Messages in this thread Patch in this message | | | From | Miklos Szeredi <> | Date | Tue, 30 Aug 2016 11:18:10 +0200 | Subject | Re: kernel BUG at net/unix/garbage.c:149!" |
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On Tue, Aug 30, 2016 at 12:37 AM, Miklos Szeredi <mszeredi@redhat.com> wrote: > On Sat, Aug 27, 2016 at 11:55 AM, Miklos Szeredi <mszeredi@redhat.com> wrote:
> crash> list -H gc_inflight_list unix_sock.link -s unix_sock.inflight | > grep counter | cut -d= -f2 | awk '{s+=$1} END {print s}' > 130 > crash> p unix_tot_inflight > unix_tot_inflight = $2 = 135 > > We've lost track of a total of five inflight sockets, so it's not a > one-off thing. Really weird... Now off to sleep, maybe I'll dream of > the solution.
Okay, found one bug: gc assumes that in-flight sockets that don't have an external ref can't gain one while unix_gc_lock is held. That is true because unix_notinflight() will be called before detaching fds, which takes unix_gc_lock. Only MSG_PEEK was somehow overlooked. That one also clones the fds, also keeping them in the skb. But through MSG_PEEK an external reference can definitely be gained without ever touching unix_gc_lock.
Not sure whether the reported bug can be explained by this. Can you confirm the MSG_PEEK was used in the setup?
Does someone want to write a stress test for SCM_RIGHTS + MSG_PEEK?
Anyway, attaching a fix that works by acquiring unix_gc_lock in case of MSG_PEEK also. It is trivially correct, but I haven't tested it.
Thanks, Miklos From: Miklos Szeredi <mszeredi@redhat.com> Subject: af_unix: fix garbage collect vs. MSG_PEEK
Gc assumes that in-flight sockets that don't have an external ref can't gain one while unix_gc_lock is held. That is true because unix_notinflight() will be called before detaching fds, which takes unix_gc_lock.
Only MSG_PEEK was somehow overlooked. That one also clones the fds, also keeping them in the skb. But through MSG_PEEK an external reference can definitely be gained without ever touching unix_gc_lock.
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com> Cc: <stable@vger.kernel.org> --- include/net/af_unix.h | 1 + net/unix/af_unix.c | 15 +++++++++++++-- net/unix/garbage.c | 6 ++++++ 3 files changed, 20 insertions(+), 2 deletions(-)
--- a/include/net/af_unix.h +++ b/include/net/af_unix.h @@ -10,6 +10,7 @@ void unix_inflight(struct user_struct *u void unix_notinflight(struct user_struct *user, struct file *fp); void unix_gc(void); void wait_for_unix_gc(void); +void unix_gc_barrier(void); struct sock *unix_get_socket(struct file *filp); struct sock *unix_peer_get(struct sock *); --- a/net/unix/af_unix.c +++ b/net/unix/af_unix.c @@ -1563,6 +1563,17 @@ static int unix_attach_fds(struct scm_co return max_level; } +static void unix_peek_fds(struct scm_cookie *scm, struct sk_buff *skb) +{ + scm->fp = scm_fp_dup(UNIXCB(skb).fp); + /* + * During garbage collection it is assumed that in-flight sockets don't + * get a new external reference. So we need to wait until current run + * finishes. + */ + unix_gc_barrier(); +} + static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds) { int err = 0; @@ -2195,7 +2206,7 @@ static int unix_dgram_recvmsg(struct soc 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; @@ -2435,7 +2446,7 @@ static int unix_stream_read_generic(stru /* 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); --- a/net/unix/garbage.c +++ b/net/unix/garbage.c @@ -266,6 +266,12 @@ void wait_for_unix_gc(void) wait_event(unix_gc_wait, gc_in_progress == false); } +void unix_gc_barrier(void) +{ + spin_lock(&unix_gc_lock); + spin_unlock(&unix_gc_lock); +} + /* The external entry point: unix_gc() */ void unix_gc(void) { | |