-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 ============================================================================= FreeBSD-EN-09:05.null Errata Notice The FreeBSD Project Topic: No zero mapping feature Category: core Module: kern Announced: 2009-10-02 Credits: John Baldwin, Konstantin Belousov, Alan Cox, and Bjoern Zeeb Affects: All supported versions of FreeBSD. Corrected: 2009-10-02 18:09:56 UTC (RELENG_8, 8.0-RC2) 2009-10-02 18:09:56 UTC (RELENG_7, 7.2-STABLE) 2009-10-02 18:09:56 UTC (RELENG_7_2, 7.2-RELEASE-p4) 2009-10-02 18:09:56 UTC (RELENG_7_1, 7.1-RELEASE-p8) 2009-10-02 18:09:56 UTC (RELENG_6, 6.4-STABLE) 2009-10-02 18:09:56 UTC (RELENG_6_4, 6.4-RELEASE-p7) 2009-10-02 18:09:56 UTC (RELENG_6_3, 6.3-RELEASE-p13) For general information regarding FreeBSD Errata Notices and Security Advisories, including descriptions of the fields above, security branches, and the following sections, please visit . I. Background In the C programming language, address 0 (NULL) is used to represent unallocated memory. NULL pointer dereferences are a common class of C programming bug in which pointers are not properly checked for NULL before being used. Dereferencing a NULL pointer normally terminates execution, via a segmentation fault for user processes, or a page fault panic in the kernel. II. Problem Description On most architectures, the FreeBSD kernel splits the process virtual memory address space into two portions: user and kernel. This improves system call performance by avoiding a full address space switch when a process enters the kernel, and improves performance for kernel access to user memory. However, in this design, address 0 is part of the user-controlled portion of the virtual address space. If the kernel dereferences a NULL pointer due to a kernel bug, a malicious process that has mapped code or data at address 0 may be able to manipulate kernel behavior. For example, if a malicious user process maps code at address 0 and then triggers a kernel bug in which a NULL function pointer is invoked, the kernel may execute that code with kernel privilege rather than panicking. III. Impact This errata patch introduces a mitigation feature in which user mapping at address 0 is disallowed, limiting the attacker's ability to convert a kernel NULL pointer dereference into a privilege escalation attack. The feature is disabled by default in FreeBSD 7 and lower, and must be enabled by setting the sysctl(8) variable security.bsd.map_at_zero to 0. In FreeBSD 8 and later feature is enabled by default. While extremely rare, certain applications may rely on mapping memory at address 0. Careful testing is advised when enabling this feature when using virtual machines, emulation technologies, and older a.out format binaries. Changing the mentioned sysctl(8) variable only affects processes started after the sysctl(8) variable was set. Processes started before the sysctl(8) variable was changed will continue to run with the setting of the sysctl(8) variable which existed when the processes was started. Consequently, to ensure that the sysctl(8) variable affects all processes, a reboot is required with the sysctl(8) variable configured as mentioned below. IV. Workaround No workaround is available. V. Solution Perform one of the following: 1) Upgrade your system to 6-STABLE, 7-STABLE, or 8-RC, or to the RELENG_7_2, RELENG_7_1, RELENG_6_4, or RELENG_6_3 security branch dated after the correction date. Enable feature as mentioned below. 2) To patch your present system: The following patches have been verified to apply to FreeBSD 6.3, 6.4, 7.1, and 7.2 systems. a) Download the relevant patch from the location below, and verify the detached PGP signature using your PGP utility. [FreeBSD 7.x] # fetch http://security.FreeBSD.org/patches/EN-09:05/null.patch # fetch http://security.FreeBSD.org/patches/EN-09:05/null.patch.asc [FreeBSD 6.x] # fetch http://security.FreeBSD.org/patches/EN-09:05/null6.patch # fetch http://security.FreeBSD.org/patches/EN-09:05/null6.patch.asc NOTE WELL: The patch for FreeBSD 7.x can be used on FreeBSD 8, but does not enable the feature by default! b) Apply the patch. # cd /usr/src # patch < /path/to/patch c) Recompile your kernel as described in and reboot the system. To actually enable the feature in FreeBSD 6.x and 7.x, add the following to either /boot/loader.conf or /etc/sysctl.conf: security.bsd.map_at_zero="0" VI. Correction details The following list contains the revision numbers of each file that was corrected in FreeBSD. CVS: Branch Revision Path - ------------------------------------------------------------------------- RELENG_6 src/sys/kern/kern_exec.c 1.275.2.9 RELENG_6_4 src/UPDATING 1.416.2.40.2.11 src/sys/conf/newvers.sh 1.69.2.18.2.13 src/sys/kern/kern_exec.c 1.275.2.8.4.2 RELENG_6_3 src/UPDATING 1.416.2.37.2.18 src/sys/conf/newvers.sh 1.69.2.15.2.17 src/sys/kern/kern_exec.c 1.275.2.8.2.1 RELENG_7 src/sys/kern/kern_exec.c 1.308.2.11 RELENG_7_2 src/UPDATING 1.507.2.23.2.7 src/sys/conf/newvers.sh 1.72.2.11.2.8 src/sys/kern/kern_exec.c 1.308.2.8.2.2 RELENG_7_1 src/UPDATING 1.507.2.13.2.11 src/sys/conf/newvers.sh 1.72.2.9.2.12 src/sys/kern/kern_exec.c 1.308.2.6.2.2 RELENG_8 src/sys/kern/kern_exec.c 1.337.2.3 src/sys/kern/init_main.c 1.303.2.2 - ------------------------------------------------------------------------- Subversion: Branch/path Revision - ------------------------------------------------------------------------- stable/6/ r197715 releng/6.4/ r197715 releng/6.3/ r197715 stable/7/ r197715 releng/7.2/ r197715 releng/7.1/ r197715 stable/8/ r197714 - ------------------------------------------------------------------------- VII. References The latest revision of this advisory is available at http://security.FreeBSD.org/advisories/FreeBSD-EN-09:05.null.asc -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.10 (FreeBSD) iD8DBQFKxltpFdaIBMps37IRAoniAJ9ENWQ431doaje7gXrAfAov5l0FKwCdFRxh rTmlD1oew/hZTMBuFKM/LSI= =+ZZf -----END PGP SIGNATURE-----