/* * mpx.c - Memory Protection eXtensions * * Copyright (c) 2014, Intel Corporation. * Qiaowei Ren * Dave Hansen */ #include #include #include #include #include #include #include #include #include #include #define CREATE_TRACE_POINTS #include static const char *mpx_mapping_name(struct vm_area_struct *vma) { return "[mpx]"; } static struct vm_operations_struct mpx_vma_ops = { .name = mpx_mapping_name, }; static int is_mpx_vma(struct vm_area_struct *vma) { return (vma->vm_ops == &mpx_vma_ops); } /* * This is really a simplified "vm_mmap". it only handles MPX * bounds tables (the bounds directory is user-allocated). * * Later on, we use the vma->vm_ops to uniquely identify these * VMAs. */ static unsigned long mpx_mmap(unsigned long len) { unsigned long ret; unsigned long addr, pgoff; struct mm_struct *mm = current->mm; vm_flags_t vm_flags; struct vm_area_struct *vma; /* Only bounds table can be allocated here */ if (len != MPX_BT_SIZE_BYTES) return -EINVAL; down_write(&mm->mmap_sem); /* Too many mappings? */ if (mm->map_count > sysctl_max_map_count) { ret = -ENOMEM; goto out; } /* Obtain the address to map to. we verify (or select) it and ensure * that it represents a valid section of the address space. */ addr = get_unmapped_area(NULL, 0, len, 0, MAP_ANONYMOUS | MAP_PRIVATE); if (addr & ~PAGE_MASK) { ret = addr; goto out; } vm_flags = VM_READ | VM_WRITE | VM_MPX | mm->def_flags | VM_MAYREAD | VM_MAYWRITE | VM_MAYEXEC; /* Set pgoff according to addr for anon_vma */ pgoff = addr >> PAGE_SHIFT; ret = mmap_region(NULL, addr, len, vm_flags, pgoff); if (IS_ERR_VALUE(ret)) goto out; vma = find_vma(mm, ret); if (!vma) { ret = -ENOMEM; goto out; } vma->vm_ops = &mpx_vma_ops; if (vm_flags & VM_LOCKED) { up_write(&mm->mmap_sem); mm_populate(ret, len); return ret; } out: up_write(&mm->mmap_sem); return ret; } enum reg_type { REG_TYPE_RM = 0, REG_TYPE_INDEX, REG_TYPE_BASE, }; static int get_reg_offset(struct insn *insn, struct pt_regs *regs, enum reg_type type) { int regno = 0; static const int regoff[] = { offsetof(struct pt_regs, ax), offsetof(struct pt_regs, cx), offsetof(struct pt_regs, dx), offsetof(struct pt_regs, bx), offsetof(struct pt_regs, sp), offsetof(struct pt_regs, bp), offsetof(struct pt_regs, si), offsetof(struct pt_regs, di), #ifdef CONFIG_X86_64 offsetof(struct pt_regs, r8), offsetof(struct pt_regs, r9), offsetof(struct pt_regs, r10), offsetof(struct pt_regs, r11), offsetof(struct pt_regs, r12), offsetof(struct pt_regs, r13), offsetof(struct pt_regs, r14), offsetof(struct pt_regs, r15), #endif }; int nr_registers = ARRAY_SIZE(regoff); /* * Don't possibly decode a 32-bit instructions as * reading a 64-bit-only register. */ if (IS_ENABLED(CONFIG_X86_64) && !insn->x86_64) nr_registers -= 8; switch (type) { case REG_TYPE_RM: regno = X86_MODRM_RM(insn->modrm.value); if (X86_REX_B(insn->rex_prefix.value) == 1) regno += 8; break; case REG_TYPE_INDEX: regno = X86_SIB_INDEX(insn->sib.value); if (X86_REX_X(insn->rex_prefix.value) == 1) regno += 8; break; case REG_TYPE_BASE: regno = X86_SIB_BASE(insn->sib.value); if (X86_REX_B(insn->rex_prefix.value) == 1) regno += 8; break; default: pr_err("invalid register type"); BUG(); break; } if (regno > nr_registers) { WARN_ONCE(1, "decoded an instruction with an invalid register"); return -EINVAL; } return regoff[regno]; } /* * return the address being referenced be instruction * for rm=3 returning the content of the rm reg * for rm!=3 calculates the address using SIB and Disp */ static void __user *mpx_get_addr_ref(struct insn *insn, struct pt_regs *regs) { unsigned long addr, base, indx; int addr_offset, base_offset, indx_offset; insn_byte_t sib; insn_get_modrm(insn); insn_get_sib(insn); sib = insn->sib.value; if (X86_MODRM_MOD(insn->modrm.value) == 3) { addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM); if (addr_offset < 0) goto out_err; addr = regs_get_register(regs, addr_offset); } else { if (insn->sib.nbytes) { base_offset = get_reg_offset(insn, regs, REG_TYPE_BASE); if (base_offset < 0) goto out_err; indx_offset = get_reg_offset(insn, regs, REG_TYPE_INDEX); if (indx_offset < 0) goto out_err; base = regs_get_register(regs, base_offset); indx = regs_get_register(regs, indx_offset); addr = base + indx * (1 << X86_SIB_SCALE(sib)); } else { addr_offset = get_reg_offset(insn, regs, REG_TYPE_RM); if (addr_offset < 0) goto out_err; addr = regs_get_register(regs, addr_offset); } addr += insn->displacement.value; } return (void __user *)addr; out_err: return (void __user *)-1; } static int mpx_insn_decode(struct insn *insn, struct pt_regs *regs) { unsigned char buf[MAX_INSN_SIZE]; int x86_64 = !test_thread_flag(TIF_IA32); int not_copied; int nr_copied; not_copied = copy_from_user(buf, (void __user *)regs->ip, sizeof(buf)); nr_copied = sizeof(buf) - not_copied; /* * The decoder _should_ fail nicely if we pass it a short buffer. * But, let's not depend on that implementation detail. If we * did not get anything, just error out now. */ if (!nr_copied) return -EFAULT; insn_init(insn, buf, nr_copied, x86_64); insn_get_length(insn); /* * copy_from_user() tries to get as many bytes as we could see in * the largest possible instruction. If the instruction we are * after is shorter than that _and_ we attempt to copy from * something unreadable, we might get a short read. This is OK * as long as the read did not stop in the middle of the * instruction. Check to see if we got a partial instruction. */ if (nr_copied < insn->length) return -EFAULT; insn_get_opcode(insn); /* * We only _really_ need to decode bndcl/bndcn/bndcu * Error out on anything else. */ if (insn->opcode.bytes[0] != 0x0f) goto bad_opcode; if ((insn->opcode.bytes[1] != 0x1a) && (insn->opcode.bytes[1] != 0x1b)) goto bad_opcode; return 0; bad_opcode: return -EINVAL; } /* * If a bounds overflow occurs then a #BR is generated. This * function decodes MPX instructions to get violation address * and set this address into extended struct siginfo. * * Note that this is not a super precise way of doing this. * Userspace could have, by the time we get here, written * anything it wants in to the instructions. We can not * trust anything about it. They might not be valid * instructions or might encode invalid registers, etc... * * The caller is expected to kfree() the returned siginfo_t. */ siginfo_t *mpx_generate_siginfo(struct pt_regs *regs) { const struct bndreg *bndregs, *bndreg; siginfo_t *info = NULL; struct insn insn; uint8_t bndregno; int err; err = mpx_insn_decode(&insn, regs); if (err) goto err_out; /* * We know at this point that we are only dealing with * MPX instructions. */ insn_get_modrm(&insn); bndregno = X86_MODRM_REG(insn.modrm.value); if (bndregno > 3) { err = -EINVAL; goto err_out; } /* get bndregs field from current task's xsave area */ bndregs = get_xsave_field_ptr(XSTATE_BNDREGS); if (!bndregs) { err = -EINVAL; goto err_out; } /* now go select the individual register in the set of 4 */ bndreg = &bndregs[bndregno]; info = kzalloc(sizeof(*info), GFP_KERNEL); if (!info) { err = -ENOMEM; goto err_out; } /* * The registers are always 64-bit, but the upper 32 * bits are ignored in 32-bit mode. Also, note that the * upper bounds are architecturally represented in 1's * complement form. * * The 'unsigned long' cast is because the compiler * complains when casting from integers to different-size * pointers. */ info->si_lower = (void __user *)(unsigned long)bndreg->lower_bound; info->si_upper = (void __user *)(unsigned long)~bndreg->upper_bound; info->si_addr_lsb = 0; info->si_signo = SIGSEGV; info->si_errno = 0; info->si_code = SEGV_BNDERR; info->si_addr = mpx_get_addr_ref(&insn, regs); /* * We were not able to extract an address from the instruction, * probably because there was something invalid in it. */ if (info->si_addr == (void *)-1) { err = -EINVAL; goto err_out; } trace_mpx_bounds_register_exception(info->si_addr, bndreg); return info; err_out: /* info might be NULL, but kfree() handles that */ kfree(info); return ERR_PTR(err); } static __user void *mpx_get_bounds_dir(void) { const struct bndcsr *bndcsr; if (!cpu_feature_enabled(X86_FEATURE_MPX)) return MPX_INVALID_BOUNDS_DIR; /* * 32-bit binaries on 64-bit kernels are currently * unsupported. */ if (IS_ENABLED(CONFIG_X86_64) && test_thread_flag(TIF_IA32)) return MPX_INVALID_BOUNDS_DIR; /* * The bounds directory pointer is stored in a register * only accessible if we first do an xsave. */ bndcsr = get_xsave_field_ptr(XSTATE_BNDCSR); if (!bndcsr) return MPX_INVALID_BOUNDS_DIR; /* * Make sure the register looks valid by checking the * enable bit. */ if (!