Re: [PATCH v2] unix: properly account for FDs passed over unix sockets

Hannes Frederic Sowa Tue, 02 Feb 2016 13:56:37 -0800

Hi Willy,

On 02.02.2016 21:39, Willy Tarreau wrote:

On Tue, Feb 02, 2016 at 09:32:56PM +0100, Hannes Frederic Sowa wrote:

But "struct pid *" in unix_skb_parms should be enough to get us to
corresponding "struct cred *" so we can decrement the correct counter
during skb destruction.


So:

We increment current task's unix_inflight and also check the current
task's limit during attaching fds to skbs and decrement the inflight
counter via "struct pid *". This looks like it should work.


I like it as well, the principle sounds sane.

That way it's always the person who actually does the send (rather
than the opener of the socket _or_ the opener of the file that gets
passed around) that gets credited, and thanks to the cred pointer we
can then de-credit them properly.


Exactly, I try to implement that. Thanks a lot!


Thanks to you Hannes, I appreciate that you work on it, it would take
much more time to me to dig into this.

I slightly tested the attached patch. If you have the originalreproducer available could you also give it a try? Unfortunately Icurrently don't find it and am limited in time this evening.


Thanks a lot,
Hannes

diff --git a/include/net/af_unix.h b/include/net/af_unix.h
index 2a91a0561a4783..4567dbe04f274d 100644
--- a/include/net/af_unix.h
+++ b/include/net/af_unix.h
@@ -6,8 +6,8 @@
 #include <linux/mutex.h>
 #include <net/sock.h>

-void unix_inflight(struct file *fp);
-void unix_notinflight(struct file *fp);
+void unix_inflight(const struct cred *cred, struct file *fp);
+void unix_notinflight(const struct cred *cred, struct file *fp);
 void unix_gc(void);
 void wait_for_unix_gc(void);
 struct sock *unix_get_socket(struct file *filp);
diff --git a/include/net/scm.h b/include/net/scm.h
index 262532d111f51e..8bf7d496545bf8 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -21,6 +21,7 @@ struct scm_creds {
 struct scm_fp_list {
        short                   count;
        short                   max;
+       const struct cred       *cred;
        struct file             *fp[SCM_MAX_FD];
 };

diff --git a/net/core/scm.c b/net/core/scm.c
index 14596fb3717270..6b02b574e283f6 100644
--- a/net/core/scm.c
+++ b/net/core/scm.c

@@ -87,6 +87,7 @@ static int scm_fp_copy(struct cmsghdr *cmsg, structscm_fp_list **fplp)

                *fplp = fpl;
                fpl->count = 0;
                fpl->max = SCM_MAX_FD;
+               fpl->cred = NULL;
        }
        fpp = &fpl->fp[fpl->count];

@@ -107,6 +108,10 @@ static int scm_fp_copy(struct cmsghdr *cmsg, structscm_fp_list **fplp)

                *fpp++ = file;
                fpl->count++;
        }
+
+       if (fpl->cred)
+               put_cred(fpl->cred);
+       fpl->cred = get_current_cred();
        return num;
 }

@@ -119,6 +124,7 @@ void __scm_destroy(struct scm_cookie *scm)
                scm->fp = NULL;
                for (i=fpl->count-1; i>=0; i--)
                        fput(fpl->fp[i]);
+               put_cred(fpl->cred);
                kfree(fpl);
        }
 }
@@ -336,6 +342,7 @@ struct scm_fp_list *scm_fp_dup(struct scm_fp_list *fpl)
                for (i = 0; i < fpl->count; i++)
                        get_file(fpl->fp[i]);
                new_fpl->max = new_fpl->count;
+               new_fpl->cred = get_cred(fpl->cred);
        }
        return new_fpl;
 }
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 49d5093eb0553a..ba5058682419ba 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c

@@ -1496,7 +1496,7 @@ static void unix_detach_fds(struct scm_cookie*scm, struct sk_buff *skb)

        UNIXCB(skb).fp = NULL;

        for (i = scm->fp->count-1; i >= 0; i--)
-               unix_notinflight(scm->fp->fp[i]);
+               unix_notinflight(scm->fp->cred, scm->fp->fp[i]);
 }

 static void unix_destruct_scm(struct sk_buff *skb)

@@ -1561,7 +1561,7 @@ static int unix_attach_fds(struct scm_cookie *scm,struct sk_buff *skb)

                return -ENOMEM;

        for (i = scm->fp->count - 1; i >= 0; i--)
-               unix_inflight(scm->fp->fp[i]);
+               unix_inflight(scm->fp->cred, scm->fp->fp[i]);
        return max_level;
 }

diff --git a/net/unix/garbage.c b/net/unix/garbage.c
index 8fcdc2283af50c..30b03e7dddd547 100644
--- a/net/unix/garbage.c
+++ b/net/unix/garbage.c
@@ -116,7 +116,7 @@ struct sock *unix_get_socket(struct file *filp)
  * descriptor if it is for an AF_UNIX socket.
  */

-void unix_inflight(struct file *fp)
+void unix_inflight(const struct cred *cred, struct file *fp)
 {
        struct sock *s = unix_get_socket(fp);

@@ -133,11 +133,11 @@ void unix_inflight(struct file *fp)
                }
                unix_tot_inflight++;
        }
-       fp->f_cred->user->unix_inflight++;
+       cred->user->unix_inflight++;
        spin_unlock(&unix_gc_lock);
 }

-void unix_notinflight(struct file *fp)
+void unix_notinflight(const struct cred *cred, struct file *fp)
 {
        struct sock *s = unix_get_socket(fp);

@@ -152,7 +152,7 @@ void unix_notinflight(struct file *fp)
                        list_del_init(&u->link);
                unix_tot_inflight--;
        }
-       fp->f_cred->user->unix_inflight--;
+       cred->user->unix_inflight--;
        spin_unlock(&unix_gc_lock);
 }

Re: [PATCH v2] unix: properly account for FDs passed over unix sockets

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