no
way to detect if a sleeping task was interrupted by a page fault
exception or preemption before it went to sleep.

Another issue is that when dumping the stack of an interrupted task, the
unwinder has no way of knowing where the saved pt_regs registers are, so
it can't print them.

This solves those issues by encoding the pt_regs pointer in the frame
pointer on entry from an interrupt or an exception.

This patch also updates the unwinder to be able to decode it, because
otherwise the unwinder would be broken by this change.

Note that this causes a change in the behavior of the unwinder: each
instance of a pt_regs on the stack is now considered a "frame".  So
callers of unwind_get_return_address() will now get an occasional
'regs->ip' address that would have previously been skipped over.

Suggested-by: Andy Lutomirski <luto@amacapital.net>
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/8b9f84a21e39d249049e0547b559ff8da0df0988.1476973742.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

1 files changed, 68 insertions, 8 deletions

diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c
index a2456d4d286a..2221ab1678c5 100644
--- a/arch/x86/kernel/unwind_frame.c
+++ b/arch/x86/kernel/unwind_frame.c
@@ -14,6 +14,9 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
 	if (unwind_done(state))
 		return 0;
 
+	if (state->regs && user_mode(state->regs))
+		return 0;
+
 	addr = ftrace_graph_ret_addr(state->task, &state->graph_idx, *addr_p,
 				     addr_p);
 
@@ -21,6 +24,20 @@ unsigned long unwind_get_return_address(struct unwind_state *state)
 }
 EXPORT_SYMBOL_GPL(unwind_get_return_address);
 
+/*
+ * This determines if the frame pointer actually contains an encoded pointer to
+ * pt_regs on the stack.  See ENCODE_FRAME_POINTER.
+ */
+static struct pt_regs *decode_frame_pointer(unsigned long *bp)
+{
+	unsigned long regs = (unsigned long)bp;
+
+	if (!(regs & 0x1))
+		return NULL;
+
+	return (struct pt_regs *)(regs & ~0x1);
+}
+
 static bool update_stack_state(struct unwind_state *state, void *addr,
 			       size_t len)
 {
@@ -43,26 +60,59 @@ static bool update_stack_state(struct unwind_state *state, void *addr,
 
 bool unwind_next_frame(struct unwind_state *state)
 {
-	unsigned long *next_bp;
+	struct pt_regs *regs;
+	unsigned long *next_bp, *next_frame;
+	size_t next_len;
 
 	if (unwind_done(state))
 		return false;
 
-	next_bp = (unsigned long *)*state->bp;
+	/* have we reached the end? */
+	if (state->regs && user_mode(state->regs))
+		goto the_end;
+
+	/* get the next frame pointer */
+	if (state->regs)
+		next_bp = (unsigned long *)state->regs->bp;
+	else
+		next_bp = (unsigned long *)*state->bp;
+
+	/* is the next frame pointer an encoded pointer to pt_regs? */
+	regs = decode_frame_pointer(next_bp);
+	if (regs) {
+		next_frame = (unsigned long *)regs;
+		next_len = sizeof(*regs);
+	} else {
+		next_frame = next_bp;
+		next_len = FRAME_HEADER_SIZE;
+	}
 
 	/* make sure the next frame's data is accessible */
-	if (!update_stack_state(state, next_bp, FRAME_HEADER_SIZE))
+	if (!update_stack_state(state, next_frame, next_len))
 		return false;
-
 	/* move to the next frame */
-	state->bp = next_bp;
+	if (regs) {
+		state->regs = regs;
+		state->bp = NULL;
+	} else {
+		state->bp = next_bp;
+		state->regs = NULL;
+	}
+
 	return true;
+
+the_end:
+	state->stack_info.type = STACK_TYPE_UNKNOWN;
+	return false;
 }
 EXPORT_SYMBOL_GPL(unwind_next_frame);
 
 void __unwind_start(struct unwind_state *state, struct task_struct *task,
 		    struct pt_regs *regs, unsigned long *first_frame)
 {
+	unsigned long *bp, *frame;
+	size_t len;
+
 	memset(state, 0, sizeof(*state));
 	state->task = task;
 
@@ -73,12 +123,22 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	}
 
 	/* set up the starting stack frame */
-	state->bp = get_frame_pointer(task, regs);
+	bp = get_frame_pointer(task, regs);
+	regs = decode_frame_pointer(bp);
+	if (regs) {
+		state->regs = regs;
+		frame = (unsigned long *)regs;
+		len = sizeof(*regs);
+	} else {
+		state->bp = bp;
+		frame = bp;
+		len = FRAME_HEADER_SIZE;
+	}
 
 	/* initialize stack info and make sure the frame data is accessible */
-	get_stack_info(state->bp, state->task, &state->stack_info,
+	get_stack_info(frame, state->task, &state->stack_info,
 		       &state->stack_mask);
-	update_stack_state(state, state->bp, FRAME_HEADER_SIZE);
+	update_stack_state(state, frame, len);
 
 	/*
 	 * The caller can provide the address of the first frame directly