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Pine Digital Security Advisory
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Advisory ID       : PINE-CERT-20030901
Authors           : Joost Pol
Vendor Informed   : 2003-23-09
Issue date        : 2003-01-10
Application       : kernel / fhold
Version(s)        : All 4.X versions since 4.3
Platforms         : FreeBSD
Availability      : http://www.pine.nl/press/pine-cert-20030901.txt
References        : http://www.pine.nl/press/pine-cert-20030101.txt
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Synopsis

        While performing an audit for a customer, Pine Digital Security
        encountered an integer overflow condition which could lead to
        a denial of service attack or privilege escalation.

        This vulnerability is similar to the fpathconf vulnerability:

         http://www.pine.nl/press/pine-cert-20030101.txt

Versions

        FreeBSD RELENG_4.X: All versions >= 4.3 are vulnerable.

        FreeBSD RELENG_5.X: Not vulnerable.

Impact

        Serious.

        Local users can exploit this vulnerability to cause a system
        panic (denial of service) or potentially escalate their
        privileges.

Description

        Inside the readv(2) system call we spotted a condition where a
        call to fdrop() is missing. When issueing a readv(2) call with
        an oversized iovcnt parameter the function will return without
        releasing the outstanding file reference.

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

        Quoting PINE-CERT-20030101:

         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.

Exploitability

	On most vulnerable systems this vulnerability can be reliably
        exploited by local users to escalate their privileges or cause
        an effective denial of service attack.

        Refer to PINE-CERT-20030101 for more information.

Disclaimer

        Pine Digital Security does not release exploits.

Patches

        The FreeBSD Project has updated their CVS repositories.

References

        FreeBSD-SA-03:16.filedesc

        http://www.pine.nl/press/pine-cert-20030901.txt

        file://usr/include/sys/file.h
        file://usr/src/sys/kern/kern_descrip.c
        file://usr/src/sys/kern/vfs_syscalls.c
        file://usr/src/sys/kern/sys_generic.c

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