Joost Pol
06/01/03, 18:23
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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@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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--GvXjxJ+pjyke8COw--
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
See attached advisory,
Regards,
Joost Pol
--
Service Provider :: Pine Digital Security :: tel XX-XXXXXXXX :: 070-311 10 10
:: PGP id 0xF360BB93 fpr ABED 303C 0120 E057 633A 1CB1 D236 C82A F360 BB93 ::
--GvXjxJ+pjyke8COw
Content-Type: text/plain; charset=us-ascii
Content-Disposition: attachment; filename="pine-cert-20030101.txt.asc"
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
- -----------------------------------------------------------------------------
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@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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