[PATCH v2] x86/entry/64: wipe KASAN stack shadow before rewind_stack_do_exit()

From: Jann Horn
Date: Tue Aug 28 2018 - 14:42:30 EST

Reset the KASAN shadow state of the task stack before rewinding RSP.
Without this, a kernel oops will leave parts of the stack poisoned, and
code running under do_exit() can trip over such poisoned regions and cause
nonsensical false-positive KASAN reports about stack-out-of-bounds bugs.

This patch does not wipe exception stacks; if you oops on an exception
stack, you might get random KASAN false-positives from other tasks
afterwards. This is probably relatively uninteresting, since if you're
oopsing on an exception stack, you likely have bigger things to worry
about. It'd be more interesting if vmapped stacks and KASAN were
compatible, since then handle_stack_overflow() would oops from exception
stack context.

Fixes: 2deb4be28077 ("x86/dumpstack: When OOPSing, rewind the stack before do_exit()")
Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
- Rewrite, do it in C from oops_end() instead of in assembly
(Andrey Ryabinin)

arch/x86/kernel/dumpstack.c | 4 ++++
1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 9c8652974f8e..1596e6bfea6f 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -17,6 +17,7 @@
#include <linux/bug.h>
#include <linux/nmi.h>
#include <linux/sysfs.h>
+#include <linux/kasan.h>

#include <asm/cpu_entry_area.h>
#include <asm/stacktrace.h>
@@ -346,7 +347,10 @@ void oops_end(unsigned long flags, struct pt_regs *regs, int signr)
* We're not going to return, but we might be on an IST stack or
* have very little stack space left. Rewind the stack and kill
* the task.
+ * Before we rewind the stack, we have to tell KASAN that we're going to
+ * reuse the task stack and that existing poisons are invalid.
+ kasan_unpoison_task_stack(current);