arch/mips/math-emu/dsemul.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/err.h>
   3 #include <linux/slab.h>
   4 #include <linux/mm_types.h>
   5 #include <linux/sched/task.h>
   6
   7 #include <asm/branch.h>
   8 #include <asm/cacheflush.h>
   9 #include <asm/fpu_emulator.h>
  10 #include <asm/inst.h>
  11 #include <asm/mipsregs.h>
  12 #include <linux/uaccess.h>
  13
  14 /**
  15  * struct emuframe - The 'emulation' frame structure
  16  * @emul:       The instruction to 'emulate'.
  17  * @badinst:    A break instruction to cause a return to the kernel.
  18  *
  19  * This structure defines the frames placed within the delay slot emulation
  20  * page in response to a call to mips_dsemul(). Each thread may be allocated
  21  * only one frame at any given time. The kernel stores within it the
  22  * instruction to be 'emulated' followed by a break instruction, then
  23  * executes the frame in user mode. The break causes a trap to the kernel
  24  * which leads to do_dsemulret() being called unless the instruction in
  25  * @emul causes a trap itself, is a branch, or a signal is delivered to
  26  * the thread. In these cases the allocated frame will either be reused by
  27  * a subsequent delay slot 'emulation', or be freed during signal delivery or
  28  * upon thread exit.
  29  *
  30  * This approach is used because:
  31  *
  32  * - Actually emulating all instructions isn't feasible. We would need to
  33  *   be able to handle instructions from all revisions of the MIPS ISA,
  34  *   all ASEs & all vendor instruction set extensions. This would be a
  35  *   whole lot of work & continual maintenance burden as new instructions
  36  *   are introduced, and in the case of some vendor extensions may not
  37  *   even be possible. Thus we need to take the approach of actually
  38  *   executing the instruction.
  39  *
  40  * - We must execute the instruction within user context. If we were to
  41  *   execute the instruction in kernel mode then it would have access to
  42  *   kernel resources without very careful checks, leaving us with a
  43  *   high potential for security or stability issues to arise.
  44  *
  45  * - We used to place the frame on the users stack, but this requires
  46  *   that the stack be executable. This is bad for security so the
  47  *   per-process page is now used instead.
  48  *
  49  * - The instruction in @emul may be something entirely invalid for a
  50  *   delay slot. The user may (intentionally or otherwise) place a branch
  51  *   in a delay slot, or a kernel mode instruction, or something else
  52  *   which generates an exception. Thus we can't rely upon the break in
  53  *   @badinst always being hit. For this reason we track the index of the
  54  *   frame allocated to each thread, allowing us to clean it up at later
  55  *   points such as signal delivery or thread exit.
  56  *
  57  * - The user may generate a fake struct emuframe if they wish, invoking
  58  *   the BRK_MEMU break instruction themselves. We must therefore not
  59  *   trust that BRK_MEMU means there's actually a valid frame allocated
  60  *   to the thread, and must not allow the user to do anything they
  61  *   couldn't already.
  62  */
  63 struct emuframe {
  64         mips_instruction        emul;
  65         mips_instruction        badinst;
  66 };
  67
  68 static const int emupage_frame_count = PAGE_SIZE / sizeof(struct emuframe);
  69
  70 static inline __user struct emuframe *dsemul_page(void)
  71 {
  72         return (__user struct emuframe *)STACK_TOP;
  73 }
  74
  75 static int alloc_emuframe(void)
  76 {
  77         mm_context_t *mm_ctx = &current->mm->context;
  78         int idx;
  79
  80 retry:
  81         spin_lock(&mm_ctx->bd_emupage_lock);
  82
  83         /* Ensure we have an allocation bitmap */
  84         if (!mm_ctx->bd_emupage_allocmap) {
  85                 mm_ctx->bd_emupage_allocmap =
  86                         kcalloc(BITS_TO_LONGS(emupage_frame_count),
  87                                               sizeof(unsigned long),
  88                                 GFP_ATOMIC);
  89
  90                 if (!mm_ctx->bd_emupage_allocmap) {
  91                         idx = BD_EMUFRAME_NONE;
  92                         goto out_unlock;
  93                 }
  94         }
  95
  96         /* Attempt to allocate a single bit/frame */
  97         idx = bitmap_find_free_region(mm_ctx->bd_emupage_allocmap,
  98                                       emupage_frame_count, 0);
  99         if (idx < 0) {
 100                 /*
 101                  * Failed to allocate a frame. We'll wait until one becomes
 102                  * available. We unlock the page so that other threads actually
 103                  * get the opportunity to free their frames, which means
 104                  * technically the result of bitmap_full may be incorrect.
