-----BEGIN PGP SIGNED MESSAGE----- NetBSD Security Advisory 2001-008 ================================= Topic: Processes can gain "Supervisor" privileges on sh3. Version: All versions of NetBSD, on the sh3 platform only. Severity: Local users may execute code with supervisor privileges. Fixed: NetBSD-current: May 16, 2001 NetBSD-1.5 branch: May 27, 2001 Abstract ======== Missing validation of user-supplied arguments to a system call can allow user applications on the sh3 platform to execute code with supervisor privileges, bypassing normal system protections. This problem is only present on the sh3 platform, which includes the dreamcast, evbsh3, hpcsh and mmeye ports. Technical Details ================= The Hitachi Super-H architecture includes the Status Register, a processor register which provides status information such as the true/false condition bit, and which also controls properties among which is the processor mode (either user mode or privileged mode). The only component of NetBSD executing in privileged mode is the kernel; user processes are restricted to user mode and thus cannot execute privileged processor instructions or access related resources. As a part of the protection model, the contents of the Status Register can be modified only when executing in privileged mode. While reviewing code in NetBSD, it was discovered that the NetBSD "sigreturn" system call did not do validation of user-supplied Status Register contents; it was also discovered that the "process_write_regs" kernel routine, which is used by the procfs and ptrace(2) facilities, did not do appropriate validation of user-supplied Status Register contents; in short, it's possible for a user process to construct and pass an execution context which makes it resume in privileged mode. Solutions and Workarounds ========================= Systems running NetBSD/evbsh3, NetBSD/dreamcast, NetBSD/hpcsh or NetBSD/mmeye are vulnerable. Although the NetBSD Project has not distributed binary releases for these ports yet, binary snapshots of development versions are known to be in use. Systems running NetBSD-current dated from before May 16, 2001 should be upgraded to NetBSD-current dated May 16, 2001 or later. Systems running NetBSD-release-1-5 dated from before May 27, 2001 should be upgraded to NetBSD-release-1-5 dated May 27, 2001 or later. The following patches to /sys/arch/sh3/include/psl.h, /sys/arch/sh3/sh3/compat_13_machdep.c and /sys/arch/sh3/sh3/sh3_machdep.c should be applied before building a new kernel. This patch can be applied (with offset differences) to NetBSD-1.5 or NetBSD-current kernel sources using the patch(1) command. Index: include/psl.h =================================================================== RCS file: /cvsroot/syssrc/sys/arch/sh3/include/psl.h,v retrieving revision 1.1 retrieving revision 1.2 diff -u -r1.1 -r1.2 - --- include/psl.h 1999/09/13 10:31:21 1.1 +++ include/psl.h 2001/05/16 12:42:38 1.2 @@ -57,8 +57,8 @@ #define PSL_MBO 0x00000000 /* must be one bits */ #define PSL_MBZ 0x8ffffc0c /* must be zero bits */ - -#define PSL_USERSET 0 - -#define PSL_USERSTATIC (PSL_BL|PSL_RB|PSL_MD|PSL_IMASK) +#define PSL_USERSET 0 +#define PSL_USERSTATIC (PSL_BL|PSL_RB|PSL_MD|PSL_IMASK|PSL_MBO|PSL_MBZ) #ifdef _KERNEL #include Index: sh3/compat_13_machdep.c =================================================================== RCS file: /cvsroot/syssrc/sys/arch/sh3/sh3/compat_13_machdep.c,v retrieving revision 1.2 retrieving revision 1.3 diff -u -r1.2 -r1.3 - --- sh3/compat_13_machdep.c 2000/12/22 22:58:55 1.2 +++ sh3/compat_13_machdep.c 2001/05/16 12:42:38 1.3 @@ -71,16 +71,9 @@ /* Restore register context. */ tf = p->p_md.md_regs; - - /* - - * Check for security violations. If we're returning to - - * protected mode, the CPU will validate the segment registers - - * automatically and generate a trap on violations. We handle - - * the trap, rather than doing all of the checking here. - - */ - -#ifdef TODO + /* Check for security violations. */ if (((context.sc_ssr ^ tf->tf_ssr) & PSL_USERSTATIC) != 0) return (EINVAL); - -#endif tf->tf_ssr = context.sc_ssr; Index: sh3/sh3_machdep.c =================================================================== RCS file: /cvsroot/syssrc/sys/arch/sh3/sh3/sh3_machdep.c,v retrieving revision 1.12 retrieving revision 1.13 diff -u -r1.12 -r1.13 - --- sh3/sh3_machdep.c 2001/04/24 04:31:09 1.12 +++ sh3/sh3_machdep.c 2001/05/16 12:42:38 1.13 @@ -350,21 +350,13 @@ /* Restore signal context. */ tf = p->p_md.md_regs; - - { - - /* - - * Check for security violations. If we're returning to - - * protected mode, the CPU will validate the segment registers - - * automatically and generate a trap on violations. We handle - - * the trap, rather than doing all of the checking here. - - */ - -#ifdef TODO - - if (((context.sc_ssr ^ tf->tf_ssr) & PSL_USERSTATIC) != 0) { - - return (EINVAL); - - } - -#endif - - tf->tf_ssr = context.sc_ssr; - - } + /* Check for security violations. */ + if (((context.sc_ssr ^ tf->tf_ssr) & PSL_USERSTATIC) != 0) + return (EINVAL); + + tf->tf_ssr = context.sc_ssr; + tf->tf_r0 = context.sc_r0; tf->tf_r1 = context.sc_r1; tf->tf_r2 = context.sc_r2; Thanks To ========= Klaus Klein for detecting and resolving this problem. Revision History ================ 2001-05-29 Initial Release More Information ================ An up-to-date PGP signed copy of this release will be maintained at ftp://ftp.netbsd.org/pub/NetBSD/security/advisories/NetBSD-SA2001-008.txt.asc Information about NetBSD and NetBSD security can be found at http://www.NetBSD.ORG/ and http://www.NetBSD.ORG/Security/. Copyright 2001, The NetBSD Foundation, Inc. All Rights Reserved. $NetBSD: NetBSD-SA2001-008.txt,v 1.6 2001/05/29 05:58:42 lukem Exp $ -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.0.5 (NetBSD) Comment: For info see http://www.gnupg.org iQCVAwUBOxM/ij5Ru2/4N2IFAQFdxwP+J90AP+kzRCmaSrLQ1gwV1eanLLiovUEo p/nunjBUFJRWo+05uOIBjarf7lV94AVFUkqhZnf4iPoQdmBa/VZjAUBl19XA8pFl CLNADN7rH6w+yUPlugfNx9k8n5nn3w+vJFjuC+kyyOP5v5Ajld0QgHpvnqbgi/re iox48ReU/Yw= =YkBv -----END PGP SIGNATURE-----