--- /dev/null
+/*
+ * umip.c Emulation for instruction protected by the Intel User-Mode
+ * Instruction Prevention feature
+ *
+ * Copyright (c) 2017, Intel Corporation.
+ * Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
+ */
+
+#include <linux/uaccess.h>
+#include <asm/umip.h>
+#include <asm/traps.h>
+#include <asm/insn.h>
+#include <asm/insn-eval.h>
+
+/** DOC: Emulation for User-Mode Instruction Prevention (UMIP)
+ *
+ * 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 emulate the result of such instructions to provide dummy values.
+ * 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.
+ *
+ * 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 (sldt, str and smsw).
+ *
+ * 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.
+ *
+ * Given that sldt and str are not commonly used in programs that run on WineHQ
+ * or DOSEMU2, they are not emulated.
+ *
+ * The instruction smsw is emulated to return the value that the register CR0
+ * has at boot time as set in the head_32.
+ *
+ * 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.
+ *
+ * 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.
+ */
+
+#define UMIP_DUMMY_GDT_BASE 0xfffe0000
+#define UMIP_DUMMY_IDT_BASE 0xffff0000
+
+/*
+ * The SGDT and SIDT instructions store the contents of the global descriptor
+ * table and interrupt table registers, respectively. The destination is a
+ * memory operand of X+2 bytes. X bytes are used to store the base address of
+ * the table and 2 bytes are used to store the limit. In 32-bit processes, the
+ * only processes for which emulation is provided, X has a value of 4.
+ */
+#define UMIP_GDT_IDT_BASE_SIZE 4
+#define UMIP_GDT_IDT_LIMIT_SIZE 2
+
+#define UMIP_INST_SGDT 0 /* 0F 01 /0 */
+#define UMIP_INST_SIDT 1 /* 0F 01 /1 */
+#define UMIP_INST_SMSW 3 /* 0F 01 /4 */
+
+/**
+ * identify_insn() - Identify a UMIP-protected instruction
+ * @insn: Instruction structure with opcode and ModRM byte.
+ *
+ * From the opcode and ModRM.reg in @insn identify, if any, a UMIP-protected
+ * instruction that can be emulated.
+ *
+ * Returns:
+ *
+ * On success, a constant identifying a specific UMIP-protected instruction that
+ * can be emulated.
+ *
+ * -EINVAL on error or when not an UMIP-protected instruction that can be
+ * emulated.
+ */
+static int identify_insn(struct insn *insn)
+{
+ /* By getting modrm we also get the opcode. */
+ insn_get_modrm(insn);
+
+ if (!insn->modrm.nbytes)
+ return -EINVAL;
+
+ /* All the instructions of interest start with 0x0f. */
+ if (insn->opcode.bytes[0] != 0xf)
+ return -EINVAL;
+
+ if (insn->opcode.bytes[1] == 0x1) {
+ switch (X86_MODRM_REG(insn->modrm.value)) {
+ case 0:
+ return UMIP_INST_SGDT;
+ case 1:
+ return UMIP_INST_SIDT;
+ case 4:
+ return UMIP_INST_SMSW;
+ default:
+ return -EINVAL;
+ }
+ }
+
+ /* SLDT AND STR are not emulated */
+ return -EINVAL;
+}
+
+/**
+ * emulate_umip_insn() - Emulate UMIP instructions and return dummy values
+ * @insn: Instruction structure with operands
+ * @umip_inst: A constant indicating the instruction to emulate
+ * @data: Buffer into which the dummy result is stored
+ * @data_size: Size of the emulated result
+ *
+ * Emulate an instruction protected by UMIP and provide a dummy result. The
+ * result of the emulation is saved in @data. The size of the results depends
+ * on both the instruction and type of operand (register vs memory address).
+ * The size of the result is updated in @data_size. Caller is responsible
+ * of providing a @data buffer of at least UMIP_GDT_IDT_BASE_SIZE +
+ * UMIP_GDT_IDT_LIMIT_SIZE bytes.
+ *
+ * Returns:
+ *
+ * 0 on success, -EINVAL on error while emulating.
+ */
+static int emulate_umip_insn(struct insn *insn, int umip_inst,
+ unsigned char *data, int *data_size)
+{
+ unsigned long dummy_base_addr, dummy_value;
+ unsigned short dummy_limit = 0;
+
+ if (!data || !data_size || !insn)
+ return -EINVAL;
+ /*
+ * These two instructions return the base address and limit of the
+ * global and interrupt descriptor table, respectively. According to the
+ * Intel Software Development manual, the base address can be 24-bit,
+ * 32-bit or 64-bit. Limit is always 16-bit. If the operand size is
+ * 16-bit, the returned value of the base address is supposed to be a
+ * zero-extended 24-byte number. However, it seems that a 32-byte number
+ * is always returned irrespective of the operand size.
+ */
+ if (umip_inst == UMIP_INST_SGDT || umip_inst == UMIP_INST_SIDT) {
+ /* SGDT and SIDT do not use registers operands. */
+ if (X86_MODRM_MOD(insn->modrm.value) == 3)
+ return -EINVAL;
+
+ if (umip_inst == UMIP_INST_SGDT)
+ dummy_base_addr = UMIP_DUMMY_GDT_BASE;
+ else
+ dummy_base_addr = UMIP_DUMMY_IDT_BASE;
+
+ *data_size = UMIP_GDT_IDT_LIMIT_SIZE + UMIP_GDT_IDT_BASE_SIZE;
+
+ memcpy(data + 2, &dummy_base_addr, UMIP_GDT_IDT_BASE_SIZE);
+ memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);
+
+ } else if (umip_inst == UMIP_INST_SMSW) {
+ dummy_value = CR0_STATE;
+
+ /*
+ * Even though the CR0 register has 4 bytes, the number
+ * of bytes to be copied in the result buffer is determined
+ * by whether the operand is a register or a memory location.
