dsemul.c

// 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;
}