Re: Is xt_owner's owner_mt() racy with sock_orphan()? [worse with new TYPESAFE_BY_RCU file lifetime?]

[Phil Sutter <phil@nwl.cc>]

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Hi,

On Tue, Dec 05, 2023 at 06:08:29PM +0100, Jann Horn wrote:
> On Tue, Dec 5, 2023 at 5:40 PM Jann Horn <jannh@google.com> wrote:
> >
> > Hi!
> >
> > I think this code is racy, but testing that seems like a pain...
> >
> > owner_mt() in xt_owner runs in context of a NF_INET_LOCAL_OUT or
> > NF_INET_POST_ROUTING hook. It first checks that sk->sk_socket is
> > non-NULL, then checks that sk->sk_socket->file is non-NULL, then
> > accesses the ->f_cred of that file.
> >
> > I don't see anything that protects this against a concurrent
> > sock_orphan(), which NULLs out the sk->sk_socket pointer, if we're in
> 
> Ah, and all the other users of ->sk_socket in net/netfilter/ do it
> under the sk_callback_lock... so I guess the fix would be to add the
> same in owner_mt?

Sounds reasonable, although I wonder how likely a socket is to
orphan while netfilter is processing a packet it just sent.

How about the attached patch? Not sure what hash to put into a Fixes:
tag given this is a day 1 bug and ipt_owner/ip6t_owner predate git.

Thanks, Phil

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From 3e28490e43b04d49e6e7f145a70fff7dd42c8cc5 Mon Sep 17 00:00:00 2001
From: Phil Sutter <phil@nwl.cc>
Date: Tue, 5 Dec 2023 21:58:12 +0100
Subject: [nf PATCH] netfilter: xt_owner: Fix for unsafe access of sk->sk_socket

A concurrently running sock_orphan() may NULL the sk_socket pointer in
between check and deref. Follow other users (like nft_meta.c for
instance) and acquire sk_callback_lock before dereferencing sk_socket.

Reported-by: Jann Horn <jannh@google.com>
Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 net/netfilter/xt_owner.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/net/netfilter/xt_owner.c b/net/netfilter/xt_owner.c
index e85ce69924ae..50332888c8d2 100644
--- a/net/netfilter/xt_owner.c
+++ b/net/netfilter/xt_owner.c
@@ -76,18 +76,23 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 		 */
 		return false;
 
-	filp = sk->sk_socket->file;
-	if (filp == NULL)
+	read_lock_bh(&sk->sk_callback_lock);
+	filp = sk->sk_socket ? sk->sk_socket->file : NULL;
+	if (filp == NULL) {
+		read_unlock_bh(&sk->sk_callback_lock);
 		return ((info->match ^ info->invert) &
 		       (XT_OWNER_UID | XT_OWNER_GID)) == 0;
+	}
 
 	if (info->match & XT_OWNER_UID) {
 		kuid_t uid_min = make_kuid(net->user_ns, info->uid_min);
 		kuid_t uid_max = make_kuid(net->user_ns, info->uid_max);
 		if ((uid_gte(filp->f_cred->fsuid, uid_min) &&
 		     uid_lte(filp->f_cred->fsuid, uid_max)) ^
-		    !(info->invert & XT_OWNER_UID))
+		    !(info->invert & XT_OWNER_UID)) {
+			read_unlock_bh(&sk->sk_callback_lock);
 			return false;
+		}
 	}
 
 	if (info->match & XT_OWNER_GID) {
@@ -112,10 +117,13 @@ owner_mt(const struct sk_buff *skb, struct xt_action_param *par)
 			}
 		}
 
-		if (match ^ !(info->invert & XT_OWNER_GID))
+		if (match ^ !(info->invert & XT_OWNER_GID)) {
+			read_unlock_bh(&sk->sk_callback_lock);
 			return false;
+		}
 	}
 
+	read_unlock_bh(&sk->sk_callback_lock);
 	return true;
 }
 
-- 
2.41.0