[Bug 271910] bad TY_ENDDISC option can cause ppp to write beyond end of buffer

From: <bugzilla-noreply_at_freebsd.org>
Date: Thu, 08 Jun 2023 15:41:47 UTC

            Bug ID: 271910
           Summary: bad TY_ENDDISC option can cause ppp to write beyond
                    end of buffer
           Product: Base System
           Version: CURRENT
          Hardware: Any
                OS: Any
            Status: New
          Severity: Affects Some People
          Priority: ---
         Component: bin
          Assignee: bugs@FreeBSD.org
          Reporter: rtm@lcs.mit.edu
 Attachment #242687 text/plain
         mime type:

Created attachment 242687
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=242687&action=edit
send ppp an HDLC frame that causes it to overrun a buffer

This HDLC LCP Configure-Request frame:

  7e c0 21 01 01 00 06 13 02 6b 94 7e 

causes ppp/lcp.c's LcpDecodeConfig() to call memcpy() with length =
0xffffffffffffffff here:

    case TY_ENDDISC:
          memcpy(p->dl->peer.enddisc.address, opt->data + 1, opt->hdr.len - 3);

opt->hdr.len is the 02 in the frame above, so len - 3 is -1. opt->data
points into the input HDLC frame.

The "- 3" here can also lead to trouble:

      log_Printf(LogLCP, "%s %s\n", request,
                 mp_Enddisc(opt->data[0], opt->data + 1, opt->hdr.len - 3));

Here's a parsing of the frame:

    0xc0, 0x21, // LCP
    0x01, 0x01, // code=Configure-Request, ID=1
    0x00, 0x06, // length
    0x13, 0x02, // 0x13=Multilink-Endpoint-Discriminator, 2=length
    0x6b, 0x94, // HDLC checksum

Here's a backtrace from the attached demo program ppp3c.c:

#0  memcpy () at /usr/src/lib/libc/amd64/string/memmove.S:306
#1  0x00002433db2ed225 in LcpDecodeConfig (fp=0x243c0533f7e8, 
    cp=0x243c05363165 "\002", end=0x243c05363167 "", mode_type=0, 
    dec=0x243bfc109310) at /usr/src/usr.sbin/ppp/lcp.c:1216
#2  0x00002433db2e08f6 in FsmRecvConfigReq (fp=0x243c0533f7e8, 
    lhp=0x243bfc1094a8, bp=0x243c05363100) at /usr/src/usr.sbin/ppp/fsm.c:496
#3  0x00002433db2dffd0 in fsm_Input (fp=fp@entry=0x243c0533f7e8, 
    bp=bp@entry=0x243c05363100) at /usr/src/usr.sbin/ppp/fsm.c:1096
#4  0x00002433db2ebd0c in lcp_Input (bundle=<optimized out>, 
    l=<optimized out>, bp=0x243c05363100) at /usr/src/usr.sbin/ppp/lcp.c:1305
#5  0x00002433db2ee16c in Despatch (
    bundle=0x2433db3144b0 <bundle_Create.bundle>, l=0x243c0533f600, 
    bp=0x243c05363100, proto=<optimized out>)
    at /usr/src/usr.sbin/ppp/link.c:381
#6  link_PullPacket (l=0x243c0533f600, buf=<optimized out>, 
    len=<optimized out>, b=0x2433db3144b0 <bundle_Create.bundle>)
    at /usr/src/usr.sbin/ppp/link.c:323
#7  0x00002433db2c7e25 in bundle_DescriptorRead (d=<optimized out>, 
    bundle=0x2433db3144b0 <bundle_Create.bundle>, fdset=0x243c0536a140)
    at /usr/src/usr.sbin/ppp/bundle.c:546
#8  0x00002433db2f1704 in DoLoop (bundle=0x2433db3144b0 <bundle_Create.bundle>)
    at /usr/src/usr.sbin/ppp/main.c:661
#9  main (argc=3, argv=<optimized out>) at /usr/src/usr.sbin/ppp/main.c:535

(gdb) print/x *opt
$2 = {hdr = {id = 0x13, len = 0x2}, data = {0x0, 0x6, 0x13, 0x2, 0x13, 0x2, 
    0x6b, 0x94, 0xa5 <repeats 42 times>}}

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