arch/x86/lib/insn-eval.c

/*
 * Utility functions for x86 operand and address decoding
 *
 * Copyright (C) Intel Corporation 2017
 */
#include <linux/kernel.h>
#include <linux/string.h>
#include <linux/ratelimit.h>
#include <asm/inat.h>
#include <asm/insn.h>
#include <asm/insn-eval.h>

#undef pr_fmt
#define pr_fmt(fmt) "insn: " fmt

enum reg_type {
	REG_TYPE_RM = 0,
	REG_TYPE_INDEX,
	REG_TYPE_BASE,
};

static int get_reg_offset(struct insn *insn, struct pt_regs *regs,
			  enum reg_type type)
{
	int regno = 0;

	static const int regoff[] = {
		offsetof(struct pt_regs, ax),
		offsetof(struct pt_regs, cx),
		offsetof(struct pt_regs, dx),
		offsetof(struct pt_regs, bx),
		offsetof(struct pt_regs, sp),
		offsetof(struct pt_regs, bp),
		offsetof(struct pt_regs, si),
		offsetof(struct pt_regs, di),
#ifdef CONFIG_X86_64
		offsetof(struct pt_regs, r8),
		offsetof(struct pt_regs, r9),
		offsetof(struct pt_regs, r10),
		offsetof(struct pt_regs, r11),
		offsetof(struct pt_regs, r12),
		offsetof(struct pt_regs, r13),
		offsetof(struct pt_regs, r14),
		offsetof(struct pt_regs, r15),
#endif
	};
	int nr_registers = ARRAY_SIZE(regoff);
	/*
	 * Don't possibly decode a 32-bit instructions as
	 * reading a 64-bit-only register.
	 */
	if (IS_ENABLED(CONFIG_X86_64) && !insn->x86_64)
		nr_registers -= 8;

	switch (type) {
	case REG_TYPE_RM:
		regno = X86_MODRM_RM(insn->modrm.value);
		if (X86_REX_B(insn->rex_prefix.value))
			regno += 8;
		break;

	case REG_TYPE_INDEX:
		regno = X86_SIB_INDEX(insn->sib.value);
		if (X86_REX_X(insn->rex_prefix.value))
			regno += 8;

		/*
		 * If ModRM.mod != 3 and SIB.index = 4 the scale*index
		 * portion of the address computation is null. This is
		 * true only if REX.X is 0. In such a case, the SIB index
		 * is used in the address computation.
		 */
		if (X86_MODRM_MOD(insn->modrm.value) != 3 && regno == 4)
			return -EDOM;
		break;

	case REG_TYPE_BASE:
		regno = X86_SIB_BASE(insn->sib.value);
		/*
		 * If ModRM.mod is 0 and SIB.base == 5, the base of the
		 * register-indirect addressing is 0. In this case, a
		 * 32-bit displacement follows the SIB byte.
		 */
		if (!X86_MODRM_MOD(insn->modrm.value) && regno == 5)
			return -EDOM;

		if (X86_REX_B(insn->rex_prefix.value))
			regno += 8;
		break;

	default:
		pr_err_ratelimited("invalid register type: %d\n", type);
		return -EINVAL;
	}

	if (regno >= nr_registers) {
		WARN_ONCE(1, "decoded an instruction with an invalid register");
		return -EINVAL;
	}
	return regoff[regno];
}

/**
 * insn_get_modrm_rm_off() - Obtain register in r/m part of the ModRM byte
 * @insn:	Instruction containing the ModRM byte
 * @regs:	Register values as seen when entering kernel mode
 *
 * Returns:
 *
 * The register indicated by the r/m part of the ModRM byte. The
 * register is obtained as an offset from the base of pt_regs. In specific
 * cases, the returned value can be -EDOM to indicate that the particular value
 * of ModRM does not refer to a register and shall be ignored.
 */
int insn_get_modrm_rm_off(struct insn *insn, struct pt_regs *regs)
{
	return get_reg_offset(insn, regs, REG_TYPE_RM);
}

/*
 * return the address being referenced be instruction
 * for rm=3 returning the content of the rm reg
 * for rm!=3 calculates the address using SIB and Disp
 */
void __user *insn_get_addr_ref(struct insn *insn, struct pt_regs *regs)
{
	int addr_offset, base_offset, indx_offset;
	unsigned long linear_addr = -1L;
	long eff_addr, base, indx;
	insn_byte_t sib;

	insn_get_modrm(insn);
	insn_get_sib(insn);
	sib = insn->sib.value;

	if (X86_MODRM_MOD(insn->modrm.value) == 3) {
		addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
		if (addr_offset < 0)
			goto out;

		eff_addr = regs_get_register(regs, addr_offset);
	} else {
		if (insn->sib.nbytes) {
			/*
			 * Negative values in the base and index offset means
			 * an error when decoding the SIB byte. Except -EDOM,
			 * which means that the registers should not be used
			 * in the address computation.
			 */
			base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE);
			if (base_offset == -EDOM)
				base = 0;
			else if (base_offset < 0)
				goto out;
			else
				base = regs_get_register(regs, base_offset);

			indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX);

			if (indx_offset == -EDOM)
				indx = 0;
			else if (indx_offset < 0)
				goto out;
			else
				indx = regs_get_register(regs, indx_offset);

			eff_addr = base + indx * (1 << X86_SIB_SCALE(sib));
		} else {
			addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM);
			if (addr_offset < 0)
				goto out;

			eff_addr = regs_get_register(regs, addr_offset);
		}

		eff_addr += insn->displacement.value;
	}

	linear_addr = (unsigned long)eff_addr;

out:
	return (void __user *)linear_addr;
}