full-disclosure - PDS: Integer overflow in FreeBSD kernel

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From: joost at pine.nl (Joost Pol)
Subject: PDS: Integer overflow in FreeBSD kernel

See attached advisory,

Regards,

Joost Pol
-- 
Service Provider :: Pine Digital Security :: tel XX-XXXXXXXX :: 070-311 10 10 
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Pine Digital Security Advisory 
- -----------------------------------------------------------------------------
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 
- -----------------------------------------------------------------------------

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@...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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