x86/unwinder/orc: Dont bail on stack overflow

If the stack overflows into a guard page and the ORC unwinder 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.

But there is a bug that prevents unwinding from working correctly: if the
starting register state has RSP pointing into a stack guard page, the ORC
unwinder bails out immediately.

Instead of bailing out immediately check whether the next page up is a
valid check page and if so analyze that. As a result the ORC unwinder will
start the unwind.

Tested 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, but they are outside the scope of this
fix.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Borislav Petkov <bpetkov@suse.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Dave Hansen <dave.hansen@intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: David Laight <David.Laight@aculab.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: Eduardo Valentin <eduval@amazon.com>
Cc: Greg KH <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Rik van Riel <riel@redhat.com>
Cc: Will Deacon <will.deacon@arm.com>
Cc: aliguori@amazon.com
Cc: daniel.gruss@iaik.tugraz.at
Cc: hughd@google.com
Cc: keescook@google.com
Link: https://lkml.kernel.org/r/20171204150604.991389777@linutronix.de
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/arch/x86/kernel/unwind_orc.c b/arch/x86/kernel/unwind_orc.c
index a3f973b2c97a03b121fe0173dbdc9298216721e6..ff8e1132b2ae41045a882aa85c35988c333cab2c 100644 (file)
--- 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)state->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