-----BEGIN PGP SIGNED MESSAGE----- ============================================================================= FreeBSD-SA-02:35.ffs Security Advisory The FreeBSD Project Topic: local users may read and write arbitrary blocks on an FFS filesystem Category: core Module: kernel Announced: 2002-08-05 Credits: Matt Dillon , Ian Dowse , Tor Egge Affects: All releases of FreeBSD up to and including 4.6.1-RELEASE-p4 4.6-STABLE prior to the correction date Corrected: 2002-06-23 22:34:52 UTC (RELENG_4) 2002-07-31 17:55:22 UTC (RELENG_4_6) 2002-07-31 17:55:11 UTC (RELENG_4_5) 2002-07-31 17:54:57 UTC (RELENG_4_4) FreeBSD only: YES I. Background The Berkeley Fast File System (FFS) is the default filesystem used by FreeBSD. II. Problem Description A bug in the calculation of the maximum permitted FFS file size allows users to create files that are larger than FreeBSD's virtual memory system can handle. The integer overflows that result when such files are accessed may map filesystem metadata into the user file, permitting access to arbitrary filesystem blocks. The bug is encountered only on FFS filesystems with a block size of 16k or greater on the i386 architecture, or 32k or greater on the alpha architecture. Also, the filesystem must have at least 6 blocks of free space, and the user must have write access to at least one file in the filesystem. The default FreeBSD FFS filesystem block size was changed from 8k to 16k on all architectures just before 4.5-RELEASE. III. Impact Local attackers may cause a denial of service by simply corrupting the filesystem. A local attacker may also be able to read and write arbitrary files on local filesystems, allowing them to gain superuser privileges. FFS filesystems with a block size less than 16k (on the i386 architecture) or 32k (on the alpha architecture), such as those created using the default FFS filesystem block size prior to 4.5-RELEASE, are not vulnerable. The following command can be used to determine the block size used on a given filesystem: # dumpfs /some/filesystem | grep '^bsize' IV. Workaround On filesystems with 16k blocks, the bug cannot be exploited when a process has a file size resource limit (RLIMIT_FSIZE) of 63 MB or less. This can be most easily accomplished by modifying /etc/login.conf so that the appropriate login classes (typically `default') contain a field entry such as the following: :filesize=63m:\ After editing /etc/login.conf, the corresponding capability database must be rebuilt with the following command: # cap_mkdb /etc/login.conf Please see login.conf(5) for details. Note that this will not affect currently running processes, nor new processes started by users who are already logged in. The corresponding limit appropriate for filesystems with 32k or larger blocks is not known at this time, and might be smaller or larger than 63 MB. It is the responsibility of applications such as `login' and `sshd' to read and honor login.conf. Be aware that 3rd party applications that provide login functionality may or may not honor login.conf. V. Solution 1) Upgrade your vulnerable system to 4.6-STABLE; or to any of the RELENG_4_6 (4.6.1-RELEASE-p5), RELENG_4_5 (4.5-RELEASE-p14), or RELENG_4_4 (4.4-RELEASE-p21) security branches dated after the respective correction dates. 2) To patch your present system: a) Download the relevant patch from the location below, and verify the detached PGP signature using your PGP utility. The following patch has been tested to apply to all FreeBSD 4.x releases. # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:35/ffs.patch # fetch ftp://ftp.FreeBSD.org/pub/FreeBSD/CERT/patches/SA-02:35/ffs.patch.asc b) Recompile your kernel as described in http://www.freebsd.org/handbook/kernelconfig.html and reboot the system. VI. Correction details The following list contains the revision numbers of each file that was corrected in FreeBSD. Path Revision Branch - ------------------------------------------------------------------------- sys/ufs/ffs/ffs_vfsops.c RELENG_4 1.117.2.10 RELENG_4_6 1.117.2.9.2.1 RELENG_4_5 1.117.2.7.2.1 RELENG_4_4 1.117.2.3.2.1 sys/conf/newvers.sh RELENG_4_6 1.44.2.23.2.10 RELENG_4_5 1.44.2.20.2.15 RELENG_4_4 1.44.2.17.2.20 - ------------------------------------------------------------------------- -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.7 (FreeBSD) iQCVAwUBPU8ML1UuHi5z0oilAQGkWQP/fJvzkrl2ptG87Qn2pIa24kLyax5WCnca uPhq9JxIhXIxAqdIZcrEbbTyeRo/ygtsLzxDKOP0G+A2VxilVL9Ld3a32OSM+nzM uiSnVHTIxPtmkyZnwdmyTcrBki290p/W3LnZhxzfAt1vdIRD+ibOkBXNAaXFxDRz T1UzIarVqgM= =wq5s -----END PGP SIGNATURE-----