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       1                 :            : // SPDX-License-Identifier: GPL-2.0-or-later
       2                 :            : /*
       3                 :            :  * NET3:        Garbage Collector For AF_UNIX sockets
       4                 :            :  *
       5                 :            :  * Garbage Collector:
       6                 :            :  *      Copyright (C) Barak A. Pearlmutter.
       7                 :            :  *
       8                 :            :  * Chopped about by Alan Cox 22/3/96 to make it fit the AF_UNIX socket problem.
       9                 :            :  * If it doesn't work blame me, it worked when Barak sent it.
      10                 :            :  *
      11                 :            :  * Assumptions:
      12                 :            :  *
      13                 :            :  *  - object w/ a bit
      14                 :            :  *  - free list
      15                 :            :  *
      16                 :            :  * Current optimizations:
      17                 :            :  *
      18                 :            :  *  - explicit stack instead of recursion
      19                 :            :  *  - tail recurse on first born instead of immediate push/pop
      20                 :            :  *  - we gather the stuff that should not be killed into tree
      21                 :            :  *    and stack is just a path from root to the current pointer.
      22                 :            :  *
      23                 :            :  *  Future optimizations:
      24                 :            :  *
      25                 :            :  *  - don't just push entire root set; process in place
      26                 :            :  *
      27                 :            :  *  Fixes:
      28                 :            :  *      Alan Cox        07 Sept 1997    Vmalloc internal stack as needed.
      29                 :            :  *                                      Cope with changing max_files.
      30                 :            :  *      Al Viro         11 Oct 1998
      31                 :            :  *              Graph may have cycles. That is, we can send the descriptor
      32                 :            :  *              of foo to bar and vice versa. Current code chokes on that.
      33                 :            :  *              Fix: move SCM_RIGHTS ones into the separate list and then
      34                 :            :  *              skb_free() them all instead of doing explicit fput's.
      35                 :            :  *              Another problem: since fput() may block somebody may
      36                 :            :  *              create a new unix_socket when we are in the middle of sweep
      37                 :            :  *              phase. Fix: revert the logic wrt MARKED. Mark everything
      38                 :            :  *              upon the beginning and unmark non-junk ones.
      39                 :            :  *
      40                 :            :  *              [12 Oct 1998] AAARGH! New code purges all SCM_RIGHTS
      41                 :            :  *              sent to connect()'ed but still not accept()'ed sockets.
      42                 :            :  *              Fixed. Old code had slightly different problem here:
      43                 :            :  *              extra fput() in situation when we passed the descriptor via
      44                 :            :  *              such socket and closed it (descriptor). That would happen on
      45                 :            :  *              each unix_gc() until the accept(). Since the struct file in
      46                 :            :  *              question would go to the free list and might be reused...
      47                 :            :  *              That might be the reason of random oopses on filp_close()
      48                 :            :  *              in unrelated processes.
      49                 :            :  *
      50                 :            :  *      AV              28 Feb 1999
      51                 :            :  *              Kill the explicit allocation of stack. Now we keep the tree
      52                 :            :  *              with root in dummy + pointer (gc_current) to one of the nodes.
      53                 :            :  *              Stack is represented as path from gc_current to dummy. Unmark
      54                 :            :  *              now means "add to tree". Push == "make it a son of gc_current".
      55                 :            :  *              Pop == "move gc_current to parent". We keep only pointers to
      56                 :            :  *              parents (->gc_tree).
      57                 :            :  *      AV              1 Mar 1999
      58                 :            :  *              Damn. Added missing check for ->dead in listen queues scanning.
      59                 :            :  *
      60                 :            :  *      Miklos Szeredi 25 Jun 2007
      61                 :            :  *              Reimplement with a cycle collecting algorithm. This should
      62                 :            :  *              solve several problems with the previous code, like being racy
      63                 :            :  *              wrt receive and holding up unrelated socket operations.
