kern/76410: Null pointer dereference in net/bpf.c

[Eugene Stark]

>Number:         76410
>Category:       kern
>Synopsis:       Null pointer dereference in net/bpf.c
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    freebsd-bugs
>State:          open
>Quarter:        
>Keywords:       
>Date-Required:
>Class:          sw-bug
>Submitter-Id:   current-users
>Arrival-Date:   Tue Jan 18 14:40:27 GMT 2005
>Closed-Date:
>Last-Modified:
>Originator:     Eugene Stark
>Release:        4.10-RELEASE
>Organization:
SUNY at Stony Brook
>Environment:
FreeBSD laptop.starkhome.cs.sunysb.edu 4.10-RELEASE-p4 FreeBSD 4.10-RELEASE-p4 #1: Mon Jan 17 17:51:38 EST 2005     gene at laptop.starkhome.cs.sunysb.edu:/A/src/sys/compile/LAPTOP  i386

>Description:
      After applying the patch I proposed in PR-76207 for a null pointer
dereference in pci/if_xl.c, I found that it exposed a different bug in
net/bpf.c.  Specifically, if a call is made to bpfdetach() when no
devices have been successfully attached and consequently bpf_iflist
is NULL, then it is possible to exit the first loop in bpfdetach() with
bp == NULL, thus causing a NULL pointer dereference in the next statement.

It appears from the comments that the author wished to defend against
the case in which an attempt was made to detach a device that had never
successfully been attached.  However, the code does not properly handle
the situation in which this occurs when there are no other devices that
had been attached.
>How-To-Repeat:
      This occurred after applying the patch I suggest to pci/if_xl.c
in PR-76207.  Although that patch avoided a crash in the xl driver proper,
the next time I saw the error it ended up taking a null pointer dereference in bpfdetach().  So to repeat the problem, one would have to
attempt to bpfdetach() a device that has not been bpfattach()ed, in a
situation in which no other devices are attached to BPF.  The xl driver
will expose this problem under certain conditions.
>Fix:
      I have applied the following patch to net/bpf.c:

*** bpf.c.orig	Wed Dec  1 08:14:54 2004
--- bpf.c	Mon Jan 17 17:51:12 2005
***************
*** 1440,1446 ****
  	}
  
  	/* Interface wasn't attached */
! 	if (bp->bif_ifp == NULL) {
  		splx(s);
  		printf("bpfdetach: %s%d was not attached\n", ifp->if_name,
  		    ifp->if_unit);
--- 1440,1446 ----
  	}
  
  	/* Interface wasn't attached */
! 	if (bp == NULL || bp->bif_ifp == NULL) {
  		splx(s);
  		printf("bpfdetach: %s%d was not attached\n", ifp->if_name,
  		    ifp->if_unit);

>Release-Note:
>Audit-Trail:
>Unformatted: