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af_unix: fix garbage collect vs MSG_PEEK

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unix_gc() assumes that candidate sockets can never gain an external
reference (i.e.  be installed into an fd) while the unix_gc_lock is
held.  Except for MSG_PEEK this is guaranteed by modifying inflight
count under the unix_gc_lock.

MSG_PEEK does not touch any variable protected by unix_gc_lock (file
count is not), yet it needs to be serialized with garbage collection.
Do this by locking/unlocking unix_gc_lock:

 1) increment file count

 2) lock/unlock barrier to make sure incremented file count is visible
    to garbage collection

 3) install file into fd

This is a lock barrier (unlike smp_mb()) that ensures that garbage
collection is run completely before or completely after the barrier.

Cc: <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Miklos Szeredi 2021-07-28 14:47:20 +02:00 committed by Linus Torvalds
parent 7d549995d4
commit cbcf01128d
1 changed files with 49 additions and 2 deletions
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51
net/unix/af_unix.c
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@ -1526,6 +1526,53 @@ out:
return err;
}
static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb)
{
scm->fp = scm_fp_dup(UNIXCB(skb).fp);
/*
* Garbage collection of unix sockets starts by selecting a set of
* candidate sockets which have reference only from being in flight
* (total_refs == inflight_refs). This condition is checked once during
* the candidate collection phase, and candidates are marked as such, so
* that non-candidates can later be ignored. While inflight_refs is
* protected by unix_gc_lock, total_refs (file count) is not, hence this
* is an instantaneous decision.
*
* Once a candidate, however, the socket must not be reinstalled into a
* file descriptor while the garbage collection is in progress.
*
* If the above conditions are met, then the directed graph of
* candidates (*) does not change while unix_gc_lock is held.
*
* Any operations that changes the file count through file descriptors
* (dup, close, sendmsg) does not change the graph since candidates are
* not installed in fds.
*
* Dequeing a candidate via recvmsg would install it into an fd, but
* that takes unix_gc_lock to decrement the inflight count, so it's
* serialized with garbage collection.
*
* MSG_PEEK is special in that it does not change the inflight count,
* yet does install the socket into an fd. The following lock/unlock
* pair is to ensure serialization with garbage collection. It must be
* done between incrementing the file count and installing the file into
* an fd.
*
* If garbage collection starts after the barrier provided by the
* lock/unlock, then it will see the elevated refcount and not mark this
* as a candidate. If a garbage collection is already in progress
* before the file count was incremented, then the lock/unlock pair will
* ensure that garbage collection is finished before progressing to
* installing the fd.
*
* (*) A -> B where B is on the queue of A or B is on the queue of C
* which is on the queue of listening socket A.
*/
spin_lock(&unix_gc_lock);
spin_unlock(&unix_gc_lock);
}
static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
{
int err = 0;
@ -2175,7 +2222,7 @@ static int unix_dgram_recvmsg(struct socket *sock, struct msghdr *msg,
sk_peek_offset_fwd(sk, size);
if (UNIXCB(skb).fp)
scm.fp = scm_fp_dup(UNIXCB(skb).fp);
unix_peek_fds(&scm, skb);
}
err = (flags & MSG_TRUNC) ? skb->len - skip : size;
@ -2418,7 +2465,7 @@ unlock:
/* It is questionable, see note in unix_dgram_recvmsg.
*/
if (UNIXCB(skb).fp)
scm.fp = scm_fp_dup(UNIXCB(skb).fp);
unix_peek_fds(&scm, skb);
sk_peek_offset_fwd(sk, chunk);