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
dsemul_page(void)70 static inline __user struct emuframe *dsemul_page(void)
71 {
72 return (__user struct emuframe *)STACK_TOP;
73 }
74
alloc_emuframe(void)75 static int alloc_emuframe(void)
76 {
77 mm_context_t *mm_ctx = ¤t->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 = bitmap_zalloc(emupage_frame_count,
86 GFP_ATOMIC);
87 if (!mm_ctx->bd_emupage_allocmap) {
88 idx = BD_EMUFRAME_NONE;
89 goto out_unlock;
90 }
91 }
92
93 /* Attempt to allocate a single bit/frame */
94 idx = bitmap_find_free_region(mm_ctx->bd_emupage_allocmap,
95 emupage_frame_count, 0);
96 if (idx < 0) {
97 /*
98 * Failed to allocate a frame. We'll wait until one becomes
99 * available. We unlock the page so that other threads actually
100 * get the opportunity to free their frames, which means
101 * technically the result of bitmap_full may be incorrect.
102 * However the worst case is that we repeat all this and end up
103 * back here again.
104 */
105 spin_unlock(&mm_ctx->bd_emupage_lock);
106 if (!wait_event_killable(mm_ctx->bd_emupage_queue,
107 !bitmap_full(mm_ctx->bd_emupage_allocmap,
108 emupage_frame_count)))
109 goto retry;
110
111 /* Received a fatal signal - just give in */
112 return BD_EMUFRAME_NONE;
113 }
114
115 /* Success! */
116 pr_debug("allocate emuframe %d to %d\n", idx, current->pid);
117 out_unlock:
118 spin_unlock(&mm_ctx->bd_emupage_lock);
119 return idx;
120 }
121
free_emuframe(int idx,struct mm_struct * mm)122 static void free_emuframe(int idx, struct mm_struct *mm)
123 {
124 mm_context_t *mm_ctx = &mm->context;
125
126 spin_lock(&mm_ctx->bd_emupage_lock);
127
128 pr_debug("free emuframe %d from %d\n", idx, current->pid);
129 bitmap_clear(mm_ctx->bd_emupage_allocmap, idx, 1);
130
131 /* If some thread is waiting for a frame, now's its chance */
132 wake_up(&mm_ctx->bd_emupage_queue);
133
134 spin_unlock(&mm_ctx->bd_emupage_lock);
135 }
136
within_emuframe(struct pt_regs * regs)137 static bool within_emuframe(struct pt_regs *regs)
138 {
139 unsigned long base = (unsigned long)dsemul_page();
140
141 if (regs->cp0_epc < base)
142 return false;
143 if (regs->cp0_epc >= (base + PAGE_SIZE))
144 return false;
145
146 return true;
147 }
148
dsemul_thread_cleanup(struct task_struct * tsk)149 bool dsemul_thread_cleanup(struct task_struct *tsk)
150 {
151 int fr_idx;
152
153 /* Clear any allocated frame, retrieving its index */
154 fr_idx = atomic_xchg(&tsk->thread.bd_emu_frame, BD_EMUFRAME_NONE);
155
156 /* If no frame was allocated, we're done */
157 if (fr_idx == BD_EMUFRAME_NONE)
158 return false;
159
160 task_lock(tsk);
161
162 /* Free the frame that this thread had allocated */
163 if (tsk->mm)
164 free_emuframe(fr_idx, tsk->mm);
165
166 task_unlock(tsk);
167 return true;
168 }
169
dsemul_thread_rollback(struct pt_regs * regs)170 bool dsemul_thread_rollback(struct pt_regs *regs)
171 {
172 struct emuframe __user *fr;
173 int fr_idx;
174
175 /* Do nothing if we're not executing from a frame */
176 if (!within_emuframe(regs))
177 return false;
178
179 /* Find the frame being executed */
180 fr_idx = atomic_read(¤t->thread.bd_emu_frame);
181 if (fr_idx == BD_EMUFRAME_NONE)
182 return false;
183 fr = &dsemul_page()[fr_idx];
184
185 /*
186 * If the PC is at the emul instruction, roll back to the branch. If
187 * PC is at the badinst (break) instruction, we've already emulated the
188 * instruction so progress to the continue PC. If it's anything else
189 * then something is amiss & the user has branched into some other area
190 * of the emupage - we'll free the allocated frame anyway.