      64                 :            :  */
      65                 :            : 
      66                 :            : #include <linux/kernel.h>
      67                 :            : #include <linux/string.h>
      68                 :            : #include <linux/socket.h>
      69                 :            : #include <linux/un.h>
      70                 :            : #include <linux/net.h>
      71                 :            : #include <linux/fs.h>
      72                 :            : #include <linux/skbuff.h>
      73                 :            : #include <linux/netdevice.h>
      74                 :            : #include <linux/file.h>
      75                 :            : #include <linux/proc_fs.h>
      76                 :            : #include <linux/mutex.h>
      77                 :            : #include <linux/wait.h>
      78                 :            : 
      79                 :            : #include <net/sock.h>
      80                 :            : #include <net/af_unix.h>
      81                 :            : #include <net/scm.h>
      82                 :            : #include <net/tcp_states.h>
      83                 :            : 
      84                 :            : #include "scm.h"
      85                 :            : 
      86                 :            : /* Internal data structures and random procedures: */
      87                 :            : 
      88                 :            : static LIST_HEAD(gc_candidates);
      89                 :            : static DECLARE_WAIT_QUEUE_HEAD(unix_gc_wait);
      90                 :            : 
      91                 :          3 : static void scan_inflight(struct sock *x, void (*func)(struct unix_sock *),
      92                 :            :                           struct sk_buff_head *hitlist)
      93                 :            : {
      94                 :            :         struct sk_buff *skb;
      95                 :            :         struct sk_buff *next;
      96                 :            : 
      97                 :            :         spin_lock(&x->sk_receive_queue.lock);
      98                 :          3 :         skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
      99                 :            :                 /* Do we have file descriptors ? */
     100                 :          1 :                 if (UNIXCB(skb).fp) {
     101                 :            :                         bool hit = false;
     102                 :            :                         /* Process the descriptors of this socket */
     103                 :          0 :                         int nfd = UNIXCB(skb).fp->count;
     104                 :          0 :                         struct file **fp = UNIXCB(skb).fp->fp;
     105                 :            : 
     106                 :          0 :                         while (nfd--) {
     107                 :            :                                 /* Get the socket the fd matches if it indeed does so */
     108                 :          0 :                                 struct sock *sk = unix_get_socket(*fp++);
     109                 :            : 
     110                 :          0 :                                 if (sk) {
     111                 :            :                                         struct unix_sock *u = unix_sk(sk);
     112                 :            : 
     113                 :            :                                         /* Ignore non-candidates, they could
     114                 :            :                                          * have been added to the queues after
     115                 :            :                                          * starting the garbage collection
     116                 :            :                                          */
     117                 :          0 :                                         if (test_bit(UNIX_GC_CANDIDATE, &u->gc_flags)) {
     118                 :            :                                                 hit = true;
     119                 :            : 
     120                 :          0 :                                                 func(u);
     121                 :            :                                         }
     122                 :            :                                 }
     123                 :            :                         }
     124                 :          0 :                         if (hit && hitlist != NULL) {
     125                 :            :                                 __skb_unlink(skb, &x->sk_receive_queue);
     126                 :            :                                 __skb_queue_tail(hitlist, skb);
     127                 :            :                         }
     128                 :            :                 }
     129                 :            :         }
     130                 :            :         spin_unlock(&x->sk_receive_queue.lock);
     131                 :          3 : }
     132                 :            : 
     133                 :          3 : static void scan_children(struct sock *x, void (*func)(struct unix_sock *),
     134                 :            :                           struct sk_buff_head *hitlist)
     135                 :            : {
     136                 :          3 :         if (x->sk_state != TCP_LISTEN) {
     137                 :          3 :                 scan_inflight(x, func, hitlist);
     138                 :            :         } else {
     139                 :            :                 struct sk_buff *skb;
     140                 :            :                 struct sk_buff *next;
     141                 :            :                 struct unix_sock *u;
     142                 :          0 :                 LIST_HEAD(embryos);
     143                 :            : 
     144                 :            :                 /* For a listening socket collect the queued embryos
     145                 :            :                  * and perform a scan on them as well.
     146                 :            :                  */
     147                 :            :                 spin_lock(&x->sk_receive_queue.lock);
     148                 :          0 :                 skb_queue_walk_safe(&x->sk_receive_queue, skb, next) {
     149                 :          0 :                         u = unix_sk(skb->sk);
     150                 :            : 
     151                 :            :                         /* An embryo cannot be in-flight, so it's safe
     152                 :            :                          * to use the list link.
