unp gc vs socket close/shutdown race
Mateusz Guzik
mjguzik at gmail.com
Fri Jul 10 00:17:46 UTC 2015
On Wed, Jul 08, 2015 at 02:15:38AM +0200, Mateusz Guzik wrote:
> First off note the patch below is a total hack with the easiest solution
> possible so that it can be MFCed for 10.2.
>
> The issue:
>
> Closing the socket involves:
>
> if (pr->pr_flags & PR_RIGHTS && pr->pr_domain->dom_dispose != NULL)
> (*pr->pr_domain->dom_dispose)(so->so_rcv.sb_mb);
> if (pr->pr_usrreqs->pru_detach != NULL)
> (*pr->pr_usrreqs->pru_detach)(so);
>
> unp_dispose which gets rid of file descriptors stored in mbufs attached
> to the socket. It leaves the mbuf chain in place.
>
> uipc_detach actually unlinks the socket from global unp list.
>
> In particular, this means there is a socket with unusable mbufs visible
> to unp garbage collector (unp_gc). Also there is no synchronisation of
> any form performed here, so it can inspect mbufs as fds are getting
> freed (or afterwards) leading to panics. Note that uipc_detach waits for
> unp_gc to finish due to UNP_LIST_LOCK, it's the dispose func which
> causes trouble.
>
> Given that stuff should not be accessed after unp_dispose, and the
> socket is about to die I figured it would be best to mark the socket so
> that unp_gc can ignore it.
>
> Note that unp_dispose only gets a pointer to mbuf. I have not found any
> way to obtain a socket from this, which in turn results in the hack
> below.
>
> I added a new func - unp_dispose2, which is not a part of struct domain.
> dom_dispose consumers check for PR_DISPOSE2 flag and call the function
> passing the socket as an argument.
>
> unp_dispose2(struct socket *so)
> {
> struct unpcb *unp;
>
> unp = sotounpcb(so);
> UNP_LIST_LOCK();
> unp->unp_gcflag |= UNPGC_IGNORE;
> UNP_LIST_UNLOCK();
> unp_dispose(so->so_rcv.sb_mb);
> }
>
> The UNP_LIST_LOCK + UNLOCK synchronizes against unp_gc - either it sees the
> flag and ignores the socket, or gets to inspect it and unp_dispose2 waits for
> it to finish. AFAICT it is completely harmless to proceed with freeing after
> unp_gc had a look.
>
> There is a similar problem with shutdown(), but the race has a smaller window
> due to it clearing mbufs just after dispose call.
>
> In general, it feels like something else is also broken, but I don't see what.
>
> The issue can be reproduced by running this program in a loop:
> https://people.freebsd.org/~mjg/reproducers/unp-gc-panic.c
>
> With the patch below the issue seems to be fixed:
>
Deuglified the patch a little. PR_DISPOSE2 flag is introduced. When set
on a protocol, unp_dispose2 is called insted of dom_dispose.
Also fixed a copy-pasto in sorflush case. It used to partially construct
a sockbuf and clear stuff in the original. It is changed to partialy
construct the socket instead, as that's what we obtain in unp_dispose2.
sockbuf is reconstructed to the same extent, socket also gets pcb so
that unp_dispose2 can botain relevant unp pointer.
diff --git a/sys/kern/uipc_socket.c b/sys/kern/uipc_socket.c
index a431b4b..e78c81d 100644
--- a/sys/kern/uipc_socket.c
+++ b/sys/kern/uipc_socket.c
@@ -804,8 +804,13 @@ sofree(struct socket *so)
ACCEPT_UNLOCK();
VNET_SO_ASSERT(so);
- if (pr->pr_flags & PR_RIGHTS && pr->pr_domain->dom_dispose != NULL)
- (*pr->pr_domain->dom_dispose)(so->so_rcv.sb_mb);
+ if (pr->pr_flags & PR_RIGHTS) {
+ if (pr->pr_flags & PR_DISPOSE2) {
+ unp_dispose2(so);
+ } else if (pr->pr_domain->dom_dispose != NULL) {
+ (*pr->pr_domain->dom_dispose)(so->so_rcv.sb_mb);
+ }
+ }
if (pr->pr_usrreqs->pru_detach != NULL)
(*pr->pr_usrreqs->pru_detach)(so);
@@ -2356,7 +2361,7 @@ sorflush(struct socket *so)
{
struct sockbuf *sb = &so->so_rcv;
struct protosw *pr = so->so_proto;
- struct sockbuf asb;
+ struct socket aso;
VNET_SO_ASSERT(so);
@@ -2381,8 +2386,9 @@ sorflush(struct socket *so)
* and mutex data unchanged.
*/
SOCKBUF_LOCK(sb);
- bzero(&asb, offsetof(struct sockbuf, sb_startzero));
- bcopy(&sb->sb_startzero, &asb.sb_startzero,
+ bzero(&aso, sizeof(aso));
+ aso.so_pcb = so->so_pcb;
+ bcopy(&sb->sb_startzero, &aso.so_rcv.sb_startzero,
sizeof(*sb) - offsetof(struct sockbuf, sb_startzero));
bzero(&sb->sb_startzero,
sizeof(*sb) - offsetof(struct sockbuf, sb_startzero));
@@ -2393,9 +2399,14 @@ sorflush(struct socket *so)
* Dispose of special rights and flush the socket buffer. Don't call
* any unsafe routines (that rely on locks being initialized) on asb.
