Provide a software-based implementation of the priviledged no access support found in ARMv8.1. Userspace pages are mapped using a different domain number from the kernel and IO mappings. If we switch the user domain to "no access" when we enter the kernel, we can prevent the kernel from touching userspace. However, the kernel needs to be able to access userspace via the various user accessor functions. With the wrapping in the previous patch, we can temporarily enable access when the kernel needs user access, and re-disable it afterwards. This allows us to trap non-intended accesses to userspace, eg, caused by an inadvertent dereference of the LIST_POISON* values, which, with appropriate user mappings setup, can be made to succeed. This in turn can allow use-after-free bugs to be further exploited than would otherwise be possible. Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
272 lines
7 KiB
C
272 lines
7 KiB
C
/*
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* linux/arch/arm/kernel/swp_emulate.c
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*
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* Copyright (C) 2009 ARM Limited
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* __user_* functions adapted from include/asm/uaccess.h
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*
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* This program is free software; you can redistribute it and/or modify
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* it under the terms of the GNU General Public License version 2 as
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* published by the Free Software Foundation.
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*
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* Implements emulation of the SWP/SWPB instructions using load-exclusive and
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* store-exclusive for processors that have them disabled (or future ones that
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* might not implement them).
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*
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* Syntax of SWP{B} instruction: SWP{B}<c> <Rt>, <Rt2>, [<Rn>]
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* Where: Rt = destination
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* Rt2 = source
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* Rn = address
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*/
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#include <linux/init.h>
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#include <linux/kernel.h>
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#include <linux/proc_fs.h>
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#include <linux/seq_file.h>
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#include <linux/sched.h>
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#include <linux/syscalls.h>
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#include <linux/perf_event.h>
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#include <asm/opcodes.h>
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#include <asm/system_info.h>
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#include <asm/traps.h>
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#include <asm/uaccess.h>
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/*
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* Error-checking SWP macros implemented using ldrex{b}/strex{b}
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*/
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#define __user_swpX_asm(data, addr, res, temp, B) \
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__asm__ __volatile__( \
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" mov %2, %1\n" \
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"0: ldrex"B" %1, [%3]\n" \
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"1: strex"B" %0, %2, [%3]\n" \
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" cmp %0, #0\n" \
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" movne %0, %4\n" \
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"2:\n" \
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" .section .text.fixup,\"ax\"\n" \
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" .align 2\n" \
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"3: mov %0, %5\n" \
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" b 2b\n" \
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" .previous\n" \
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" .section __ex_table,\"a\"\n" \
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" .align 3\n" \
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" .long 0b, 3b\n" \
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" .long 1b, 3b\n" \
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" .previous" \
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: "=&r" (res), "+r" (data), "=&r" (temp) \
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: "r" (addr), "i" (-EAGAIN), "i" (-EFAULT) \
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: "cc", "memory")
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#define __user_swp_asm(data, addr, res, temp) \
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__user_swpX_asm(data, addr, res, temp, "")
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#define __user_swpb_asm(data, addr, res, temp) \
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__user_swpX_asm(data, addr, res, temp, "b")
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/*
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* Macros/defines for extracting register numbers from instruction.
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*/
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#define EXTRACT_REG_NUM(instruction, offset) \
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(((instruction) & (0xf << (offset))) >> (offset))
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#define RN_OFFSET 16
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#define RT_OFFSET 12
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#define RT2_OFFSET 0
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/*
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* Bit 22 of the instruction encoding distinguishes between
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* the SWP and SWPB variants (bit set means SWPB).
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*/
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#define TYPE_SWPB (1 << 22)
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static unsigned long swpcounter;
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static unsigned long swpbcounter;
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static unsigned long abtcounter;
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static pid_t previous_pid;
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#ifdef CONFIG_PROC_FS
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static int proc_status_show(struct seq_file *m, void *v)
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{
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seq_printf(m, "Emulated SWP:\t\t%lu\n", swpcounter);
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seq_printf(m, "Emulated SWPB:\t\t%lu\n", swpbcounter);
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seq_printf(m, "Aborted SWP{B}:\t\t%lu\n", abtcounter);
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if (previous_pid != 0)
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seq_printf(m, "Last process:\t\t%d\n", previous_pid);
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return 0;
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}
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static int proc_status_open(struct inode *inode, struct file *file)
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{
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return single_open(file, proc_status_show, PDE_DATA(inode));
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}
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static const struct file_operations proc_status_fops = {
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.open = proc_status_open,
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.read = seq_read,
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.llseek = seq_lseek,
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.release = single_release,
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};
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#endif
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/*
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* Set up process info to signal segmentation fault - called on access error.
