arch/mips/math-emu/dsemul.c - linux - Git at Google

 // SPDX-License-Identifier: GPL-2.0
 #include <linux/err.h>
 #include <linux/slab.h>
 #include <linux/mm_types.h>
 #include <linux/sched/task.h>

 #include <asm/branch.h>
 #include <asm/cacheflush.h>
 #include <asm/fpu_emulator.h>
 #include <asm/inst.h>
 #include <asm/mipsregs.h>
 #include <linux/uaccess.h>

 /**
  * struct emuframe - The 'emulation' frame structure
  * @emul:	The instruction to 'emulate'.
  * @badinst:	A break instruction to cause a return to the kernel.
  *
  * This structure defines the frames placed within the delay slot emulation
  * page in response to a call to mips_dsemul(). Each thread may be allocated
  * only one frame at any given time. The kernel stores within it the
  * instruction to be 'emulated' followed by a break instruction, then
  * executes the frame in user mode. The break causes a trap to the kernel
  * which leads to do_dsemulret() being called unless the instruction in
  * @emul causes a trap itself, is a branch, or a signal is delivered to
  * the thread. In these cases the allocated frame will either be reused by
  * a subsequent delay slot 'emulation', or be freed during signal delivery or
  * upon thread exit.
  *
  * This approach is used because:
  *
  * - Actually emulating all instructions isn't feasible. We would need to
  *   be able to handle instructions from all revisions of the MIPS ISA,
  *   all ASEs & all vendor instruction set extensions. This would be a
  *   whole lot of work & continual maintenance burden as new instructions
  *   are introduced, and in the case of some vendor extensions may not
  *   even be possible. Thus we need to take the approach of actually
  *   executing the instruction.
  *
  * - We must execute the instruction within user context. If we were to
  *   execute the instruction in kernel mode then it would have access to
  *   kernel resources without very careful checks, leaving us with a
  *   high potential for security or stability issues to arise.
  *
  * - We used to place the frame on the users stack, but this requires
  *   that the stack be executable. This is bad for security so the
  *   per-process page is now used instead.
  *
  * - The instruction in @emul may be something entirely invalid for a
  *   delay slot. The user may (intentionally or otherwise) place a branch
  *   in a delay slot, or a kernel mode instruction, or something else
  *   which generates an exception. Thus we can't rely upon the break in
  *   @badinst always being hit. For this reason we track the index of the
  *   frame allocated to each thread, allowing us to clean it up at later
  *   points such as signal delivery or thread exit.
  *
  * - The user may generate a fake struct emuframe if they wish, invoking
  *   the BRK_MEMU break instruction themselves. We must therefore not
  *   trust that BRK_MEMU means there's actually a valid frame allocated
  *   to the thread, and must not allow the user to do anything they
  *   couldn't already.
  */
 struct emuframe {
 	mips_instruction	emul;
 	mips_instruction	badinst;
 };

 static const int emupage_frame_count = PAGE_SIZE / sizeof(struct emuframe);

 static inline __user struct emuframe *dsemul_page(void)
 {
 	return (__user struct emuframe *)STACK_TOP;
 }

 static int alloc_emuframe(void)
 {
 	mm_context_t *mm_ctx = &current->mm->context;
 	int idx;

 retry:
 	spin_lock(&mm_ctx->bd_emupage_lock);

 	/* Ensure we have an allocation bitmap */
 	if (!mm_ctx->bd_emupage_allocmap) {
 		mm_ctx->bd_emupage_allocmap = bitmap_zalloc(emupage_frame_count,
 							    GFP_ATOMIC);
 		if (!mm_ctx->bd_emupage_allocmap) {
 			idx = BD_EMUFRAME_NONE;
 			goto out_unlock;
 		}
 	}

 	/* Attempt to allocate a single bit/frame */
 	idx = bitmap_find_free_region(mm_ctx->bd_emupage_allocmap,
 				      emupage_frame_count, 0);
 	if (idx < 0) {
 		/*
 		 * Failed to allocate a frame. We'll wait until one becomes
 		 * available. We unlock the page so that other threads actually
 		 * get the opportunity to free their frames, which means
 		 * technically the result of bitmap_full may be incorrect.
 		 * However the worst case is that we repeat all this and end up
 		 * back here again.
 		 */
 		spin_unlock(&mm_ctx->bd_emupage_lock);
 		if (!wait_event_killable(mm_ctx->bd_emupage_queue,
 			!bitmap_full(mm_ctx->bd_emupage_allocmap,
 				     emupage_frame_count)))
 			goto retry;

