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Diffstat (limited to '')
| -rw-r--r-- | arch/mips/math-emu/dsemul.c | 305 |
1 files changed, 305 insertions, 0 deletions
diff --git a/arch/mips/math-emu/dsemul.c b/arch/mips/math-emu/dsemul.c new file mode 100644 index 000000000..e02bd20b6 --- /dev/null +++ b/arch/mips/math-emu/dsemul.c @@ -0,0 +1,305 @@ +// SPDX-License-Identifier: GPL-2.0 +#include <linux/err.h> +#include <linux/slab.h> +#include <linux/mm_types.h> +#include <linux/sched/task.h> + +#include <asm/branch.h> +#include <asm/cacheflush.h> +#include <asm/fpu_emulator.h> +#include <asm/inst.h> +#include <asm/mipsregs.h> +#include <linux/uaccess.h> + +/** + * struct emuframe - The 'emulation' frame structure + * @emul: The instruction to 'emulate'. + * @badinst: A break instruction to cause a return to the kernel. + * + * 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; +}; + +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 = bitmap_zalloc(emupage_frame_count, + 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; + + bitmap_free(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; + unsigned long fr_uaddr; + struct emuframe fr; + int fr_idx, ret; + + /* NOP is easy */ + if (ir == 0) + return -1; + + /* microMIPS instructions */ + if (isa16) { + union mips_instruction insn = { .word = ir }; + + /* NOP16 aka MOVE16 $0, $0 */ + if ((ir >> 16) == MM_NOP16) + return -1; + + /* ADDIUPC */ + if (insn.mm_a_format.opcode == mm_addiupc_op) { + unsigned int rs; + s32 v; + + rs = (((insn.mm_a_format.rs + 0xe) & 0xf) + 2); + v = regs->cp0_epc & ~3; + v += insn.mm_a_format.simmediate << 2; + regs->regs[rs] = (long)v; + return -1; + } + } + + pr_debug("dsemul 0x%08lx cont at 0x%08lx\n", regs->cp0_epc, cont_pc); + + /* 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; + + /* Retrieve the appropriately encoded break instruction */ + break_math = BREAK_MATH(isa16); + + /* Write the instructions to the frame */ + if (isa16) { + union mips_instruction _emul = { + .halfword = { ir >> 16, ir } + }; + union mips_instruction _badinst = { + .halfword = { break_math >> 16, break_math } + }; + + fr.emul = _emul.word; + fr.badinst = _badinst.word; + } else { + fr.emul = ir; + fr.badinst = break_math; + } + + /* Write the frame to user memory */ + fr_uaddr = (unsigned long)&dsemul_page()[fr_idx]; + ret = access_process_vm(current, fr_uaddr, &fr, sizeof(fr), + FOLL_FORCE | FOLL_WRITE); + if (unlikely(ret != sizeof(fr))) { + 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 = fr_uaddr | isa16; + + return 0; +} + +bool do_dsemulret(struct pt_regs *xcp) +{ + /* Cleanup the allocated frame, returning if there wasn't one */ + if (!dsemul_thread_cleanup(current)) { + MIPS_FPU_EMU_INC_STATS(errors); + return false; + } + + /* Set EPC to return to post-branch instruction */ + 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 true; +} |