xfrm6: fix uninitialized saddr in xfrm6_get_saddr()

xfrm6_get_saddr() does not check the return value of
ipv6_dev_get_saddr(). When ipv6_dev_get_saddr() fails to find a suitable
source address (returns -EADDRNOTAVAIL), saddr->in6 is left
uninitialized, but xfrm6_get_saddr() still returns 0 (success).

This causes the caller xfrm_tmpl_resolve_one() to use the uninitialized
address in xfrm_state_find(), triggering KMSAN warning:

=====================================================
BUG: KMSAN: uninit-value in xfrm_state_find+0x2424/0xa940
 xfrm_state_find+0x2424/0xa940
 xfrm_resolve_and_create_bundle+0x906/0x5a20
 xfrm_lookup_with_ifid+0xcc0/0x3770
 xfrm_lookup_route+0x63/0x2b0
 ip_route_output_flow+0x1ce/0x270
 udp_sendmsg+0x2ce1/0x3400
 inet_sendmsg+0x1ef/0x2a0
 __sock_sendmsg+0x278/0x3d0
 __sys_sendto+0x593/0x720
 __x64_sys_sendto+0x130/0x200
 x64_sys_call+0x332b/0x3e70
 do_syscall_64+0xd3/0xf80
 entry_SYSCALL_64_after_hwframe+0x77/0x7f

Local variable tmp.i.i created at:
 xfrm_resolve_and_create_bundle+0x3e3/0x5a20
 xfrm_lookup_with_ifid+0xcc0/0x3770
=====================================================

Fix by checking the return value of ipv6_dev_get_saddr() and propagating
the error.

Fixes: a1e59ab ("[XFRM]: Fix wildcard as tunnel source")
Reported-by: syzbot+e136d86d34b42399a8b1@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/68bf1024.a70a0220.7a912.02c2.GAE@google.com/T/
Signed-off-by: Jiayuan Chen <jiayuan.chen@shopee.com>
Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Author: mrpre

net/ipv6/xfrm6_policy.c

@@ -57,6 +57,7 @@ static int xfrm6_get_saddr(xfrm_address_t *saddr,
 	struct dst_entry *dst;
 	struct net_device *dev;
 	struct inet6_dev *idev;
+	int err;
 
 	dst = xfrm6_dst_lookup(params);
 	if (IS_ERR(dst))
@@ -68,9 +69,11 @@ static int xfrm6_get_saddr(xfrm_address_t *saddr,
 		return -EHOSTUNREACH;
 	}
 	dev = idev->dev;
-	ipv6_dev_get_saddr(dev_net(dev), dev, &params->daddr->in6, 0,
-			   &saddr->in6);
+	err = ipv6_dev_get_saddr(dev_net(dev), dev, &params->daddr->in6, 0,
+				 &saddr->in6);
 	dst_release(dst);
+	if (err)
+		return -EHOSTUNREACH;
 	return 0;
 }