[PATCH 1/2] x86/dumpstack: on oops do not rewind stack for kthread

[Roman Pen <roman.penyaev@profitbricks.com>]

kthread uses stack and keeps completion structure on it to be woken up
on vfork_done completion.

In commit 2deb4be28 Andy Lutomirski rewinds the stack unconditionally
and further completion of task->vfork_done for any kthread leads to stack
corruption (or infinite spin on attempt to spin lock on garbage memory).

Signed-off-by: Roman Pen <roman.penyaev@profitbricks.com>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: x86@kernel.org
Cc: linux-kernel@vger.kernel.org
---
 arch/x86/kernel/dumpstack.c | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/arch/x86/kernel/dumpstack.c b/arch/x86/kernel/dumpstack.c
index e0648f7..74be764 100644
--- a/arch/x86/kernel/dumpstack.c
+++ b/arch/x86/kernel/dumpstack.c
@@ -250,9 +250,14 @@ 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.
+	 * the task.  But kthread is a special case, since kthread uses
+	 * stack to keep completion structure to be woken on vfork_done
+	 * completion.
 	 */
-	rewind_stack_do_exit(signr);
+	if (current->flags & PF_KTHREAD)
+		do_exit(signr);
+	else
+		rewind_stack_do_exit(signr);
 }
 NOKPROBE_SYMBOL(oops_end);
 
-- 
2.9.3