 		/* Received a fatal signal - just give in */
 		return BD_EMUFRAME_NONE;
 	}

 	/* Success! */
 	pr_debug("allocate emuframe %d to %d\n", idx, current->pid);
 out_unlock:
 	spin_unlock(&mm_ctx->bd_emupage_lock);
 	return idx;
 }

 static void free_emuframe(int idx, struct mm_struct *mm)
 {
 	mm_context_t *mm_ctx = &mm->context;

 	spin_lock(&mm_ctx->bd_emupage_lock);

 	pr_debug("free emuframe %d from %d\n", idx, current->pid);
 	bitmap_clear(mm_ctx->bd_emupage_allocmap, idx, 1);

 	/* If some thread is waiting for a frame, now's its chance */
 	wake_up(&mm_ctx->bd_emupage_queue);

 	spin_unlock(&mm_ctx->bd_emupage_lock);
 }

 static bool within_emuframe(struct pt_regs *regs)
 {
 	unsigned long base = (unsigned long)dsemul_page();

 	if (regs->cp0_epc < base)
 		return false;
 	if (regs->cp0_epc >= (base + PAGE_SIZE))
 		return false;

 	return true;
 }

 bool dsemul_thread_cleanup(struct task_struct *tsk)
 {
 	int fr_idx;

 	/* Clear any allocated frame, retrieving its index */
 	fr_idx = atomic_xchg(&tsk->thread.bd_emu_frame, BD_EMUFRAME_NONE);

 	/* If no frame was allocated, we're done */
 	if (fr_idx == BD_EMUFRAME_NONE)
 		return false;

 	task_lock(tsk);

 	/* Free the frame that this thread had allocated */
 	if (tsk->mm)
 		free_emuframe(fr_idx, tsk->mm);

 	task_unlock(tsk);
 	return true;
 }

 bool dsemul_thread_rollback(struct pt_regs *regs)
 {
 	struct emuframe __user *fr;
 	int fr_idx;

 	/* Do nothing if we're not executing from a frame */
 	if (!within_emuframe(regs))
 		return false;

 	/* Find the frame being executed */
 	fr_idx = atomic_read(&current->thread.bd_emu_frame);
 	if (fr_idx == BD_EMUFRAME_NONE)
 		return false;
 	fr = &dsemul_page()[fr_idx];

 	/*
 	 * If the PC is at the emul instruction, roll back to the branch. If
 	 * PC is at the badinst (break) instruction, we've already emulated the
 	 * instruction so progress to the continue PC. If it's anything else
 	 * then something is amiss & the user has branched into some other area
 	 * of the emupage - we'll free the allocated frame anyway.
 	 */
 	if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->emul)
 		regs->cp0_epc = current->thread.bd_emu_branch_pc;
 	else if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->badinst)
 		regs->cp0_epc = current->thread.bd_emu_cont_pc;

 	atomic_set(&current->thread.bd_emu_frame, BD_EMUFRAME_NONE);
 	free_emuframe(fr_idx, current->mm);
 	return true;
 }

 void dsemul_mm_cleanup(struct mm_struct *mm)
 {
 	mm_context_t *mm_ctx = &mm->context;

 	bitmap_free(mm_ctx->bd_emupage_allocmap);
 }

 int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
 		unsigned long branch_pc, unsigned long cont_pc)
 {
 	int isa16 = get_isa16_mode(regs->cp0_epc);
 	mips_instruction break_math;
 	unsigned long fr_uaddr;
 	struct emuframe fr;
 	int fr_idx, ret;

 	/* NOP is easy */
 	if (ir == 0)
 		return -1;

 	/* microMIPS instructions */
 	if (isa16) {
 		union mips_instruction insn = { .word = ir };

 		/* NOP16 aka MOVE16 $0, $0 */
 		if ((ir >> 16) == MM_NOP16)
 			return -1;

 		/* ADDIUPC */
 		if (insn.mm_a_format.opcode == mm_addiupc_op) {
 			unsigned int rs;
 			s32 v;

 			rs = (((insn.mm_a_format.rs + 0xe) & 0xf) + 2);
 			v = regs->cp0_epc & ~3;
 			v += insn.mm_a_format.simmediate << 2;
 			regs->regs[rs] = (long)v;
 			return -1;
 		}
 	}

 	pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc);

