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Message-ID: <CAM_iQpUFCnmu36L0hwrK+xv9gBWKtcq44nOVGNEyR=o9QDx7Pg@mail.gmail.com>
Date: Thu, 31 Dec 2020 10:53:59 -0800
From: Cong Wang <xiyou.wangcong@...il.com>
To: Chinmay Agarwal <chinagar@...eaurora.org>
Cc: Linux Kernel Network Developers <netdev@...r.kernel.org>,
sharathv@...eaurora.org
Subject: Re: Race Condition Observed in ARP Processing.
On Tue, Dec 29, 2020 at 8:06 AM Chinmay Agarwal <chinagar@...eaurora.org> wrote:
>
> Hi All,
>
> We found a crash while performing some automated stress tests on a 5.4 kernel based device.
>
> We found out that it there is a freed neighbour address which was still part of the gc_list and was leading to crash.
> Upon adding some debugs and checking neigh_put/neigh_hold/neigh_destroy calls stacks, looks like there is a possibility of a Race condition happening in the code.
[...]
> The crash may have been due to out of order ARP replies.
> As neighbour is marked dead should we go ahead with updating our ARP Tables?
I think you are probably right, we should just do unlock and return
in __neigh_update() when hitting if (neigh->dead) branch. Something
like below:
diff --git a/net/core/neighbour.c b/net/core/neighbour.c
index 9500d28a43b0..0ce592f585c8 100644
--- a/net/core/neighbour.c
+++ b/net/core/neighbour.c
@@ -1250,6 +1250,7 @@ static int __neigh_update(struct neighbour
*neigh, const u8 *lladdr,
goto out;
if (neigh->dead) {
NL_SET_ERR_MSG(extack, "Neighbor entry is now dead");
+ new = old;
goto out;
}
But given the old state probably contains NUD_PERMANENT, I guess
you hit the following branch instead:
if (!(flags & NEIGH_UPDATE_F_ADMIN) &&
(old & (NUD_NOARP | NUD_PERMANENT)))
goto out;
So we may have to check ->dead before this. Please double check.
This bug is probably introduced by commit 9c29a2f55ec05cc8b525ee.
Can you make a patch and send it out formally after testing?
Thanks!
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