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 = &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 = 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(&current->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(&current->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(&current->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(&current->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