/*
 * umip.c Emulation for instruction protected by the 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>
#include <linux/ratelimit.h>

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

/** DOC: Emulation for User-Mode Instruction Prevention (UMIP)
 *
 * User-Mode Instruction Prevention is a security feature present in recent
 * x86 processors that, when enabled, prevents a group of instructions (SGDT,
 * SIDT, SLDT, SMSW and STR) from being run in user mode by issuing a general
 * protection fault if the instruction is executed with CPL > 0.
 *
 * 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.
 *
 * The instruction SMSW is emulated to return the value that the register CR0
 * has at boot time as set in the head_32.
 * SLDT and STR are emulated to return the values that the kernel programmatically
 * assigns:
 * - SLDT returns (GDT_ENTRY_LDT * 8) if an LDT has been set, 0 if not.
 * - STR returns (GDT_ENTRY_TSS * 8).
 *
 * Emulation is provided for both 32-bit and 64-bit processes.
 *
 * 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 0xfffffffffffe0000ULL
#define UMIP_DUMMY_IDT_BASE 0xffffffffffff0000ULL

/*
 * 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 X
 * has a value of 4, in 64-bit processes X has a value of 8.
 */
#define UMIP_GDT_IDT_BASE_SIZE_64BIT 8
#define UMIP_GDT_IDT_BASE_SIZE_32BIT 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  2       /* 0F 01 /4 */
#define UMIP_INST_SLDT  3       /* 0F 00 /0 */
#define UMIP_INST_STR   4       /* 0F 00 /1 */

static const char * const umip_insns[5] = {
        [UMIP_INST_SGDT] = "SGDT",
        [UMIP_INST_SIDT] = "SIDT",
        [UMIP_INST_SMSW] = "SMSW",
        [UMIP_INST_SLDT] = "SLDT",
        [UMIP_INST_STR] = "STR",
};

#define umip_pr_err(regs, fmt, ...) \
        umip_printk(regs, KERN_ERR, fmt, ##__VA_ARGS__)
#define umip_pr_warn(regs, fmt, ...) \
        umip_printk(regs, KERN_WARNING, fmt,  ##__VA_ARGS__)

/**
 * umip_printk() - Print a rate-limited message
 * @regs:       Register set with the context in which the warning is printed
 * @log_level:  Kernel log level to print the message
 * @fmt:        The text string to print
 *
 * Print the text contained in @fmt. The print rate is limited to bursts of 5
 * messages every two minutes. The purpose of this customized version of
 * printk() is to print messages when user space processes use any of the
 * UMIP-protected instructions. Thus, the printed text is prepended with the
 * task name and process ID number of the current task as well as the
 * instruction and stack pointers in @regs as seen when entering kernel mode.
 *
 * Returns:
 *
 * None.
 */
static __printf(3, 4)
void umip_printk(const struct pt_regs *regs, const char *log_level,
                 const char *fmt, ...)
{
        /* Bursts of 5 messages every two minutes */
        static DEFINE_RATELIMIT_STATE(ratelimit, 2 * 60 * HZ, 5);
        struct task_struct *tsk = current;
        struct va_format vaf;
        va_list args;

        if (!__ratelimit(&ratelimit))
                return;

        va_start(args, fmt);
        vaf.fmt = fmt;
        vaf.va = &args;
        printk("%s" pr_fmt("%s[%d] ip:%lx sp:%lx: %pV"), log_level, tsk->comm,
               task_pid_nr(tsk), regs->ip, regs->sp, &vaf);
        va_end(args);
}

/**
 * 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;
                }
        } else if (insn->opcode.bytes[1] == 0x0) {
                if (X86_MODRM_REG(insn->modrm.value) == 0)
                        return UMIP_INST_SLDT;
                else if (X86_MODRM_REG(insn->modrm.value) == 1)
                        return UMIP_INST_STR;
                else
                        return -EINVAL;
        } else {
                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
 * @x86_64:     true if process is 64-bit, false otherwise
 *
 * 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, bool x86_64)
{
        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) {
                u64 dummy_base_addr;
                u16 dummy_limit = 0;

                /* 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;

                /*
                 * 64-bit processes use the entire dummy base address.
                 * 32-bit processes use the lower 32 bits of the base address.
                 * dummy_base_addr is always 64 bits, but we memcpy the correct
                 * number of bytes from it to the destination.
                 */
                if (x86_64)
                        *data_size = UMIP_GDT_IDT_BASE_SIZE_64BIT;
                else
                        *data_size = UMIP_GDT_IDT_BASE_SIZE_32BIT;

                memcpy(data + 2, &dummy_base_addr, *data_size);