 	/* Allocate a frame if we don't already have one */
 	fr_idx = atomic_read(&current->thread.bd_emu_frame);
 	if (fr_idx == BD_EMUFRAME_NONE)
 		fr_idx = alloc_emuframe();
 	if (fr_idx == BD_EMUFRAME_NONE)
 		return SIGBUS;

 	/* Retrieve the appropriately encoded break instruction */
 	break_math = BREAK_MATH(isa16);

 	/* Write the instructions to the frame */
 	if (isa16) {
 		union mips_instruction _emul = {
 			.halfword = { ir >> 16, ir }
 		};
 		union mips_instruction _badinst = {
 			.halfword = { break_math >> 16, break_math }
 		};

 		fr.emul = _emul.word;
 		fr.badinst = _badinst.word;
 	} else {
 		fr.emul = ir;
 		fr.badinst = break_math;
 	}

 	/* Write the frame to user memory */
 	fr_uaddr = (unsigned long)&dsemul_page()[fr_idx];
 	ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr),
 				FOLL_FORCE | FOLL_WRITE);
 	if (unlikely(ret != sizeof(fr))) {
 		MIPS_FPU_EMU_INC_STATS(errors);
 		free_emuframe(fr_idx, current->mm);
 		return SIGBUS;
 	}

 	/* Record the PC of the branch, PC to continue from & frame index */
 	current->thread.bd_emu_branch_pc = branch_pc;
 	current->thread.bd_emu_cont_pc = cont_pc;
 	atomic_set(&current->thread.bd_emu_frame, fr_idx);

 	/* Change user register context to execute the frame */
 	regs->cp0_epc = fr_uaddr | isa16;

 	return 0;
 }

 bool do_dsemulret(struct pt_regs *xcp)
 {
 	/* Cleanup the allocated frame, returning if there wasn't one */
 	if (!dsemul_thread_cleanup(current)) {
 		MIPS_FPU_EMU_INC_STATS(errors);
 		return false;
 	}

 	/* Set EPC to return to post-branch instruction */
 	xcp->cp0_epc = current->thread.bd_emu_cont_pc;
 	pr_debug("dsemulret to 0x%08lx\n", xcp->cp0_epc);
 	MIPS_FPU_EMU_INC_STATS(ds_emul);
 	return true;
 }
	// SPDX-License-Identifier: GPL-2.0
	#include <linux/err.h>
	#include <linux/slab.h>
	#include <linux/mm_types.h>
	#include <linux/sched/task.h>

	#include <asm/branch.h>
	#include <asm/cacheflush.h>
	#include <asm/fpu_emulator.h>
	#include <asm/inst.h>
	#include <asm/mipsregs.h>
	#include <linux/uaccess.h>

	/**
	* struct emuframe - The 'emulation' frame structure
	* @emul: The instruction to 'emulate'.
	* @badinst: A break instruction to cause a return to the kernel.
	*
	* This structure defines the frames placed within the delay slot emulation
	* page in response to a call to mips_dsemul(). Each thread may be allocated
	* only one frame at any given time. The kernel stores within it the
	* instruction to be 'emulated' followed by a break instruction, then
	* executes the frame in user mode. The break causes a trap to the kernel
	* which leads to do_dsemulret() being called unless the instruction in
	* @emul causes a trap itself, is a branch, or a signal is delivered to
	* the thread. In these cases the allocated frame will either be reused by
	* a subsequent delay slot 'emulation', or be freed during signal delivery or
	* upon thread exit.
	*
	* This approach is used because:
	*
	* - Actually emulating all instructions isn't feasible. We would need to
	* be able to handle instructions from all revisions of the MIPS ISA,
	* all ASEs & all vendor instruction set extensions. This would be a
	* whole lot of work & continual maintenance burden as new instructions
	* are introduced, and in the case of some vendor extensions may not
	* even be possible. Thus we need to take the approach of actually
	* executing the instruction.
	*
	* - We must execute the instruction within user context. If we were to
	* execute the instruction in kernel mode then it would have access to
	* kernel resources without very careful checks, leaving us with a
	* high potential for security or stability issues to arise.
	*
	* - We used to place the frame on the users stack, but this requires
	* that the stack be executable. This is bad for security so the
	* per-process page is now used instead.
	*
	* - The instruction in @emul may be something entirely invalid for a
	* delay slot. The user may (intentionally or otherwise) place a branch
	* in a delay slot, or a kernel mode instruction, or something else
	* which generates an exception. Thus we can't rely upon the break in
	* @badinst always being hit. For this reason we track the index of the
	* frame allocated to each thread, allowing us to clean it up at later
	* points such as signal delivery or thread exit.
	*
	* - The user may generate a fake struct emuframe if they wish, invoking
	* the BRK_MEMU break instruction themselves. We must therefore not
	* trust that BRK_MEMU means there's actually a valid frame allocated
	* to the thread, and must not allow the user to do anything they
	* couldn't already.
	*/
	struct emuframe {
	mips_instruction emul;
	mips_instruction badinst;
	};

