[PATCH] netlink: Fix netlink_recvmsg() handling of error from netlink_dump().

[PATCH] netlink: Fix netlink_recvmsg() handling of error from netlink_dump().

[Ben Pfaff]

netlink_dump() returns a negative errno value on error.  Until now,
netlink_recvmsg() directly recorded that negative value in sk->sk_err,
but that's wrong since sk_err takes positive errno values.  (This manifests
as userspace receiving a positive return value from the recv() system call,
falsely indicating success.)

This bug was introduced in the commit that started checking the
netlink_dump() return value, commit b44d211 (netlink: handle errors from
netlink_dump()).

Multithreaded Netlink dumps are one way to trigger this behavior in
practice, as described in the commit message for the userspace workaround
posted here:
    http://openvswitch.org/pipermail/dev/2014-June/042339.html

Signed-off-by: Ben Pfaff <blp@nicira.com>
---
 net/netlink/af_netlink.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 15c731f..b7514b3 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -2483,7 +2483,7 @@ static int netlink_recvmsg(struct kiocb *kiocb, struct socket *sock,
 	    atomic_read(&sk->sk_rmem_alloc) <= sk->sk_rcvbuf / 2) {
 		ret = netlink_dump(sk);
 		if (ret) {
-			sk->sk_err = ret;
+			sk->sk_err = -ret;
 			sk->sk_error_report(sk);
 		}
 	}
-- 
1.7.10.4

Re: [PATCH] netlink: Fix netlink_recvmsg() handling of error from netlink_dump().

[David Miller]

From: Ben Pfaff <blp@nicira.com>
Date: Mon, 30 Jun 2014 15:35:04 -0700

> netlink_dump() returns a negative errno value on error.  Until now,
> netlink_recvmsg() directly recorded that negative value in sk->sk_err,
> but that's wrong since sk_err takes positive errno values.  (This manifests
> as userspace receiving a positive return value from the recv() system call,
> falsely indicating success.)
> 
> This bug was introduced in the commit that started checking the
> netlink_dump() return value, commit b44d211 (netlink: handle errors from
> netlink_dump()).
> 
> Multithreaded Netlink dumps are one way to trigger this behavior in
> practice, as described in the commit message for the userspace workaround
> posted here:
>     http://openvswitch.org/pipermail/dev/2014-June/042339.html
> 
> Signed-off-by: Ben Pfaff <blp@nicira.com>

There's another place, later in this file, doing the same exact thing.

Please respin this patch, fixing that location too.

Thanks.

Re: [PATCH] netlink: Fix netlink_recvmsg() handling of error from netlink_dump().

[Ben Pfaff]

On Mon, Jul 07, 2014 at 07:37:35PM -0700, David Miller wrote:
> From: Ben Pfaff <blp@nicira.com>
> Date: Mon, 30 Jun 2014 15:35:04 -0700
> 
> > netlink_dump() returns a negative errno value on error.  Until now,
> > netlink_recvmsg() directly recorded that negative value in sk->sk_err,
> > but that's wrong since sk_err takes positive errno values.  (This manifests
> > as userspace receiving a positive return value from the recv() system call,
> > falsely indicating success.)
> > 
> > This bug was introduced in the commit that started checking the
> > netlink_dump() return value, commit b44d211 (netlink: handle errors from
> > netlink_dump()).
> > 
> > Multithreaded Netlink dumps are one way to trigger this behavior in
> > practice, as described in the commit message for the userspace workaround
> > posted here:
> >     http://openvswitch.org/pipermail/dev/2014-June/042339.html
> > 
> > Signed-off-by: Ben Pfaff <blp@nicira.com>
> 
> There's another place, later in this file, doing the same exact thing.
> 
> Please respin this patch, fixing that location too.

Thanks for pointing that out, I sent out v2.