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SubjectRe: [PATCH net] af_unix: fix garbage collect vs. MSG_PEEK
On Mon, Jul 26, 2021 at 05:36:21PM +0200, Greg Kroah-Hartman wrote:
> From: Miklos Szeredi <>
> Gc assumes that in-flight sockets that don't have an external ref can't

I think this commit log could be expanded. I had to really study things
to even beging to understand what was going on. I assume "Gc" here means
specifically unix_gc()?

> 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.

In reading the code, I *think* what is being protected by unix_gc_lock is
user->unix_inflight, u->inflight, unix_tot_inflight, and gc_inflight_list?

I note that unix_tot_inflight isn't an atomic but is read outside of
locking by unix_release_sock() and wait_for_unix_gc(), which seems wrong
(or at least inefficient).

But regardless, are the "external references" the f_count (i.e. get_file()
of u->sk.sk_socket->file) being changed by scm_fp_dup() and read by
unix_gc() (i.e. file_count())? It seems the test in unix_gc() is for
the making sure f_count isn't out of sync with u->inflight (is this the
corresponding "internal" reference?):

total_refs = file_count(u->sk.sk_socket->file);
inflight_refs = atomic_long_read(&u->inflight);

BUG_ON(inflight_refs < 1);
BUG_ON(total_refs < inflight_refs);
if (total_refs == inflight_refs) {

> 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.

The idea appears to be that all scm_fp_dup() callers need to refresh the
u->inflight counts which is what unix_attach_fds() and unix_detach_fds()
do. Why is lock/unlock sufficient for unix_peek_fds()?

I assume the rationale is because MSG_PEEK uses a temporary scm, which
only gets fput() clean-up on destroy ("inflight" is neither incremented
nor decremented at any point in the scm lifetime).

But I don't see why any of this helps.

fget(), spin_lock(), inflight++, spin_unlock()
spin_lock(), inflight--, spin_unlock(), fput()
fget(), spin_lock(), spin_unlock()
spin_lock(), "total_refs == inflight_refs" to hitlist,
spin_unlock(), free hitlist skbs

Doesn't this mean total_refs and inflight_refs can still get out of
sync? What keeps an skb from being "visible" to unix_peek_fds() between
the unix_gx() spin_unlock() and the unix_peek_fds() fget()?

A: unix_gx():
find "total_refs == inflight_refs", add to hitlist
B: unix_peek_fds():
A: unix_gc():
walk hitlist and free(skb)
B: unix_peek_fds():
*use freed skb*

I feel like I must be missing something since the above race would
appear to exist even for unix_attach_fds()/unix_detach_fds():

A: unix_gx():
find "total_refs == inflight_refs", add to hitlist
B: unix_attach_fds():
A: unix_gc():
walk hitlist and free(skb)
B: unix_attach_fds():
*use freed skb*

I'm assuming I'm missing a top-level usage count on skb that is held by
callers, which means the skb isn't actually freed by unix_gc(). But I
return to not understanding why adding the lock/unlock helps.

What are the expected locking semantics here?


> Signed-off-by: Miklos Szeredi <>
> Cc: <>
> Signed-off-by: Greg Kroah-Hartman <>
> ---
> net/unix/af_unix.c | 16 ++++++++++++++--
> 1 file changed, 14 insertions(+), 2 deletions(-)
> Note, this is a resend of this old submission that somehow fell through
> the cracks:
> and was never submitted "properly" and this issue never seemed to get
> resolved properly.
> I've cleaned it up and made the change much smaller and localized to
> only one file. I kept Miklos's authorship as he did the hard work on
> this, I just removed lines and fixed a formatting issue :)
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 23c92ad15c61..cdea997aa5bf 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -1526,6 +1526,18 @@ static int unix_getname(struct socket *sock, struct sockaddr *uaddr, int peer)
> return err;
> }
> +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.
> + */
> + spin_lock(&unix_gc_lock);
> + spin_unlock(&unix_gc_lock);
> +}

Kees Cook

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