+ * If operand is a register, return as many bytes as the operand
+ * size. If operand is memory, return only the two least
+ * siginificant bytes of CR0.
+ */
+ if (X86_MODRM_MOD(insn->modrm.value) == 3)
+ *data_size = insn->opnd_bytes;
+ else
+ *data_size = 2;
+
+ memcpy(data, &dummy_value, *data_size);
+ /* STR and SLDT are not emulated */
+ } else {
+ return -EINVAL;
+ }
+
+ return 0;
+}
+
+/**
+ * fixup_umip_exception() - Fixup a general protection fault caused by UMIP
+ * @regs: Registers as saved when entering the #GP handler
+ *
+ * The instructions sgdt, sidt, str, smsw, sldt cause a general protection
+ * fault if executed with CPL > 0 (i.e., from user space). If the offending
+ * user-space process is not in long mode, this function fixes the exception
+ * up and provides dummy results for sgdt, sidt and smsw; str and sldt are not
+ * fixed up. Also long mode user-space processes are not fixed up.
+ *
+ * If operands are memory addresses, results are copied to user-space memory as
+ * indicated by the instruction pointed by eIP using the registers indicated in
+ * the instruction operands. If operands are registers, results are copied into
+ * the context that was saved when entering kernel mode.
+ *
+ * Returns:
+ *
+ * True if emulation was successful; false if not.
+ */
+bool fixup_umip_exception(struct pt_regs *regs)
+{
+ int not_copied, nr_copied, reg_offset, dummy_data_size, umip_inst;
+ unsigned long seg_base = 0, *reg_addr;
+ /* 10 bytes is the maximum size of the result of UMIP instructions */
+ unsigned char dummy_data[10] = { 0 };
+ unsigned char buf[MAX_INSN_SIZE];
+ void __user *uaddr;
+ struct insn insn;
+ char seg_defs;
+
+ if (!regs)
+ return false;
+
+ /* Do not emulate 64-bit processes. */
+ if (user_64bit_mode(regs))
+ return false;
+
+ /*
+ * If not in user-space long mode, a custom code segment could be in
+ * use. This is true in protected mode (if the process defined a local
+ * descriptor table), or virtual-8086 mode. In most of the cases
+ * seg_base will be zero as in USER_CS.
+ */
+ if (!user_64bit_mode(regs))
+ seg_base = insn_get_seg_base(regs, INAT_SEG_REG_CS);
+
+ if (seg_base == -1L)
+ return false;
+
+ not_copied = copy_from_user(buf, (void __user *)(seg_base + regs->ip),
+ sizeof(buf));
+ nr_copied = sizeof(buf) - not_copied;
+
+ /*
+ * The copy_from_user above could have failed if user code is protected
+ * by a memory protection key. Give up on emulation in such a case.
+ * Should we issue a page fault?
+ */
+ if (!nr_copied)
+ return false;
+
+ insn_init(&insn, buf, nr_copied, user_64bit_mode(regs));
+
+ /*
+ * Override the default operand and address sizes with what is specified
+ * in the code segment descriptor. The instruction decoder only sets
+ * the address size it to either 4 or 8 address bytes and does nothing
+ * for the operand bytes. This OK for most of the cases, but we could
+ * have special cases where, for instance, a 16-bit code segment
+ * descriptor is used.
+ * If there is an address override prefix, the instruction decoder
+ * correctly updates these values, even for 16-bit defaults.
+ */
+ seg_defs = insn_get_code_seg_params(regs);
+ if (seg_defs == -EINVAL)
+ return false;
+
+ insn.addr_bytes = INSN_CODE_SEG_ADDR_SZ(seg_defs);
+ insn.opnd_bytes = INSN_CODE_SEG_OPND_SZ(seg_defs);
+
+ insn_get_length(&insn);
+ if (nr_copied < insn.length)
+ return false;
+
+ umip_inst = identify_insn(&insn);
+ if (umip_inst < 0)
+ return false;
+
+ if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size))
+ return false;
+
+ /*
+ * If operand is a register, write result to the copy of the register
+ * value that was pushed to the stack when entering into kernel mode.
+ * Upon exit, the value we write will be restored to the actual hardware
+ * register.
+ */
+ if (X86_MODRM_MOD(insn.modrm.value) == 3) {
+ reg_offset = insn_get_modrm_rm_off(&insn, regs);
+
+ /*
+ * Negative values are usually errors. In memory addressing,
+ * the exception is -EDOM. Since we expect a register operand,
+ * all negative values are errors.
+ */
+ if (reg_offset < 0)
+ return false;
+
+ reg_addr = (unsigned long *)((unsigned long)regs + reg_offset);
+ memcpy(reg_addr, dummy_data, dummy_data_size);
+ } else {
+ uaddr = insn_get_addr_ref(&insn, regs);
+ if ((unsigned long)uaddr == -1L)
+ return false;
+
+ nr_copied = copy_to_user(uaddr, dummy_data, dummy_data_size);
+ if (nr_copied > 0)
+ return false;
+ }
+
+ /* increase IP to let the program keep going */
+ regs->ip += insn.length;
+ return true;
+}
projects / openwrt / staging / blogic.git / commitdiff