[PATCH 01/21] x86/unwinder/orc: Don't bail on stack overflow

[Ingo Molnar <mingo@kernel.org>]

From: Andy Lutomirski <luto@kernel.org>

If we overflow the stack into a guard page and then try to unwind it
with ORC, it should work well: by construction, there can't be any
meaningful data in the guard page because no writes to the guard page
will have succeeded.

This patch fixes a bug that unwinding from working correctly: if the
starting register state has RSP pointing into a stack guard page, the
ORC unwinder bails out immediately.  This patch fixes that: the ORC
unwinder will start the unwind.

I tested this by intentionally overflowing the task stack.  The
result is an accurate call trace instead of a trace consisting
purely of '?' entries.

There are a few other bugs that are triggered if the unwinder
encounters a stack overflow after the first step, and Josh has WIP
patches to fix those as well.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.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/927042950d7f1a7007dd0f58538966a593508f8b.1511715954.git.luto@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 arch/x86/kernel/unwind_orc.c | 14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index a3f973b2c97a..7f6e3935666b 100644
--- a/arch/x86/kernel/unwind_orc.c
+++ b/arch/x86/kernel/unwind_orc.c
@@ -553,8 +553,18 @@ void __unwind_start(struct unwind_state *state, struct task_struct *task,
 	}
 
 	if (get_stack_info((unsigned long *)state->sp, state->task,
-			   &state->stack_info, &state->stack_mask))
-		return;
+			   &state->stack_info, &state->stack_mask)) {
+		/*
+		 * We weren't on a valid stack.  It's possible that
+		 * we overflowed a valid stack into a guard page.
+		 * See if the next page up is valid so that we can
+		 * generate some kind of backtrace if this happens.
+		 */
+		void *next_page = (void *)PAGE_ALIGN((unsigned long)regs->sp);
+		if (get_stack_info(next_page, state->task, &state->stack_info,
+				   &state->stack_mask))
+			return;
+	}
 
 	/*
 	 * The caller can provide the address of the first frame directly
-- 
2.14.1