                *data_size += UMIP_GDT_IDT_LIMIT_SIZE;
                memcpy(data, &dummy_limit, UMIP_GDT_IDT_LIMIT_SIZE);

        } else if (umip_inst == UMIP_INST_SMSW || umip_inst == UMIP_INST_SLDT ||
                   umip_inst == UMIP_INST_STR) {
                unsigned long dummy_value;

                if (umip_inst == UMIP_INST_SMSW) {
                        dummy_value = CR0_STATE;
                } else if (umip_inst == UMIP_INST_STR) {
                        dummy_value = GDT_ENTRY_TSS * 8;
                } else if (umip_inst == UMIP_INST_SLDT) {
#ifdef CONFIG_MODIFY_LDT_SYSCALL
                        down_read(&current->mm->context.ldt_usr_sem);
                        if (current->mm->context.ldt)
                                dummy_value = GDT_ENTRY_LDT * 8;
                        else
                                dummy_value = 0;
                        up_read(&current->mm->context.ldt_usr_sem);
#else
                        dummy_value = 0;
#endif
                }

                /*
                 * For these 3 instructions, 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
                 * significant bytes.
                 */
                if (X86_MODRM_MOD(insn->modrm.value) == 3)
                        *data_size = insn->opnd_bytes;
                else
                        *data_size = 2;

                memcpy(data, &dummy_value, *data_size);
        } else {
                return -EINVAL;
        }

        return 0;
}

/**
 * force_sig_info_umip_fault() - Force a SIGSEGV with SEGV_MAPERR
 * @addr:       Address that caused the signal
 * @regs:       Register set containing the instruction pointer
 *
 * Force a SIGSEGV signal with SEGV_MAPERR as the error code. This function is
 * intended to be used to provide a segmentation fault when the result of the
 * UMIP emulation could not be copied to the user space memory.
 *
 * Returns: none
 */
static void force_sig_info_umip_fault(void __user *addr, struct pt_regs *regs)
{
        struct task_struct *tsk = current;

        tsk->thread.cr2         = (unsigned long)addr;
        tsk->thread.error_code  = X86_PF_USER | X86_PF_WRITE;
        tsk->thread.trap_nr     = X86_TRAP_PF;

        force_sig_fault(SIGSEGV, SEGV_MAPERR, addr);

        if (!(show_unhandled_signals && unhandled_signal(tsk, SIGSEGV)))
                return;

        umip_pr_err(regs, "segfault in emulation. error%x\n",
                    X86_PF_USER | X86_PF_WRITE);
}

/**
 * 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 and SLDT cause a general protection
 * fault if executed with CPL > 0 (i.e., from user space). This function fixes
 * the exception up and provides dummy results for SGDT, SIDT and SMSW; STR
 * and SLDT 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 nr_copied, reg_offset, dummy_data_size, umip_inst;
        /* 10 bytes is the maximum size of the result of UMIP instructions */
        unsigned char dummy_data[10] = { 0 };
        unsigned char buf[MAX_INSN_SIZE];
        unsigned long *reg_addr;
        void __user *uaddr;
        struct insn insn;

        if (!regs)
                return false;

        /*
         * Give up on emulation if fetching the instruction failed. Should a
         * page fault or a #GP be issued?
         */
        nr_copied = insn_fetch_from_user(regs, buf);
        if (nr_copied <= 0)
                return false;

        if (!insn_decode_from_regs(&insn, regs, buf, nr_copied))
                return false;

        umip_inst = identify_insn(&insn);
        if (umip_inst < 0)
                return false;

        umip_pr_warn(regs, "%s instruction cannot be used by applications.\n",
                        umip_insns[umip_inst]);

        umip_pr_warn(regs, "For now, expensive software emulation returns the result.\n");

        if (emulate_umip_insn(&insn, umip_inst, dummy_data, &dummy_data_size,
                              user_64bit_mode(regs)))
                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) {
                        /*
                         * If copy fails, send a signal and tell caller that
                         * fault was fixed up.
                         */
                        force_sig_info_umip_fault(uaddr, regs);
                        return true;
                }
        }

        /* increase IP to let the program keep going */
        regs->ip += insn.length;
        return true;
}