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Date:   Sun, 26 Nov 2017 09:06:27 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     X86 ML <x86@...nel.org>
Cc:     Borislav Petkov <bpetkov@...e.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Brian Gerst <brgerst@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v2] x86/orc: Don't bail on stack overflow

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@...nel.org>
---

Changes from v1: better changelog

I'm not going to send Josh's patch yet.  He said he'd split it up
nicely, so I'll let him :)

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
-- 
2.13.6

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