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

From: Roman Pen
Date: Wed Sep 21 2016 - 11:50:10 EST

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@xxxxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Brian Gerst <brgerst@xxxxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
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);