     153                 :            :                          */
     154                 :          0 :                         BUG_ON(!list_empty(&u->link));
     155                 :            :                         list_add_tail(&u->link, &embryos);
     156                 :            :                 }
     157                 :            :                 spin_unlock(&x->sk_receive_queue.lock);
     158                 :            : 
     159                 :          0 :                 while (!list_empty(&embryos)) {
     160                 :          0 :                         u = list_entry(embryos.next, struct unix_sock, link);
     161                 :          0 :                         scan_inflight(&u->sk, func, hitlist);
     162                 :          0 :                         list_del_init(&u->link);
     163                 :            :                 }
     164                 :            :         }
     165                 :          3 : }
     166                 :            : 
     167                 :          0 : static void dec_inflight(struct unix_sock *usk)
     168                 :            : {
     169                 :          0 :         atomic_long_dec(&usk->inflight);
     170                 :          0 : }
     171                 :            : 
     172                 :          0 : static void inc_inflight(struct unix_sock *usk)
     173                 :            : {
     174                 :          0 :         atomic_long_inc(&usk->inflight);
     175                 :          0 : }
     176                 :            : 
     177                 :          0 : static void inc_inflight_move_tail(struct unix_sock *u)
     178                 :            : {
     179                 :          0 :         atomic_long_inc(&u->inflight);
     180                 :            :         /* If this still might be part of a cycle, move it to the end
     181                 :            :          * of the list, so that it's checked even if it was already
     182                 :            :          * passed over
     183                 :            :          */
     184                 :          0 :         if (test_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags))
     185                 :          0 :                 list_move_tail(&u->link, &gc_candidates);
     186                 :          0 : }
     187                 :            : 
     188                 :            : static bool gc_in_progress;
     189                 :            : #define UNIX_INFLIGHT_TRIGGER_GC 16000
     190                 :            : 
     191                 :          3 : void wait_for_unix_gc(void)
     192                 :            : {
     193                 :            :         /* If number of inflight sockets is insane,
     194                 :            :          * force a garbage collect right now.
     195                 :            :          */
     196                 :          3 :         if (unix_tot_inflight > UNIX_INFLIGHT_TRIGGER_GC && !gc_in_progress)
     197                 :          0 :                 unix_gc();
     198                 :          3 :         wait_event(unix_gc_wait, gc_in_progress == false);
     199                 :          3 : }
     200                 :            : 
     201                 :            : /* The external entry point: unix_gc() */
     202                 :          3 : void unix_gc(void)
     203                 :            : {
     204                 :            :         struct unix_sock *u;
     205                 :            :         struct unix_sock *next;
     206                 :            :         struct sk_buff_head hitlist;
     207                 :            :         struct list_head cursor;
     208                 :          3 :         LIST_HEAD(not_cycle_list);
     209                 :            : 
     210                 :            :         spin_lock(&unix_gc_lock);
     211                 :            : 
     212                 :            :         /* Avoid a recursive GC. */
     213                 :          3 :         if (gc_in_progress)
     214                 :            :                 goto out;
     215                 :            : 
     216                 :          3 :         gc_in_progress = true;
     217                 :            :         /* First, select candidates for garbage collection.  Only
     218                 :            :          * in-flight sockets are considered, and from those only ones
     219                 :            :          * which don't have any external reference.
     220                 :            :          *
     221                 :            :          * Holding unix_gc_lock will protect these candidates from
     222                 :            :          * being detached, and hence from gaining an external
     223                 :            :          * reference.  Since there are no possible receivers, all
     224                 :            :          * buffers currently on the candidates' queues stay there
     225                 :            :          * during the garbage collection.
     226                 :            :          *
     227                 :            :          * We also know that no new candidate can be added onto the
     228                 :            :          * receive queues.  Other, non candidate sockets _can_ be
     229                 :            :          * added to queue, so we must make sure only to touch
     230                 :            :          * candidates.
