arm64: unwind: reference pt_regs via embedded stack frame

As it turns out, the unwind code is slightly broken, and probably has
been for a while. The problem is in the dumping of the exception stack,
which is intended to dump the contents of the pt_regs struct at each
level in the call stack where an exception was taken and routed to a
routine marked as __exception (which means its stack frame is right
below the pt_regs struct on the stack).

'Right below the pt_regs struct' is ill defined, though: the unwind
code assigns 'frame pointer + 0x10' to the .sp member of the stackframe
struct at each level, and dump_backtrace() happily dereferences that as
the pt_regs pointer when encountering an __exception routine. However,
the actual size of the stack frame created by this routine (which could
be one of many __exception routines we have in the kernel) is not known,
and so frame.sp is pretty useless to figure out where struct pt_regs
really is.

So it seems the only way to ensure that we can find our struct pt_regs
when walking the stack frames is to put it at a known fixed offset of
the stack frame pointer that is passed to such __exception routines.
The simplest way to do that is to put it inside pt_regs itself, which is
the main change implemented by this patch. As a bonus, doing this allows
us to get rid of a fair amount of cruft related to walking from one stack
to the other, which is especially nice since we intend to introduce yet
another stack for overflow handling once we add support for vmapped
stacks. It also fixes an inconsistency where we only add a stack frame
pointing to ELR_EL1 if we are executing from the IRQ stack but not when
we are executing from the task stack.

To consistly identify exceptions regs even in the presence of exceptions
taken from entry code, we must check whether the next frame was created
by entry text, rather than whether the current frame was crated by
exception text.

To avoid backtracing using PCs that fall in the idmap, or are controlled
by userspace, we must explcitly zero the FP and LR in startup paths, and
must ensure that the frame embedded in pt_regs is zeroed upon entry from
EL0. To avoid these NULL entries showin in the backtrace, unwind_frame()
is updated to avoid them.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
[Mark: compare current frame against .entry.text, avoid bogus PCs]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: James Morse <james.morse@arm.com>
Cc: Will Deacon <will.deacon@arm.com>

1 files changed, 6 insertions, 27 deletions

diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c
index beaf51fb3088..81d9262acaf0 100644
--- a/arch/arm64/kernel/stacktrace.c
+++ b/arch/arm64/kernel/stacktrace.c
@@ -76,34 +76,13 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame)
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 
 	/*
-	 * Check whether we are going to walk through from interrupt stack
-	 * to task stack.
-	 * If we reach the end of the stack - and its an interrupt stack,
-	 * unpack the dummy frame to find the original elr.
-	 *
-	 * Check the frame->fp we read from the bottom of the irq_stack,
-	 * and the original task stack pointer are both in current->stack.
+	 * Frames created upon entry from EL0 have NULL FP and PC values, so
+	 * don't bother reporting these. Frames created by __noreturn functions
+	 * might have a valid FP even if PC is bogus, so only terminate where
+	 * both are NULL.
 	 */
-	if (frame->sp == IRQ_STACK_PTR()) {
-		struct pt_regs *irq_args;
-		unsigned long orig_sp = IRQ_STACK_TO_TASK_STACK(frame->sp);
-
-		if (object_is_on_stack((void *)orig_sp) &&
-		   object_is_on_stack((void *)frame->fp)) {
-			frame->sp = orig_sp;
-
-			/* orig_sp is the saved pt_regs, find the elr */
-			irq_args = (struct pt_regs *)orig_sp;
-			frame->pc = irq_args->pc;
-		} else {
-			/*
-			 * This frame has a non-standard format, and we
-			 * didn't fix it, because the data looked wrong.
-			 * Refuse to output this frame.
-			 */
-			return -EINVAL;
-		}
-	}
+	if (!frame->fp && !frame->pc)
+		return -EINVAL;
 
 	return 0;
 }