	static const int emupage_frame_count = PAGE_SIZE / sizeof(struct emuframe);

	static inline __user struct emuframe *dsemul_page(void)
	{
	return (__user struct emuframe *)STACK_TOP;
	}

	static int alloc_emuframe(void)
	{
	mm_context_t *mm_ctx = &current->mm->context;
	int idx;

	retry:
	spin_lock(&mm_ctx->bd_emupage_lock);

	/* Ensure we have an allocation bitmap */
	if (!mm_ctx->bd_emupage_allocmap) {
	mm_ctx->bd_emupage_allocmap = bitmap_zalloc(emupage_frame_count,
	GFP_ATOMIC);
	if (!mm_ctx->bd_emupage_allocmap) {
	idx = BD_EMUFRAME_NONE;
	goto out_unlock;
	}
	}

	/* Attempt to allocate a single bit/frame */
	idx = bitmap_find_free_region(mm_ctx->bd_emupage_allocmap,
	emupage_frame_count, 0);
	if (idx < 0) {
	/*
	* Failed to allocate a frame. We'll wait until one becomes
	* available. We unlock the page so that other threads actually
	* get the opportunity to free their frames, which means
	* technically the result of bitmap_full may be incorrect.
	* However the worst case is that we repeat all this and end up
	* back here again.
	*/
	spin_unlock(&mm_ctx->bd_emupage_lock);
	if (!wait_event_killable(mm_ctx->bd_emupage_queue,
	!bitmap_full(mm_ctx->bd_emupage_allocmap,
	emupage_frame_count)))
	goto retry;

	/* Received a fatal signal - just give in */
	return BD_EMUFRAME_NONE;
	}

	/* Success! */
	pr_debug("allocate emuframe %d to %d\n", idx, current->pid);
	out_unlock:
	spin_unlock(&mm_ctx->bd_emupage_lock);
	return idx;
	}

	static void free_emuframe(int idx, struct mm_struct *mm)
	{
	mm_context_t *mm_ctx = &mm->context;

	spin_lock(&mm_ctx->bd_emupage_lock);

	pr_debug("free emuframe %d from %d\n", idx, current->pid);
	bitmap_clear(mm_ctx->bd_emupage_allocmap, idx, 1);

	/* If some thread is waiting for a frame, now's its chance */
	wake_up(&mm_ctx->bd_emupage_queue);

	spin_unlock(&mm_ctx->bd_emupage_lock);
	}

	static bool within_emuframe(struct pt_regs *regs)
	{
	unsigned long base = (unsigned long)dsemul_page();

	if (regs->cp0_epc < base)
	return false;
	if (regs->cp0_epc >= (base + PAGE_SIZE))
	return false;

	return true;
	}

	bool dsemul_thread_cleanup(struct task_struct *tsk)
	{
	int fr_idx;

	/* Clear any allocated frame, retrieving its index */
	fr_idx = atomic_xchg(&tsk->thread.bd_emu_frame, BD_EMUFRAME_NONE);

	/* If no frame was allocated, we're done */
	if (fr_idx == BD_EMUFRAME_NONE)
	return false;

	task_lock(tsk);

	/* Free the frame that this thread had allocated */
	if (tsk->mm)
	free_emuframe(fr_idx, tsk->mm);

	task_unlock(tsk);
	return true;
	}

	bool dsemul_thread_rollback(struct pt_regs *regs)
	{
	struct emuframe __user *fr;
	int fr_idx;

