CVE-2018-8897

A statement in the System Programming Guide of the Intel 64 and IA-32
Architectures Software Developer’s 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.

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