On Wed, Feb 22, 2017 at 10:36:50PM -0800, Ricardo Neri wrote:
> + /*
> + * A negative offset generally means a error, except
> + * -EDOM, which means that the contents of the register
> + * should not be used as
User_mode Instruction Prevention (UMIP) is enabled by setting/clearing a
bit in %cr4.
It makes sense to enable UMIP at some point while booting, before user
spaces come up. Like SMAP and SMEP, is not critical to have it enabled
very early during boot. This is because UMIP is relevant only when
If the User-Mode Instruction Prevention CPU feature is available and
enabled, a general protection fault will be issued if the instructions
sgdt, sldt, sidt, str or smsw are executed from user-mode context
(CPL > 0). If the fault was caused by any of the instructions protected
by UMIP,
Tasks running in virtual-8086 mode or in protected mode with code
segment descriptors that specify 16-bit default address sizes via the
D bit will use 16-bit addressing form encodings as described in the Intel
64 and IA-32 Architecture Software Developer's Manual Volume 2A Section
2.1.5. 16-bit
fixup_umip_exception will be called from do_general_protection. If the
former returns false, the latter will issue a SIGSEGV with SEND_SIG_PRIV.
However, when emulation is successful but the emulated result cannot be
copied to user space memory, it is more accurate to issue a SIGSEGV with
When computing a linear address and segmentation is used, we need to know
the base address of the segment involved in the computation. In most of
the cases, it will be sufficient to use USER_DS, which has a base of 0.
However, it may be possible that a user space program defines its own
segments
These functions read the default values of the address and operand sizes
as specified in the segment descriptor. This information is determined
from the D and L bits. Hence, it can be used for both IA-32e 64-bit and
32-bit legacy modes. For virtual-8086 mode, the default address and
operand sizes
The function insn_get_reg_offset takes as argument an enumeration that
indicates the type of offset that is returned: the R/M part of the ModRM
byte, the index of the SIB byte or the base of the SIB byte. Callers of
this function would need the definition of such enumeration. This is not
needed.
The segment descriptor contains information that is relevant to how linear
address need to be computed. It contains the default size of addresses as
well as the base address of the segment. Thus, given a segment selector,
we ought look at segment descriptor to correctly calculate the linear
Certain user space programs that run on virtual-8086 mode may utilize
instructions protected by the User-Mode Instruction Prevention (UMIP)
security feature present in new Intel processors: SGDT, SIDT and SMSW. In
such a case, a general protection fault is issued if UMIP is enabled. When
such a
Other kernel submodules can benefit from using the utility functions
defined in mpx.c to obtain the addresses and values of operands contained
in the general purpose registers. An instance of this is the emulation code
used for instructions protected by the Intel User-Mode Instruction
Prevention
insn_get_addr_ref returns the effective address as defined by the
section 3.7.5.1 Vol 1 of the Intel 64 and IA-32 Architectures Software
Developer's Manual. In order to truly give the linear address, we must
add the effective address to the segment base as described by the segment
descriptor.
In
Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when a SIB byte is used and the
base of the SIB byte points to R/EBP (i.e., base = 5) and the mod part
of the ModRM byte is zero, the value of such register will not be used
as part of the
Section 2.2.1.2 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when memory addressing is used
(i.e., mod part of ModR/M is not 3), a SIB byte is used and the index of
the SIB byte points to the R/ESP (i.e., index = 4), the index should not be
used in the
This is v4 of this series. Again, it took me a while to complete the
updates as support for 16-bit address encodings for protected mode
required extra rework. The two previous submissions can be found here [1],
here [2] and here [3].
=== What is UMIP?
User-Mode Instruction Prevention (UMIP) is a
The feature User-Mode Instruction Prevention present in recent Intel
processor prevents a group of instructions from being executed with
CPL > 0. Otherwise, a general protection fault is issued.
Rather than relaying this fault to the user space (in the form of a SIGSEGV
signal), the instructions
Section 2.2.1.3 of the Intel 64 and IA-32 Architectures Software
Developer's Manual volume 2A states that when the mod part of the ModRM
byte is zero and R/EBP is specified in the R/M part of such bit, the value
of the aforementioned register should not be used in the address
computation. Instead,
With segmentation, the base address of the segment descriptor is needed
to compute a linear address. The segment descriptor used in the address
computation depends on either any segment override prefixes in the in the
instruction or the default segment determined by the registers involved
in the
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