*/
- if (pr->pr_flags & PR_RIGHTS && pr->pr_domain->dom_dispose != NULL)
- (*pr->pr_domain->dom_dispose)(asb.sb_mb);
- sbrelease_internal(&asb, so);
+ if (pr->pr_flags & PR_RIGHTS) {
+ if (pr->pr_flags & PR_DISPOSE2) {
+ unp_dispose2(&aso);
+ } else if (pr->pr_domain->dom_dispose != NULL) {
+ (*pr->pr_domain->dom_dispose)(aso.so_rcv.sb_mb);
+ }
+ }
+ sbrelease_internal(&aso.so_rcv, so);
}
/*
diff --git a/sys/kern/uipc_usrreq.c b/sys/kern/uipc_usrreq.c
index acf9fe9..98ef6dc 100644
--- a/sys/kern/uipc_usrreq.c
+++ b/sys/kern/uipc_usrreq.c
@@ -302,14 +302,15 @@ static struct protosw localsw[] = {
{
.pr_type = SOCK_STREAM,
.pr_domain = &localdomain,
- .pr_flags = PR_CONNREQUIRED|PR_WANTRCVD|PR_RIGHTS,
+ .pr_flags = PR_CONNREQUIRED|PR_WANTRCVD|PR_RIGHTS|
+ PR_DISPOSE2,
.pr_ctloutput = &uipc_ctloutput,
.pr_usrreqs = &uipc_usrreqs_stream
},
{
.pr_type = SOCK_DGRAM,
.pr_domain = &localdomain,
- .pr_flags = PR_ATOMIC|PR_ADDR|PR_RIGHTS,
+ .pr_flags = PR_ATOMIC|PR_ADDR|PR_RIGHTS|PR_DISPOSE2,
.pr_ctloutput = &uipc_ctloutput,
.pr_usrreqs = &uipc_usrreqs_dgram
},
@@ -323,7 +324,7 @@ static struct protosw localsw[] = {
* that supports both atomic record writes and control data.
*/
.pr_flags = PR_ADDR|PR_ATOMIC|PR_CONNREQUIRED|PR_WANTRCVD|
- PR_RIGHTS,
+ PR_RIGHTS|PR_DISPOSE2,
.pr_ctloutput = &uipc_ctloutput,
.pr_usrreqs = &uipc_usrreqs_seqpacket,
},
@@ -2193,8 +2194,7 @@ unp_gc_process(struct unpcb *unp)
struct socket *so;
struct file *fp;
- /* Already processed. */
- if (unp->unp_gcflag & UNPGC_SCANNED)
+ if (unp->unp_gcflag & (UNPGC_SCANNED | UNPGC_IGNORE))
return;
fp = unp->unp_file;
@@ -2252,11 +2252,11 @@ unp_gc(__unused void *arg, int pending)
unp_taskcount++;
UNP_LIST_LOCK();
/*
- * First clear all gc flags from previous runs.
+ * First clear all gc flags from previous runs, apart from UNPGC_IGNORE.
*/
for (head = heads; *head != NULL; head++)
LIST_FOREACH(unp, *head, unp_link)
- unp->unp_gcflag = 0;
+ unp->unp_gcflag = unp->unp_gcflag & UNPGC_IGNORE;
/*
* Scan marking all reachable sockets with UNPGC_REF. Once a socket
@@ -2333,6 +2333,24 @@ unp_dispose(struct mbuf *m)
unp_scan(m, unp_freerights);
}
+/*
+ * XXX A hack working around a difenciency in domain API.
+ * dom_dispose handler does not get the socket it is supposed to operate on,
+ * which makes it very problematic to synchronize against unp_gc, which in turn
+ * can trip over data as we are freeing it.
+ */
+void
+unp_dispose2(struct socket *so)
+{
+ struct unpcb *unp;
+
+ unp = sotounpcb(so);
+ UNP_LIST_LOCK();
+ unp->unp_gcflag |= UNPGC_IGNORE;
+ UNP_LIST_UNLOCK();
+ unp_dispose(so->so_rcv.sb_mb);
+}
+
static void
unp_scan(struct mbuf *m0, void (*op)(struct filedescent **, int))
{
diff --git a/sys/sys/protosw.h b/sys/sys/protosw.h
index 55db0e3..af45532 100644
--- a/sys/sys/protosw.h
+++ b/sys/sys/protosw.h
@@ -120,6 +120,7 @@ struct protosw {
#define PR_RIGHTS 0x10 /* passes capabilities */
#define PR_IMPLOPCL 0x20 /* implied open/close */
#define PR_LASTHDR 0x40 /* enforce ipsec policy; last header */
+#define PR_DISPOSE2 0x80 /* call unp_dispose2 */
/*
* In earlier BSD network stacks, a single pr_usrreq() function pointer was
diff --git a/sys/sys/socket.h b/sys/sys/socket.h
index 18e2de1..e927cdc 100644
--- a/sys/sys/socket.h
+++ b/sys/sys/socket.h
@@ -666,6 +666,8 @@ void so_unlock(struct socket *so);
void so_listeners_apply_all(struct socket *so, void (*func)(struct socket *, void *), void *arg);
+void unp_dispose2(struct socket *so);
+
#endif
diff --git a/sys/sys/unpcb.h b/sys/sys/unpcb.h
index ba63f30..ead9f0a 100644
--- a/sys/sys/unpcb.h
+++ b/sys/sys/unpcb.h
@@ -106,6 +106,7 @@ struct unpcb {
#define UNPGC_REF 0x1 /* unpcb has external ref. */
#define UNPGC_DEAD 0x2 /* unpcb might be dead. */
#define UNPGC_SCANNED 0x4 /* Has been scanned. */
+#define UNPGC_IGNORE 0x4 /* Someone will clear it. */
/*
* These flags are used to handle non-atomicity in connect() and bind()
--
Mateusz Guzik <mjguzik gmail.com>
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