 105                  * However the worst case is that we repeat all this and end up
 106                  * back here again.
 107                  */
 108                 spin_unlock(&mm_ctx->bd_emupage_lock);
 109                 if (!wait_event_killable(mm_ctx->bd_emupage_queue,
 110                         !bitmap_full(mm_ctx->bd_emupage_allocmap,
 111                                      emupage_frame_count)))
 112                         goto retry;
 113
 114                 /* Received a fatal signal - just give in */
 115                 return BD_EMUFRAME_NONE;
 116         }
 117
 118         /* Success! */
 119         pr_debug("allocate emuframe %d to %d\n", idx, current->pid);
 120 out_unlock:
 121         spin_unlock(&mm_ctx->bd_emupage_lock);
 122         return idx;
 123 }
 124
 125 static void free_emuframe(int idx, struct mm_struct *mm)
 126 {
 127         mm_context_t *mm_ctx = &mm->context;
 128
 129         spin_lock(&mm_ctx->bd_emupage_lock);
 130
 131         pr_debug("free emuframe %d from %d\n", idx, current->pid);
 132         bitmap_clear(mm_ctx->bd_emupage_allocmap, idx, 1);
 133
 134         /* If some thread is waiting for a frame, now's its chance */
 135         wake_up(&mm_ctx->bd_emupage_queue);
 136
 137         spin_unlock(&mm_ctx->bd_emupage_lock);
 138 }
 139
 140 static bool within_emuframe(struct pt_regs *regs)
 141 {
 142         unsigned long base = (unsigned long)dsemul_page();
 143
 144         if (regs->cp0_epc < base)
 145                 return false;
 146         if (regs->cp0_epc >= (base + PAGE_SIZE))
 147                 return false;
 148
 149         return true;
 150 }
 151
 152 bool dsemul_thread_cleanup(struct task_struct *tsk)
 153 {
 154         int fr_idx;
 155
 156         /* Clear any allocated frame, retrieving its index */
 157         fr_idx = atomic_xchg(&tsk->thread.bd_emu_frame, BD_EMUFRAME_NONE);
 158
 159         /* If no frame was allocated, we're done */
 160         if (fr_idx == BD_EMUFRAME_NONE)
 161                 return false;
 162
 163         task_lock(tsk);
 164
 165         /* Free the frame that this thread had allocated */
 166         if (tsk->mm)
 167                 free_emuframe(fr_idx, tsk->mm);
 168
 169         task_unlock(tsk);
 170         return true;
 171 }
 172
 173 bool dsemul_thread_rollback(struct pt_regs *regs)
 174 {
 175         struct emuframe __user *fr;
 176         int fr_idx;
 177
 178         /* Do nothing if we're not executing from a frame */
 179         if (!within_emuframe(regs))
 180                 return false;
 181
 182         /* Find the frame being executed */
 183         fr_idx = atomic_read(&current->thread.bd_emu_frame);
 184         if (fr_idx == BD_EMUFRAME_NONE)
 185                 return false;
 186         fr = &dsemul_page()[fr_idx];
 187
 188         /*
 189          * If the PC is at the emul instruction, roll back to the branch. If
 190          * PC is at the badinst (break) instruction, we've already emulated the
 191          * instruction so progress to the continue PC. If it's anything else
 192          * then something is amiss & the user has branched into some other area
 193          * of the emupage - we'll free the allocated frame anyway.
