CVE-2018-8897

A statement in the System Programming Guide of the Intel 64 and IA-32 Architectures Software Developers Manual (SDM) was mishandled in the development of some or all operating-system kernels, resulting in unexpected behavior for #DB exceptions that are deferred by MOV SS or POP SS, as demonstrated by (for example) privilege escalation in Windows, macOS, some Xen configurations, or FreeBSD, or a Linux kernel crash. The MOV to SS and POP SS instructions inhibit interrupts (including NMIs), data breakpoints, and single step trap exceptions until the instruction boundary following the next instruction (SDM Vol. 3A; section 6.8.3). (The inhibited data breakpoints are those on memory accessed by the MOV to SS or POP to SS instruction itself.) Note that debug exceptions are not inhibited by the interrupt enable (EFLAGS.IF) system flag (SDM Vol. 3A; section 2.3). If the instruction following the MOV to SS or POP to SS instruction is an instruction like SYSCALL, SYSENTER, INT 3, etc. that transfers control to the operating system at CPL < 3, the debug exception is delivered after the transfer to CPL < 3 is complete. OS kernels may not expect this order of events and may therefore experience unexpected behavior when it occurs.

Weakness

The product contains a code sequence that can run concurrently with other code, and the code sequence requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence that is operating concurrently.

Affected Software

Extended Description

This can have security implications when the expected synchronization is in security-critical code, such as recording whether a user is authenticated or modifying important state information that should not be influenced by an outsider. A race condition occurs within concurrent environments, and is effectively a property of a code sequence. Depending on the context, a code sequence may be in the form of a function call, a small number of instructions, a series of program invocations, etc. A race condition violates these properties, which are closely related:

A race condition exists when an “interfering code sequence” can still access the shared resource, violating exclusivity. Programmers may assume that certain code sequences execute too quickly to be affected by an interfering code sequence; when they are not, this violates atomicity. For example, the single “x++” statement may appear atomic at the code layer, but it is actually non-atomic at the instruction layer, since it involves a read (the original value of x), followed by a computation (x+1), followed by a write (save the result to x). The interfering code sequence could be “trusted” or “untrusted.” A trusted interfering code sequence occurs within the product; it cannot be modified by the attacker, and it can only be invoked indirectly. An untrusted interfering code sequence can be authored directly by the attacker, and typically it is external to the vulnerable product.

Potential Mitigations

• Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring.
• Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).

Related Attack Patterns

• https://cwe.mitre.org/data/definitions/26.html
• https://cwe.mitre.org/data/definitions/29.html

References

• http://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/commit/?id=d8ba61ba58c88d5207c1ba2f7d9a2280e7d03be9
• http://openwall.com/lists/oss-security/2018/05/08/1
• http://openwall.com/lists/oss-security/2018/05/08/4
• http://www.huawei.com/en/psirt/security-advisories/huawei-sa-20190921-01-debug-en
• http://www.securityfocus.com/bid/104071
• http://www.securitytracker.com/id/1040744
• http://www.securitytracker.com/id/1040849
• http://www.securitytracker.com/id/1040861
• http://www.securitytracker.com/id/1040866
• http://www.securitytracker.com/id/1040882
• https://access.redhat.com/errata/RHSA-2018:1318
• https://access.redhat.com/errata/RHSA-2018:1319
• https://access.redhat.com/errata/RHSA-2018:1345
• https://access.redhat.com/errata/RHSA-2018:1346
• https://access.redhat.com/errata/RHSA-2018:1347
• https://access.redhat.com/errata/RHSA-2018:1348
• https://access.redhat.com/errata/RHSA-2018:1349
• https://access.redhat.com/errata/RHSA-2018:1350
• https://access.redhat.com/errata/RHSA-2018:1351
• https://access.redhat.com/errata/RHSA-2018:1352
• https://access.redhat.com/errata/RHSA-2018:1353
• https://access.redhat.com/errata/RHSA-2018:1354
• https://access.redhat.com/errata/RHSA-2018:1355
• https://access.redhat.com/errata/RHSA-2018:1524
• https://bugzilla.redhat.com/show_bug.cgi?id=1567074
• https://github.com/can1357/CVE-2018-8897/
• https://github.com/torvalds/linux/commit/d8ba61ba58c88d5207c1ba2f7d9a2280e7d03be9
• https://help.ecostruxureit.com/display/public/UADCE725/Security+fixes+in+StruxureWare+Data+Center+Expert+v7.6.0
• https://lists.debian.org/debian-lts-announce/2018/05/msg00015.html
• https://lists.debian.org/debian-lts-announce/2018/06/msg00000.html
• https://lists.debian.org/debian-lts-announce/2018/11/msg00013.html
• https://patchwork.kernel.org/patch/10386677/
• https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2018-8897
• https://security.netapp.com/advisory/ntap-20180927-0002/
• https://support.apple.com/HT208742
• https://support.citrix.com/article/CTX234679
• https://svnweb.freebsd.org/base?view=revision\u0026revision=333368
• https://usn.ubuntu.com/3641-1/
• https://usn.ubuntu.com/3641-2/
• https://www.debian.org/security/2018/dsa-4196
• https://www.debian.org/security/2018/dsa-4201
• https://www.exploit-db.com/exploits/44697/
• https://www.exploit-db.com/exploits/45024/
• https://www.freebsd.org/security/advisories/FreeBSD-SA-18:06.debugreg.asc
• https://www.kb.cert.org/vuls/id/631579
• https://www.synology.com/support/security/Synology_SA_18_21
• https://www.triplefault.io/2018/05/spurious-db-exceptions-with-pop-ss.html
• https://xenbits.xen.org/xsa/advisory-260.html