191 */
192 if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->emul)
193 regs->cp0_epc = current->thread.bd_emu_branch_pc;
194 else if (msk_isa16_mode(regs->cp0_epc) == (unsigned long)&fr->badinst)
195 regs->cp0_epc = current->thread.bd_emu_cont_pc;
196
197 atomic_set(¤t->thread.bd_emu_frame, BD_EMUFRAME_NONE);
198 free_emuframe(fr_idx, current->mm);
199 return true;
200 }
201
dsemul_mm_cleanup(struct mm_struct * mm)202 void dsemul_mm_cleanup(struct mm_struct *mm)
203 {
204 mm_context_t *mm_ctx = &mm->context;
205
206 bitmap_free(mm_ctx->bd_emupage_allocmap);
207 }
208
mips_dsemul(struct pt_regs * regs,mips_instruction ir,unsigned long branch_pc,unsigned long cont_pc)209 int mips_dsemul(struct pt_regs *regs, mips_instruction ir,
210 unsigned long branch_pc, unsigned long cont_pc)
211 {
212 int isa16 = get_isa16_mode(regs->cp0_epc);
213 mips_instruction break_math;
214 unsigned long fr_uaddr;
215 struct emuframe fr;
216 int fr_idx, ret;
217
218 /* NOP is easy */
219 if (ir == 0)
220 return -1;
221
222 /* microMIPS instructions */
223 if (isa16) {
224 union mips_instruction insn = { .word = ir };
225
226 /* NOP16 aka MOVE16 $0, $0 */
227 if ((ir >> 16) == MM_NOP16)
228 return -1;
229
230 /* ADDIUPC */
231 if (insn.mm_a_format.opcode == mm_addiupc_op) {
232 unsigned int rs;
233 s32 v;
234
235 rs = (((insn.mm_a_format.rs + 0xe) & 0xf) + 2);
236 v = regs->cp0_epc & ~3;
237 v += insn.mm_a_format.simmediate << 2;
238 regs->regs[rs] = (long)v;
239 return -1;
240 }
241 }
242
243 pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc);
244
245 /* Allocate a frame if we don't already have one */
246 fr_idx = atomic_read(¤t->thread.bd_emu_frame);
247 if (fr_idx == BD_EMUFRAME_NONE)
248 fr_idx = alloc_emuframe();
249 if (fr_idx == BD_EMUFRAME_NONE)
250 return SIGBUS;
251
252 /* Retrieve the appropriately encoded break instruction */
253 break_math = BREAK_MATH(isa16);
254
255 /* Write the instructions to the frame */
256 if (isa16) {
257 union mips_instruction _emul = {
258 .halfword = { ir >> 16, ir }
259 };
260 union mips_instruction _badinst = {
261 .halfword = { break_math >> 16, break_math }
262 };
263
264 fr.emul = _emul.word;
265 fr.badinst = _badinst.word;
266 } else {
267 fr.emul = ir;
268 fr.badinst = break_math;
269 }
270
271 /* Write the frame to user memory */
272 fr_uaddr = (unsigned long)&dsemul_page()[fr_idx];
273 ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr),
274 FOLL_FORCE | FOLL_WRITE);
275 if (unlikely(ret != sizeof(fr))) {
276 MIPS_FPU_EMU_INC_STATS(errors);
277 free_emuframe(fr_idx, current->mm);
278 return SIGBUS;
279 }
280
281 /* Record the PC of the branch, PC to continue from & frame index */
282 current->thread.bd_emu_branch_pc = branch_pc;
283 current->thread.bd_emu_cont_pc = cont_pc;
284 atomic_set(¤t->thread.bd_emu_frame, fr_idx);
285
286 /* Change user register context to execute the frame */
287 regs->cp0_epc = fr_uaddr | isa16;
288
289 return 0;
290 }
291
do_dsemulret(struct pt_regs * xcp)292 bool do_dsemulret(struct pt_regs *xcp)
293 {
294 /* Cleanup the allocated frame, returning if there wasn't one */
295 if (!dsemul_thread_cleanup(current)) {
296 MIPS_FPU_EMU_INC_STATS(errors);
297 return false;
298 }
299
300 /* Set EPC to return to post-branch instruction */
301 xcp->cp0_epc = current->thread.bd_emu_cont_pc;
302 pr_debug("dsemulret to 0x%08lx\n", xcp->cp0_epc);
303 MIPS_FPU_EMU_INC_STATS(ds_emul);
304 return true;
305 }
306