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Pine Digital Security Advisory 
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Advisory ID       : PINE-CERT-20030101
Authors           : Joost Pol
Vendor Informed   : 2002-12-30 
Issue date        : 2003-01-06 
Application       : Kernel 
Version(s)        : Various
Platforms         : FreeBSD 
Availability      : http://www.pine.nl/press/pine-cert-20030101.txt 
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Synopsis

  While performing an audit for a customer, Pine Digital Security 
  found an integer overflow in the FreeBSD kernel.

Versions

  FreeBSD:

   RELENG_4   (aka -STABLE)   : not vulnerable after 20021111
   RELENG_5_0 (aka -CURRENT)      : vulnerable in lseek(2) (199)
   All current -RELEASE versions  : vulnerable in fpathconf(2) (192)

        OpenBSD/NetBSD: not vulnerable

Impact

  Serious.

  This integer overflow could cause a system panic, resulting in a 
  Denial-of-Service (DoS). Also, this could be used to escalate 
  privileges or cause the system to disclose (sensitive) files.

Description

  Inside the FreeBSD kernel each file (socket, device or regular 
  file) opened is represented by a file structure (sys/file.h).

        Amongst other members this structure holds a reference counter 
  (int f_count). This reference counter is increased by the fhold() 
  function and decreased by the fdrop() function. (both in sys/file.h)

  For example, when a file is open(2)ed or dup(2)ed the reference 
  counter is increased and when the file is close(2)ed again the
  reference counter is decreased. Once the reference counter reaches 
  zero, the file structure itself is deallocated.

  Most system calls which perform (blocking) operations on a file
  will issue a fhold() call to prevent the file from being closed
  in the middle of an operation. Once the operation is finished the 
  (extra) reference will be released again by issuing a fdrop() call.
  
  Inside the fpathconf(2) (192) system call we spotted a condition
  where a call to fdrop() is missing. When issueing a fpathconf(2)
  call on a socket it will return with an error condition but it 
  will not release the extra file reference.

       Due to the missing fdrop() call inside the fpathconf(2) system
        call is it possible to overflow the reference counter of the
  file structure (int f_count).

  FreeBSD -CURRENT suffers from the same problem in another syscall.

Exploitability

  Causing a system panic and privilege escalation have been confirmed.
  
  Exploitation of this bug will take time, depending on machine 
  speed and system limits this could vary between hours and days. 

  1. System Panic
  
  A system panic can be caused by issuing around 2^31 calls to 
  fpathconf(2) with a filedescriptor which references a socket. 
  The reference counter (int f_count) will wrap to a negative
  value and this will cause a panic in close(2).
  
  2. Privilege Escalation
  
  It is also possible (although more difficult) to gain root access
  using this bug. One would open(2) a socket and dup(2) it. One would 
  then issue around 2^32 - 1 calls to fpathconf(2) causing the reference 
  counter to wrap to 1. 

  After closing the original filedescriptor the file structure will 
  be deallocated. At this point the dup(2)ed file descriptor is still 
  hanging around with a file structure pointing to unallocated memory. 
 
  The final step, which is left as an excercise for the reader, is 
  to have a sensitive file (like /etc/skeykeys) opened and 
  allocated at the previously freed location. 

  Once this happens the dup(2)ed file descriptor is still hanging 
  around providing access to this file. This could result in the 
  escalation of user privileges. 

Disclaimer

  Pine Digital Security does not release exploits.

  Although the missing fdrop() call in fpathconf(2) was noticed
  before by Nakamura Takayuki <tak@st.rim.or.jp> its impact
  was severely underestimated.

Patches

  FreeBSD CVS should be updated.

References 

  http://www.pine.nl/press/pine-cert-20030101.txt
  file://usr/include/sys/file.h
  file://usr/src/sys/kern/kern_descrip.c
  file://usr/src/sys/kern/vfs_syscalls.c
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