lib/43825: p->cc can go negative in libpcap

[guy%alum.mit.edu@localhost]

>Number:         43825
>Category:       lib
>Synopsis:       p->cc can go negative in libpcap
>Confidential:   no
>Severity:       non-critical
>Priority:       low
>Responsible:    lib-bug-people
>State:          open
>Class:          sw-bug
>Submitter-Id:   net
>Arrival-Date:   Wed Sep 01 08:15:01 +0000 2010
>Originator:     Guy Harris
>Release:        Top of tree
>Organization:
>Environment:
>Description:
In pcap_read_bpf(), ep is set based on the return value of read(), but read() 
from a BPF device doesn't necessarily return a value that's a multiple of the 
alignment value for BPF_WORDALIGN(). However, whenever we increment bp, we 
round up the increment value by a value rounded up by BPF_WORDALIGN(), so we 
could increment bp past ep after processing the last packet in the buffer.

(The patch also fixes the case where it returns due to a pcap_breakloop() call, 
so it doesn't re-process packets.)
>How-To-Repeat:
Run a program that opens a capture device with a timeout of 0, in a loop, calls 
pcap_dispatch() with a cnt argument of 1, and reports when it returns a value 
of 0. The timeout of 0 means that the read() that libpcap does shouldn't return 
until there's packet data, so a timeout won't cause pcap_dispatch() to return 
0. Do a large amount of network data transfer, to fill up the BPF bucket; 
notice that, on occasion, the program will report that pcap_dispatch() returns 
0.
>Fix:

--- pcap-bpf.c.dist     2010-09-01 01:09:26.000000000 -0700
+++ pcap-bpf.c  2010-09-01 01:10:50.000000000 -0700
@@ -248,14 +248,28 @@
                 * return the number of packets we've processed so far.
                 */
                if (p->break_loop) {
+                       p->bp = bp;
+                       p->cc = ep - bp;
+                       /*
+                        * ep is set based on the return value of read(),
+                        * but read() from a BPF device doesn't necessarily
+                        * return a value that's a multiple of the alignment
+                        * value for BPF_WORDALIGN().  However, whenever we
+                        * increment bp, we round up the increment value by
+                        * a value rounded up by BPF_WORDALIGN(), so we
+                        * could increment bp past ep after processing the
+                        * last packet in the buffer.
+                        *
+                        * We treat ep < bp as an indication that this
+                        * happened, and just set p->cc to 0.
+                        */
+                       if (p->cc < 0)
+                               p->cc = 0;
                        if (n == 0) {
                                p->break_loop = 0;
                                return (-2);
-                       } else {
-                               p->bp = bp;
-                               p->cc = ep - bp;
+                       } else
                                return (n);
-                       }
                }
 
                caplen = bhp->bh_caplen;
@@ -306,6 +320,11 @@
                        if (++n >= cnt && cnt > 0) {
                                p->bp = bp;
                                p->cc = ep - bp;
+                               /*
+                                * See comment above about p->cc < 0.
+                                */
+                               if (p->cc < 0)
+                                       p->cc = 0;
                                return (n);
                        }
                } else {