     231                 :            :          */
     232                 :          3 :         list_for_each_entry_safe(u, next, &gc_inflight_list, link) {
     233                 :            :                 long total_refs;
     234                 :            :                 long inflight_refs;
     235                 :            : 
     236                 :          3 :                 total_refs = file_count(u->sk.sk_socket->file);
     237                 :            :                 inflight_refs = atomic_long_read(&u->inflight);
     238                 :            : 
     239                 :          3 :                 BUG_ON(inflight_refs < 1);
     240                 :          3 :                 BUG_ON(total_refs < inflight_refs);
     241                 :          3 :                 if (total_refs == inflight_refs) {
     242                 :            :                         list_move_tail(&u->link, &gc_candidates);
     243                 :            :                         __set_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
     244                 :            :                         __set_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
     245                 :            :                 }
     246                 :            :         }
     247                 :            : 
     248                 :            :         /* Now remove all internal in-flight reference to children of
     249                 :            :          * the candidates.
     250                 :            :          */
     251                 :          3 :         list_for_each_entry(u, &gc_candidates, link)
     252                 :          3 :                 scan_children(&u->sk, dec_inflight, NULL);
     253                 :            : 
     254                 :            :         /* Restore the references for children of all candidates,
     255                 :            :          * which have remaining references.  Do this recursively, so
     256                 :            :          * only those remain, which form cyclic references.
     257                 :            :          *
     258                 :            :          * Use a "cursor" link, to make the list traversal safe, even
     259                 :            :          * though elements might be moved about.
     260                 :            :          */
     261                 :            :         list_add(&cursor, &gc_candidates);
     262                 :          3 :         while (cursor.next != &gc_candidates) {
     263                 :            :                 u = list_entry(cursor.next, struct unix_sock, link);
     264                 :            : 
     265                 :            :                 /* Move cursor to after the current position. */
     266                 :          3 :                 list_move(&cursor, &u->link);
     267                 :            : 
     268                 :          3 :                 if (atomic_long_read(&u->inflight) > 0) {
     269                 :            :                         list_move_tail(&u->link, &not_cycle_list);
     270                 :            :                         __clear_bit(UNIX_GC_MAYBE_CYCLE, &u->gc_flags);
     271                 :          3 :                         scan_children(&u->sk, inc_inflight_move_tail, NULL);
     272                 :            :                 }
     273                 :            :         }
     274                 :            :         list_del(&cursor);
     275                 :            : 
     276                 :            :         /* Now gc_candidates contains only garbage.  Restore original
     277                 :            :          * inflight counters for these as well, and remove the skbuffs
     278                 :            :          * which are creating the cycle(s).
     279                 :            :          */
     280                 :            :         skb_queue_head_init(&hitlist);
     281                 :          3 :         list_for_each_entry(u, &gc_candidates, link)
     282                 :          0 :                 scan_children(&u->sk, inc_inflight, &hitlist);
     283                 :            : 
     284                 :            :         /* not_cycle_list contains those sockets which do not make up a
     285                 :            :          * cycle.  Restore these to the inflight list.
     286                 :            :          */
     287                 :          3 :         while (!list_empty(&not_cycle_list)) {
     288                 :          3 :                 u = list_entry(not_cycle_list.next, struct unix_sock, link);
     289                 :            :                 __clear_bit(UNIX_GC_CANDIDATE, &u->gc_flags);
     290                 :          3 :                 list_move_tail(&u->link, &gc_inflight_list);
     291                 :            :         }
     292                 :            : 
     293                 :            :         spin_unlock(&unix_gc_lock);
     294                 :            : 
     295                 :            :         /* Here we are. Hitlist is filled. Die. */
     296                 :          3 :         __skb_queue_purge(&hitlist);
     297                 :            : 
     298                 :            :         spin_lock(&unix_gc_lock);
     299                 :            : 
     300                 :            :         /* All candidates should have been detached by now. */
     301                 :          3 :         BUG_ON(!list_empty(&gc_candidates));
     302                 :          3 :         gc_in_progress = false;
     303                 :          3 :         wake_up(&unix_gc_wait);
     304                 :            : 
     305                 :            :  out:
     306                 :            :         spin_unlock(&unix_gc_lock);
     307                 :          3 : }