1 files changed, 223 insertions, 109 deletions

diff --git a/arch/mips/math-emu/dsemul.c b/arch/mips/math-emu/dsemul.c
index 47074887e..4a094f7ac 100644
--- a/arch/mips/math-emu/dsemul.c
+++ b/arch/mips/math-emu/dsemul.c
@@ -1,3 +1,6 @@
+#include <linux/err.h>
+#include <linux/slab.h>
+
 #include <asm/branch.h>
 #include <asm/cacheflush.h>
 #include <asm/fpu_emulator.h>
@@ -5,43 +8,211 @@
 #include <asm/mipsregs.h>
 #include <asm/uaccess.h>
 
-#include "ieee754.h"
-
-/*
- * Emulate the arbitrary instruction ir at xcp->cp0_epc.  Required when
- * we have to emulate the instruction in a COP1 branch delay slot.  Do
- * not change cp0_epc due to the instruction
+/**
+ * struct emuframe - The 'emulation' frame structure
+ * @emul:	The instruction to 'emulate'.
+ * @badinst:	A break instruction to cause a return to the kernel.
  *
- * According to the spec:
- * 1) it shouldn't be a branch :-)
- * 2) it can be a COP instruction :-(
- * 3) if we are tring to run a protected memory space we must take
- *    special care on memory access instructions :-(
- */
-
-/*
- * "Trampoline" return routine to catch exception following
- *  execution of delay-slot instruction execution.
+ * 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;
-	mips_instruction	cookie;
-	unsigned long		epc;
 };
 
-/*
- * Set up an emulation frame for instruction IR, from a delay slot of
- * a branch jumping to CPC.  Return 0 if successful, -1 if no emulation
- * required, otherwise a signal number causing a frame setup failure.
- */
-int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
+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 =
+			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(&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;
+
+	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;
 	struct emuframe __user *fr;
-	int err;
+	int err, fr_idx;
 
 	/* NOP is easy */
 	if (ir == 0)
@@ -68,30 +239,20 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 		}
 	}
 
-	pr_debug("dsemul %lx %lx\n", regs->cp0_epc, cpc);
+	pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc);
 
-	/*
-	 * The strategy is to push the instruction onto the user stack
-	 * and put a trap after it which we can catch and jump to
-	 * the required address any alternative apart from full
-	 * instruction emulation!!.
-	 *
-	 * Algorithmics used a system call instruction, and
-	 * borrowed that vector.  MIPS/Linux version is a bit
-	 * more heavyweight in the interests of portability and
-	 * multiprocessor support.  For Linux we use a BREAK 514
-	 * instruction causing a breakpoint exception.
-	 */
-	break_math = BREAK_MATH(isa16);
-
-	/* Ensure that the two instructions are in the same cache line */
-	fr = (struct emuframe __user *)
-		((regs->regs[29] - sizeof(struct emuframe)) & ~0x7);
-
-	/* Verify that the stack pointer is not completely insane */
-	if (unlikely(!access_ok(VERIFY_WRITE, fr, sizeof(struct emuframe))))
+	/* 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;
+	fr = &dsemul_page()[fr_idx];
+
+	/* Retrieve the appropriately encoded break instruction */
+	break_math = BREAK_MATH(isa16);
 
+	/* Write the instructions to the frame */
 	if (isa16) {
 		err = __put_user(ir >> 16,
 				 (u16 __user *)(&fr->emul));
@@ -106,84 +267,37 @@ int mips_dsemul(struct pt_regs *regs, mips_instruction ir, unsigned long cpc)
 		err |= __put_user(break_math, &fr->badinst);
 	}
 
-	err |= __put_user((mips_instruction)BD_COOKIE, &fr->cookie);
-	err |= __put_user(cpc, &fr->epc);
-
 	if (unlikely(err)) {
 		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 = (unsigned long)&fr->emul | isa16;
 
+	/* Ensure the icache observes our newly written frame */
 	flush_cache_sigtramp((unsigned long)&fr->emul);
 
 	return 0;
 }
 
-int do_dsemulret(struct pt_regs *xcp)
+bool do_dsemulret(struct pt_regs *xcp)
 {
-	int isa16 = get_isa16_mode(xcp->cp0_epc);
-	struct emuframe __user *fr;
-	unsigned long epc;
-	u32 insn, cookie;
-	int err = 0;
-	u16 instr[2];
-
-	fr = (struct emuframe __user *)
-		(msk_isa16_mode(xcp->cp0_epc) - sizeof(mips_instruction));
-
-	/*
-	 * If we can't even access the area, something is very wrong, but we'll
-	 * leave that to the default handling
-	 */
-	if (!access_ok(VERIFY_READ, fr, sizeof(struct emuframe)))
-		return 0;
-
-	/*
-	 * Do some sanity checking on the stackframe:
-	 *
-	 *  - Is the instruction pointed to by the EPC an BREAK_MATH?
-	 *  - Is the following memory word the BD_COOKIE?
-	 */
-	if (isa16) {
-		err = __get_user(instr[0],
-				 (u16 __user *)(&fr->badinst));
-		err |= __get_user(instr[1],
-				  (u16 __user *)((long)(&fr->badinst) + 2));
-		insn = (instr[0] << 16) | instr[1];
-	} else {
-		err = __get_user(insn, &fr->badinst);
-	}
-	err |= __get_user(cookie, &fr->cookie);
-
-	if (unlikely(err ||
-		     insn != BREAK_MATH(isa16) || cookie != BD_COOKIE)) {
+	/* Cleanup the allocated frame, returning if there wasn't one */
+	if (!dsemul_thread_cleanup(current)) {
 		MIPS_FPU_EMU_INC_STATS(errors);
-		return 0;
-	}
-
-	/*
-	 * At this point, we are satisfied that it's a BD emulation trap.  Yes,
-	 * a user might have deliberately put two malformed and useless
-	 * instructions in a row in his program, in which case he's in for a
-	 * nasty surprise - the next instruction will be treated as a
-	 * continuation address!  Alas, this seems to be the only way that we
-	 * can handle signals, recursion, and longjmps() in the context of
-	 * emulating the branch delay instruction.
-	 */
-
-	pr_debug("dsemulret\n");
-
-	if (__get_user(epc, &fr->epc)) {		/* Saved EPC */
-		/* This is not a good situation to be in */
-		force_sig(SIGBUS, current);
-
-		return 0;
+		return false;
 	}
 
 	/* Set EPC to return to post-branch instruction */
-	xcp->cp0_epc = epc;
+	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 1;
+	return true;
 }