x86/umip: Add emulation code for UMIP instructions

The feature User-Mode Instruction Prevention present in recent Intel
processor prevents a group of instructions (sgdt, sidt, sldt, smsw, and
str) from being executed with CPL > 0. Otherwise, a general protection
fault is issued.

Rather than relaying to the user space the general protection fault caused
by the UMIP-protected instructions (in the form of a SIGSEGV signal), it
can be trapped and the instruction emulated to provide a dummy result.
This allows to both conserve the current kernel behavior and not reveal the
system resources that UMIP intends to protect (i.e., the locations of the
global descriptor and interrupt descriptor tables, the segment selectors of
the local descriptor table, the value of the task state register and the
contents of the CR0 register).

This emulation is needed because certain applications (e.g., WineHQ and
DOSEMU2) rely on this subset of instructions to function. Given that sldt
and str are not commonly used in programs that run on WineHQ or DOSEMU2,
they are not emulated. Also, emulation is provided only for 32-bit
processes; 64-bit processes that attempt to use the instructions that UMIP
protects will receive the SIGSEGV signal issued as a consequence of the
general protection fault.

The instructions protected by UMIP can be split in two groups. Those which
return a kernel memory address (sgdt and sidt) and those which return a
value (smsw, sldt and str; the last two not emulated).

For the instructions that return a kernel memory address, applications such
as WineHQ rely on the result being located in the kernel memory space, not
the actual location of the table. The result is emulated as a hard-coded
value that lies close to the top of the kernel memory. The limit for the
GDT and the IDT are set to zero.

The instruction smsw is emulated to return the value that the register CR0
has at boot time as set in the head_32.

Care is taken to appropriately emulate the results when segmentation is
used. That is, rather than relying on USER_DS and USER_CS, the function
insn_get_addr_ref() inspects the segment descriptor pointed by the
registers in pt_regs. This ensures that we correctly obtain the segment
base address and the address and operand sizes even if the user space
application uses a local descriptor table.

Signed-off-by: Ricardo Neri <[email protected]>
Reviewed-by: Thomas Gleixner <[email protected]>
Cc: Andrew Morton <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Chen Yucong <[email protected]>
Cc: Chris Metcalf <[email protected]>
Cc: Dave Hansen <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: Fenghua Yu <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Huang Rui <[email protected]>
Cc: Jiri Slaby <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Josh Poimboeuf <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Masami Hiramatsu <[email protected]>
Cc: Michael S. Tsirkin <[email protected]>
Cc: Paolo Bonzini <[email protected]>
Cc: Paul Gortmaker <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Ravi V. Shankar <[email protected]>
Cc: Shuah Khan <[email protected]>
Cc: Tony Luck <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: [email protected]
Link: http://lkml.kernel.org/r/1509935277-22138-8-git-send-email-ricardo.neri-calderon@linux.intel.com
Signed-off-by: Ingo Molnar <[email protected]>

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