 194          */
 195         if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->emul)
 196                 regs->cp0_epc = current->thread.bd_emu_branch_pc;
 197         else if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->badinst)
 198                 regs->cp0_epc = current->thread.bd_emu_cont_pc;
 199
 200         atomic_set(&current->thread.bd_emu_frame, BD_EMUFRAME_NONE);
 201         free_emuframe(fr_idx, current->mm);
 202         return true;
 203 }
 204
 205 void dsemul_mm_cleanup(struct mm_struct *mm)
 206 {
 207         mm_context_t *mm_ctx = &mm->context;
 208
 209         kfree(mm_ctx->bd_emupage_allocmap);
 210 }
 211
 212 int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
 213                 unsigned long branch_pc, unsigned long cont_pc)
 214 {
 215         int isa16 = get_isa16_mode(regs->cp0_epc);
 216         mips_instruction break_math;
 217         struct emuframe __user *fr;
 218         int err, fr_idx;
 219
 220         /* NOP is easy */
 221         if (ir == 0)
 222                 return -1;
 223
 224         /* microMIPS instructions */
 225         if (isa16) {
 226                 union mips_instruction insn = { .word = ir };
 227
 228                 /* NOP16 aka MOVE16 $0, $0 */
 229                 if ((ir >> 16) == MM_NOP16)
 230                         return -1;
 231
 232                 /* ADDIUPC */
 233                 if (insn.mm_a_format.opcode == mm_addiupc_op) {
 234                         unsigned int rs;
 235                         s32 v;
 236
 237                         rs = (((insn.mm_a_format.rs + 0xe) & 0xf) + 2);
 238                         v = regs->cp0_epc & ~3;
 239                         v += insn.mm_a_format.simmediate << 2;
 240                         regs->regs[rs] = (long)v;
 241                         return -1;
 242                 }
 243         }
 244
 245         pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc);
 246
 247         /* Allocate a frame if we don't already have one */
 248         fr_idx = atomic_read(&current->thread.bd_emu_frame);
 249         if (fr_idx == BD_EMUFRAME_NONE)
 250                 fr_idx = alloc_emuframe();
 251         if (fr_idx == BD_EMUFRAME_NONE)
 252                 return SIGBUS;
 253         fr = &dsemul_page()[fr_idx];
 254
 255         /* Retrieve the appropriately encoded break instruction */
 256         break_math = BREAK_MATH(isa16);
 257
 258         /* Write the instructions to the frame */
 259         if (isa16) {
 260                 err = __put_user(ir >> 16,
 261                                  (u16 __user *)(&fr->emul));
 262                 err |= __put_user(ir & 0xffff,
 263                                   (u16 __user *)((long)(&fr->emul) + 2));
 264                 err |= __put_user(break_math >> 16,
 265                                   (u16 __user *)(&fr->badinst));
 266                 err |= __put_user(break_math & 0xffff,
 267                                   (u16 __user *)((long)(&fr->badinst) + 2));
 268         } else {
 269                 err = __put_user(ir, &fr->emul);
 270                 err |= __put_user(break_math, &fr->badinst);
 271         }
 272
 273         if (unlikely(err)) {
 274                 MIPS_FPU_EMU_INC_STATS(errors);
 275                 free_emuframe(fr_idx, current->mm);
 276                 return SIGBUS;
 277         }
 278
 279         /* Record the PC of the branch, PC to continue from & frame index */
 280         current->thread.bd_emu_branch_pc = branch_pc;
 281         current->thread.bd_emu_cont_pc = cont_pc;
 282         atomic_set(&current->thread.bd_emu_frame, fr_idx);
 283
 284         /* Change user register context to execute the frame */
 285         regs->cp0_epc = (unsigned long)&fr->emul | isa16;
 286
 287         /* Ensure the icache observes our newly written frame */
 288         flush_cache_sigtramp((unsigned long)&fr->emul);
 289
 290         return 0;
 291 }
 292
 293 bool do_dsemulret(struct pt_regs *xcp)
 294 {
 295         /* Cleanup the allocated frame, returning if there wasn't one */
 296         if (!dsemul_thread_cleanup(current)) {
 297                 MIPS_FPU_EMU_INC_STATS(errors);
 298                 return false;
 299         }
 300
 301         /* Set EPC to return to post-branch instruction */
 302         xcp->cp0_epc = current->thread.bd_emu_cont_pc;
 303         pr_debug("dsemulret to 0x%08lx\n", xcp->cp0_epc);
 304         MIPS_FPU_EMU_INC_STATS(ds_emul);
 305         return true;
 306 }