x86/entry/64: Wipe KASAN stack shadow before rewind_stack_do_exit()

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 does not wipe the exception stacks; if an oops happens on an exception
stack, it might result in random KASAN false-positives from other tasks
afterwards. This is probably relatively uninteresting, since if the kernel
oopses on an exception stack, there are most likely 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: 5c45f29b56f9 ("x86/dumpstack: When OOPSing, rewind the stack before do_exit()")
Signed-off-by: Jann Horn <jannh@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Andrey Ryabinin <aryabinin@virtuozzo.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: kasan-dev@googlegroups.com
Cc: stable@vger.kernel.org
Link: https://lkml.kernel.org/r/20180828184033.93712-1-jannh@google.com

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index 9c8652974f8ed1f6a8ed20f97ac843ec332706f2..1596e6bfea6fc8742769d0cb2faba1e784ee4b25 100644 (file)
--- 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);
        rewind_stack_do_exit(signr);
 }
 NOKPROBE_SYMBOL(oops_end);