// 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 = ¤t->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 =
kcalloc(BITS_TO_LONGS(emupage_frame_count),
sizeof(unsigned long),
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(¤t->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(¤t->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;
kfree(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(¤t->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(¤t->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;
}