path: root/net/unix
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authorDaniel Borkmann <dborkman@redhat.com>2013-10-17 22:51:31 +0200
committerDavid S. Miller <davem@davemloft.net>2013-10-19 18:50:15 -0400
commit90c6bd34f884cd9cee21f1d152baf6c18bcac949 (patch)
treeef74f89ab35407814970f8d8f7e80f3e04b8d3b0 /net/unix
parent66c562efbf35e98c42789f73cf39ad5f28abf6be (diff)
net: unix: inherit SOCK_PASS{CRED, SEC} flags from socket to fix race
In the case of credentials passing in unix stream sockets (dgram sockets seem not affected), we get a rather sparse race after commit 16e5726 ("af_unix: dont send SCM_CREDENTIALS by default"). We have a stream server on receiver side that requests credential passing from senders (e.g. nc -U). Since we need to set SO_PASSCRED on each spawned/accepted socket on server side to 1 first (as it's not inherited), it can happen that in the time between accept() and setsockopt() we get interrupted, the sender is being scheduled and continues with passing data to our receiver. At that time SO_PASSCRED is neither set on sender nor receiver side, hence in cmsg's SCM_CREDENTIALS we get eventually pid:0, uid:65534, gid:65534 (== overflow{u,g}id) instead of what we actually would like to see. On the sender side, here nc -U, the tests in maybe_add_creds() invoked through unix_stream_sendmsg() would fail, as at that exact time, as mentioned, the sender has neither SO_PASSCRED on his side nor sees it on the server side, and we have a valid 'other' socket in place. Thus, sender believes it would just look like a normal connection, not needing/requesting SO_PASSCRED at that time. As reverting 16e5726 would not be an option due to the significant performance regression reported when having creds always passed, one way/trade-off to prevent that would be to set SO_PASSCRED on the listener socket and allow inheriting these flags to the spawned socket on server side in accept(). It seems also logical to do so if we'd tell the listener socket to pass those flags onwards, and would fix the race. Before, strace: recvmsg(4, {msg_name(0)=NULL, msg_iov(1)=[{"blub\n", 4096}], msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=0, uid=65534, gid=65534}}, msg_flags=0}, 0) = 5 After, strace: recvmsg(4, {msg_name(0)=NULL, msg_iov(1)=[{"blub\n", 4096}], msg_controllen=32, {cmsg_len=28, cmsg_level=SOL_SOCKET, cmsg_type=SCM_CREDENTIALS{pid=11580, uid=1000, gid=1000}}, msg_flags=0}, 0) = 5 Signed-off-by: Daniel Borkmann <dborkman@redhat.com> Cc: Eric Dumazet <edumazet@google.com> Cc: Eric W. Biederman <ebiederm@xmission.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net/unix')
1 files changed, 10 insertions, 0 deletions
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 86de99ad2976..c1f403bed683 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1246,6 +1246,15 @@ static int unix_socketpair(struct socket *socka, struct socket *sockb)
return 0;
+static void unix_sock_inherit_flags(const struct socket *old,
+ struct socket *new)
+ if (test_bit(SOCK_PASSCRED, &old->flags))
+ set_bit(SOCK_PASSCRED, &new->flags);
+ if (test_bit(SOCK_PASSSEC, &old->flags))
+ set_bit(SOCK_PASSSEC, &new->flags);
static int unix_accept(struct socket *sock, struct socket *newsock, int flags)
struct sock *sk = sock->sk;
@@ -1280,6 +1289,7 @@ static int unix_accept(struct socket *sock, struct socket *newsock, int flags)
/* attach accepted sock to socket */
newsock->state = SS_CONNECTED;
+ unix_sock_inherit_flags(sock, newsock);
sock_graft(tsk, newsock);
return 0;

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