	/* Do nothing if we're not executing from a frame */
	if (!within_emuframe(regs))
	return false;

	/* Find the frame being executed */
	fr_idx = atomic_read(&current->thread.bd_emu_frame);
	if (fr_idx == BD_EMUFRAME_NONE)
	return false;
	fr = &dsemul_page()[fr_idx];

	/*
	* If the PC is at the emul instruction, roll back to the branch. If
	* PC is at the badinst (break) instruction, we've already emulated the
	* instruction so progress to the continue PC. If it's anything else
	* then something is amiss & the user has branched into some other area
	* of the emupage - we'll free the allocated frame anyway.
	*/
	if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->emul)
	regs->cp0_epc = current->thread.bd_emu_branch_pc;
	else if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->badinst)
	regs->cp0_epc = current->thread.bd_emu_cont_pc;

	atomic_set(&current->thread.bd_emu_frame, BD_EMUFRAME_NONE);
	free_emuframe(fr_idx, current->mm);
	return true;
	}

	void dsemul_mm_cleanup(struct mm_struct *mm)
	{
	mm_context_t *mm_ctx = &mm->context;

	bitmap_free(mm_ctx->bd_emupage_allocmap);
	}

	int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
	unsigned long branch_pc, unsigned long cont_pc)
	{
	int isa16 = get_isa16_mode(regs->cp0_epc);
	mips_instruction break_math;
	unsigned long fr_uaddr;
	struct emuframe fr;
	int fr_idx, ret;

	/* NOP is easy */
	if (ir == 0)
	return -1;

	/* microMIPS instructions */
	if (isa16) {
	union mips_instruction insn = { .word = ir };

	/* NOP16 aka MOVE16 $0, $0 */
	if ((ir >> 16) == MM_NOP16)
	return -1;

	/* ADDIUPC */
	if (insn.mm_a_format.opcode == mm_addiupc_op) {
	unsigned int rs;
	s32 v;

	rs = (((insn.mm_a_format.rs + 0xe) & 0xf) + 2);
	v = regs->cp0_epc & ~3;
	v += insn.mm_a_format.simmediate << 2;
	regs->regs[rs] = (long)v;
	return -1;
	}
	}

	pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc);

	/* Allocate a frame if we don't already have one */
	fr_idx = atomic_read(&current->thread.bd_emu_frame);
	if (fr_idx == BD_EMUFRAME_NONE)
	fr_idx = alloc_emuframe();
	if (fr_idx == BD_EMUFRAME_NONE)
	return SIGBUS;

	/* Retrieve the appropriately encoded break instruction */
	break_math = BREAK_MATH(isa16);

	/* Write the instructions to the frame */
	if (isa16) {
	union mips_instruction _emul = {
	.halfword = { ir >> 16, ir }
	};
	union mips_instruction _badinst = {
	.halfword = { break_math >> 16, break_math }
	};

	fr.emul = _emul.word;
	fr.badinst = _badinst.word;
	} else {
	fr.emul = ir;
	fr.badinst = break_math;
	}

	/* Write the frame to user memory */
	fr_uaddr = (unsigned long)&dsemul_page()[fr_idx];
	ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr),
	FOLL_FORCE \| FOLL_WRITE);
	if (unlikely(ret != sizeof(fr))) {
	MIPS_FPU_EMU_INC_STATS(errors);
	free_emuframe(fr_idx, current->mm);
	return SIGBUS;
	}

	/* Record the PC of the branch, PC to continue from & frame index */
	current->thread.bd_emu_branch_pc = branch_pc;
	current->thread.bd_emu_cont_pc = cont_pc;
	atomic_set(&current->thread.bd_emu_frame, fr_idx);

	/* Change user register context to execute the frame */
	regs->cp0_epc = fr_uaddr \| isa16;

	return 0;
	}

	bool do_dsemulret(struct pt_regs *xcp)
	{
	/* Cleanup the allocated frame, returning if there wasn't one */
	if (!dsemul_thread_cleanup(current)) {
	MIPS_FPU_EMU_INC_STATS(errors);
	return false;
	}

	/* Set EPC to return to post-branch instruction */
	xcp->cp0_epc = current->thread.bd_emu_cont_pc;
	pr_debug("dsemulret to 0x%08lx\n", xcp->cp0_epc);
	MIPS_FPU_EMU_INC_STATS(ds_emul);
	return true;
	}