Re: [PATCH 0/2] kprobes: Fix kretprobe incorrect stacking order problem

[Andrea Righi]

On Mon, Jan 07, 2019 at 02:59:18PM -0500, Steven Rostedt wrote:
> On Mon, 7 Jan 2019 20:52:09 +0100
> Andrea Righi <righi.andrea@xxxxxxxxx> wrote:
> 
> > > Ug, kretprobe calls spinlocks in the callback? I wonder if we can
> > > remove them.
> > > 
> > > I'm guessing this is a different issue than the one that this patch
> > > fixes. This sounds like we are calling kretprobe from kretprobe?
> > > 
> > > -- Steve  
> > 
> > kretprobe_trampoline()
> >   -> trampoline_handler()
> >     -> kretprobe_hash_lock()
> >       -> raw_spin_lock_irqsave()  
> > 
> > If we put a kretprobe to raw_spin_lock_irqsave() it looks like
> > kretprobe is going to call kretprobe...
> 
> Right, but we should be able to add some recursion protection to stop
> that. I have similar protection in the ftrace code.

If we assume that __raw_spin_lock/unlock*() are always inlined a
possible way to prevent this recursion could be to use directly those
functions to do locking from the kretprobe trampoline.

But I'm not sure if that's a safe assumption... if not I'll see if I can
find a better solution.

Thanks,

From: Andrea Righi <righi.andrea@xxxxxxxxx>
Subject: [PATCH] kprobes: prevent recursion deadlock with kretprobe and
 spinlocks

kretprobe_trampoline() is using a spinlock to protect the hash of
kretprobes. Adding a kretprobe to the spinlock functions may cause
a recursion deadlock where kretprobe is calling itself:

 kretprobe_trampoline()
   -> trampoline_handler()
     -> kretprobe_hash_lock()
       -> raw_spin_lock_irqsave()
         -> _raw_spin_lock_irqsave()
 kretprobe_trampoline from _raw_spin_lock_irqsave => DEADLOCK

 kretprobe_trampoline()
   -> trampoline_handler()
     -> recycle_rp_inst()
       -> raw_spin_lock()
         -> _raw_spin_lock()
 kretprobe_trampoline from _raw_spin_lock => DEADLOCK

Use the corresponding inlined spinlock functions to prevent this
recursion.

Signed-off-by: Andrea Righi <righi.andrea@xxxxxxxxx>
---
 kernel/kprobes.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/kernel/kprobes.c b/kernel/kprobes.c
index f4ddfdd2d07e..b89bef5e3d80 100644
--- a/kernel/kprobes.c
+++ b/kernel/kprobes.c
@@ -1154,9 +1154,9 @@ void recycle_rp_inst(struct kretprobe_instance *ri,
 	hlist_del(&ri->hlist);
 	INIT_HLIST_NODE(&ri->hlist);
 	if (likely(rp)) {
-		raw_spin_lock(&rp->lock);
+		__raw_spin_lock(&rp->lock);
 		hlist_add_head(&ri->hlist, &rp->free_instances);
-		raw_spin_unlock(&rp->lock);
+		__raw_spin_unlock(&rp->lock);
 	} else
 		/* Unregistering */
 		hlist_add_head(&ri->hlist, head);
@@ -1172,7 +1172,7 @@ __acquires(hlist_lock)
 
 	*head = &kretprobe_inst_table[hash];
 	hlist_lock = kretprobe_table_lock_ptr(hash);
-	raw_spin_lock_irqsave(hlist_lock, *flags);
+	*flags = __raw_spin_lock_irqsave(hlist_lock);
 }
 NOKPROBE_SYMBOL(kretprobe_hash_lock);
 
@@ -1193,7 +1193,7 @@ __releases(hlist_lock)
 	raw_spinlock_t *hlist_lock;
 
 	hlist_lock = kretprobe_table_lock_ptr(hash);
-	raw_spin_unlock_irqrestore(hlist_lock, *flags);
+	__raw_spin_unlock_irqrestore(hlist_lock, *flags);
 }
 NOKPROBE_SYMBOL(kretprobe_hash_unlock);
 
-- 
2.17.1