From security-advisories@freebsd.org Mon Aug 28 18:07:04 2000 From: FreeBSD Security Advisories To: BUGTRAQ@SECURITYFOCUS.COM Date: Mon, 28 Aug 2000 12:02:33 -0700 Subject: [BUGTRAQ] FreeBSD Security Advisory: FreeBSD-SA-00:41.elf -----BEGIN PGP SIGNED MESSAGE----- ============================================================================= FreeBSD-SA-00:41 Security Advisory FreeBSD, Inc. Topic: Malformed ELF images can cause a system hang Category: core Module: kernel Announced: 2000-08-28 Credits: Adam McDougall Affects: FreeBSD 3.x, 4.x and 5.x prior to the correction date Corrected: 2000-07-25 (FreeBSD 5.0-CURRENT) 2000-07-23 (FreeBSD 4.0-STABLE) FreeBSD only: Yes I. Background The ELF binary format is used for binary executable programs on modern versions of FreeBSD. II. Problem Description The ELF image activator did not perform sufficient sanity checks on the ELF image header, and when confronted with an invalid or truncated header it suffered a sign overflow bug which caused the CPU to enter into a very long loop in the kernel. The result of this is that the system will appear to lock up for an extended period of time before control returns. This bug can be exploited by unprivileged local users. This vulnerability is not present in FreeBSD 4.1-RELEASE, although 3.5-RELEASE and 3.5.1-RELEASE are vulnerable. III. Impact Local users can cause the system to lock up for an extended period of time (15 minutes or more, depending on CPU speed), during which time the system is completely unresponsive to local and remote users. IV. Workaround None available. V. Solution One of the following: 1) Upgrade your vulnerable FreeBSD system to 4.1-RELEASE, 4.1-STABLE or 5.0-CURRENT after the respective correction dates. FreeBSD 3.5-STABLE has not yet been fixed due to logistical difficulties (and the patch below does not apply cleanly). Consider upgrading to 4.1-RELEASE if this is a concern - this advisory will be reissued once the patch has been applied to the 3.x branch. 2) Apply the patch below and recompile your kernel. Either save this advisory to a file, or download the patch and detached PGP signature from the following locations, and verify the signature using your PGP utility. ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:41/elf.patch ftp://ftp.freebsd.org/pub/FreeBSD/CERT/patches/SA-00:41/elf.patch.asc # cd /usr/src/sys/kern # patch -p < /path/to/patch_or_advisory [ Recompile your kernel as described in http://www.freebsd.org/handbook/kernelconfig.html and reboot the system ] --- imgact_elf.c 2000/04/30 18:51:39 1.75 +++ imgact_elf.c 2000/07/23 22:19:49 1.78 @@ -190,6 +190,21 @@ object = vp->v_object; error = 0; + /* + * It's necessary to fail if the filsz + offset taken from the + * header is greater than the actual file pager object's size. + * If we were to allow this, then the vm_map_find() below would + * walk right off the end of the file object and into the ether. + * + * While I'm here, might as well check for something else that + * is invalid: filsz cannot be greater than memsz. + */ + if ((off_t)filsz + offset > object->un_pager.vnp.vnp_size || + filsz > memsz) { + uprintf("elf_load_section: truncated ELF file\n"); + return (ENOEXEC); + } + map_addr = trunc_page((vm_offset_t)vmaddr); file_addr = trunc_page(offset); @@ -341,6 +356,12 @@ } error = exec_map_first_page(imgp); + /* + * Also make certain that the interpreter stays the same, so set + * its VTEXT flag, too. + */ + if (error == 0) + nd.ni_vp->v_flag |= VTEXT; VOP_UNLOCK(nd.ni_vp, 0, p); if (error) goto fail; @@ -449,6 +470,17 @@ /* * From this point on, we may have resources that need to be freed. */ + + /* + * Yeah, I'm paranoid. There is every reason in the world to get + * VTEXT now since from here on out, there are places we can have + * a context switch. Better safe than sorry; I really don't want + * the file to change while it's being loaded. + */ + simple_lock(&imgp->vp->v_interlock); + imgp->vp->v_flag |= VTEXT; + simple_unlock(&imgp->vp->v_interlock); + if ((error = exec_extract_strings(imgp)) != 0) goto fail; @@ -610,9 +642,6 @@ imgp->auxargs = elf_auxargs; imgp->interpreted = 0; - /* don't allow modifying the file while we run it */ - imgp->vp->v_flag |= VTEXT; - fail: return error; } -----BEGIN PGP SIGNATURE----- Version: 2.6.2 iQCVAwUBOaq1hlUuHi5z0oilAQGpvgQAoaeqjoU1QppgQ+yXF7KOL6EfTQ9mrdEe zKQ6vU//hc1ejKx9C4zmQybflQIpkHS2TMNAfXuvFG74hvETwa8cpVqolJU29CCf FKlGTCAGCSzosWrndBuvakKqjeVvvQR4JydVhkO04neVEfbUXkich/2PT+3h3dKW GuW3coG8nYE= =2w2A -----END PGP SIGNATURE-----