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*/
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static void set_segfault(struct pt_regs *regs, unsigned long addr)
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{
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siginfo_t info;
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down_read(¤t->mm->mmap_sem);
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if (find_vma(current->mm, addr) == NULL)
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info.si_code = SEGV_MAPERR;
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else
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info.si_code = SEGV_ACCERR;
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up_read(¤t->mm->mmap_sem);
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info.si_signo = SIGSEGV;
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info.si_errno = 0;
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info.si_addr = (void *) instruction_pointer(regs);
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pr_debug("SWP{B} emulation: access caused memory abort!\n");
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arm_notify_die("Illegal memory access", regs, &info, 0, 0);
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abtcounter++;
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}
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static int emulate_swpX(unsigned int address, unsigned int *data,
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unsigned int type)
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{
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unsigned int res = 0;
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if ((type != TYPE_SWPB) && (address & 0x3)) {
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/* SWP to unaligned address not permitted */
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pr_debug("SWP instruction on unaligned pointer!\n");
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return -EFAULT;
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}
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while (1) {
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unsigned long temp;
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unsigned int __ua_flags;
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__ua_flags = uaccess_save_and_enable();
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if (type == TYPE_SWPB)
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__user_swpb_asm(*data, address, res, temp);
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else
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__user_swp_asm(*data, address, res, temp);
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uaccess_restore(__ua_flags);
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if (likely(res != -EAGAIN) || signal_pending(current))
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break;
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cond_resched();
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}
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if (res == 0) {
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if (type == TYPE_SWPB)
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swpbcounter++;
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else
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swpcounter++;
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}
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return res;
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}
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/*
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* swp_handler logs the id of calling process, dissects the instruction, sanity
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* checks the memory location, calls emulate_swpX for the actual operation and
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* deals with fixup/error handling before returning
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*/
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static int swp_handler(struct pt_regs *regs, unsigned int instr)
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{
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unsigned int address, destreg, data, type;
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unsigned int res = 0;
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perf_sw_event(PERF_COUNT_SW_EMULATION_FAULTS, 1, regs, regs->ARM_pc);
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res = arm_check_condition(instr, regs->ARM_cpsr);
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switch (res) {
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case ARM_OPCODE_CONDTEST_PASS:
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break;
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case ARM_OPCODE_CONDTEST_FAIL:
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/* Condition failed - return to next instruction */
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regs->ARM_pc += 4;
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return 0;
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case ARM_OPCODE_CONDTEST_UNCOND:
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/* If unconditional encoding - not a SWP, undef */
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return -EFAULT;
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default:
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return -EINVAL;
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}
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if (current->pid != previous_pid) {
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pr_debug("\"%s\" (%ld) uses deprecated SWP{B} instruction\n",
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current->comm, (unsigned long)current->pid);
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previous_pid = current->pid;
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}
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address = regs->uregs[EXTRACT_REG_NUM(instr, RN_OFFSET)];
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data = regs->uregs[EXTRACT_REG_NUM(instr, RT2_OFFSET)];
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destreg = EXTRACT_REG_NUM(instr, RT_OFFSET);
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type = instr & TYPE_SWPB;
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pr_debug("addr in r%d->0x%08x, dest is r%d, source in r%d->0x%08x)\n",
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EXTRACT_REG_NUM(instr, RN_OFFSET), address,
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destreg, EXTRACT_REG_NUM(instr, RT2_OFFSET), data);
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/* Check access in reasonable access range for both SWP and SWPB */
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if (!access_ok(VERIFY_WRITE, (address & ~3), 4)) {
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pr_debug("SWP{B} emulation: access to %p not allowed!\n",
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(void *)address);
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res = -EFAULT;
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} else {
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res = emulate_swpX(address, &data, type);
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}
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if (res == 0) {
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/*
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* On successful emulation, revert the adjustment to the PC
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* made in kernel/traps.c in order to resume execution at the
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* instruction following the SWP{B}.
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*/
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regs->ARM_pc += 4;
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regs->uregs[destreg] = data;
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} else if (res == -EFAULT) {
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/*
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* Memory errors do not mean emulation failed.
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* Set up signal info to return SEGV, then return OK
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*/
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set_segfault(regs, address);
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}
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return 0;
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}
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/*
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* Only emulate SWP/SWPB executed in ARM state/User mode.
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* The kernel must be SWP free and SWP{B} does not exist in Thumb/ThumbEE.
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*/
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static struct undef_hook swp_hook = {
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.instr_mask = 0x0fb00ff0,
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.instr_val = 0x01000090,
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.cpsr_mask = MODE_MASK | PSR_T_BIT | PSR_J_BIT,
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.cpsr_val = USR_MODE,
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.fn = swp_handler
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};
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/*
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* Register handler and create status file in /proc/cpu
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* Invoked as late_initcall, since not needed before init spawned.
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*/
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static int __init swp_emulation_init(void)
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{
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if (cpu_architecture() < CPU_ARCH_ARMv7)
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return 0;
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#ifdef CONFIG_PROC_FS
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if (!proc_create("cpu/swp_emulation", S_IRUGO, NULL, &proc_status_fops))
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return -ENOMEM;
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#endif /* CONFIG_PROC_FS */
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pr_notice("Registering SWP/SWPB emulation handler\n");
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register_undef_hook(&swp_hook);
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return 0;
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}
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late_initcall(swp_emulation_init);
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