+#include <linux/err.h>
+#include <linux/slab.h>
+
#include <asm/branch.h>
#include <asm/cacheflush.h>
#include <asm/fpu_emulator.h>
#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 = ¤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;
struct emuframe __user *fr;
- int err;
+ int err, fr_idx;
/* NOP is easy */
if (ir == 0)
}
}
- 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(¤t->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));
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(¤t->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;
- MIPS_FPU_EMU_INC_STATS(ds_emul);
- return 1;
+ xcp->cp0_epc = current->thread.bd_emu_cont_pc;
+ pr_debug("dsemulret to 0x%08lx\n", xcp->cp0_epc);
+ return true;
}
projects / cascardo / linux.git / blobdiff