(bndcsr->bndcfgu & MPX_BNDCFG_ENABLE_FLAG)) return MPX_INVALID_BOUNDS_DIR; /* * Lastly, mask off the low bits used for configuration * flags, and return the address of the bounds table. */ return (void __user *)(unsigned long) (bndcsr->bndcfgu & MPX_BNDCFG_ADDR_MASK); } int mpx_enable_management(void) { void __user *bd_base = MPX_INVALID_BOUNDS_DIR; struct mm_struct *mm = current->mm; int ret = 0; /* * runtime in the userspace will be responsible for allocation of * the bounds directory. Then, it will save the base of the bounds * directory into XSAVE/XRSTOR Save Area and enable MPX through * XRSTOR instruction. * * The copy_xregs_to_kernel() beneath get_xsave_field_ptr() is * expected to be relatively expensive. Storing the bounds * directory here means that we do not have to do xsave in the * unmap path; we can just use mm->bd_addr instead. */ bd_base = mpx_get_bounds_dir(); down_write(&mm->mmap_sem); mm->bd_addr = bd_base; if (mm->bd_addr == MPX_INVALID_BOUNDS_DIR) ret = -ENXIO; up_write(&mm->mmap_sem); return ret; } int mpx_disable_management(void) { struct mm_struct *mm = current->mm; if (!cpu_feature_enabled(X86_FEATURE_MPX)) return -ENXIO; down_write(&mm->mmap_sem); mm->bd_addr = MPX_INVALID_BOUNDS_DIR; up_write(&mm->mmap_sem); return 0; } /* * With 32-bit mode, MPX_BT_SIZE_BYTES is 4MB, and the size of each * bounds table is 16KB. With 64-bit mode, MPX_BT_SIZE_BYTES is 2GB, * and the size of each bounds table is 4MB. */ static int allocate_bt(long __user *bd_entry) { unsigned long expected_old_val = 0; unsigned long actual_old_val = 0; unsigned long bt_addr; int ret = 0; /* * Carve the virtual space out of userspace for the new * bounds table: */ bt_addr = mpx_mmap(MPX_BT_SIZE_BYTES); if (IS_ERR((void *)bt_addr)) return PTR_ERR((void *)bt_addr); /* * Set the valid flag (kinda like _PAGE_PRESENT in a pte) */ bt_addr = bt_addr | MPX_BD_ENTRY_VALID_FLAG; /* * Go poke the address of the new bounds table in to the * bounds directory entry out in userspace memory. Note: * we may race with another CPU instantiating the same table. * In that case the cmpxchg will see an unexpected * 'actual_old_val'. * * This can fault, but that's OK because we do not hold * mmap_sem at this point, unlike some of the other part * of the MPX code that have to pagefault_disable(). */ ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry, expected_old_val, bt_addr); if (ret) goto out_unmap; /* * The user_atomic_cmpxchg_inatomic() will only return nonzero * for faults, *not* if the cmpxchg itself fails. Now we must * verify that the cmpxchg itself completed successfully. */ /* * We expected an empty 'expected_old_val', but instead found * an apparently valid entry. Assume we raced with another * thread to instantiate this table and desclare succecss. */ if (actual_old_val & MPX_BD_ENTRY_VALID_FLAG) { ret = 0; goto out_unmap; } /* * We found a non-empty bd_entry but it did not have the * VALID_FLAG set. Return an error which will result in * a SEGV since this probably means that somebody scribbled * some invalid data in to a bounds table. */ if (expected_old_val != actual_old_val) { ret = -EINVAL; goto out_unmap; } return 0; out_unmap: vm_munmap(bt_addr & MPX_BT_ADDR_MASK, MPX_BT_SIZE_BYTES); return ret; } /* * When a BNDSTX instruction attempts to save bounds to a bounds * table, it will first attempt to look up the table in the * first-level bounds directory. If it does not find a table in * the directory, a #BR is generated and we get here in order to * allocate a new table. * * With 32-bit mode, the size of BD is 4MB, and the size of each * bound table is 16KB. With 64-bit mode, the size of BD is 2GB, * and the size of each bound table is 4MB. */ static int do_mpx_bt_fault(void) { unsigned long bd_entry, bd_base; const struct bndcsr *bndcsr; bndcsr = get_xsave_field_ptr(XSTATE_BNDCSR); if (!bndcsr) return -EINVAL; /* * Mask off the preserve and enable bits */ bd_base = bndcsr->bndcfgu & MPX_BNDCFG_ADDR_MASK; /* * The hardware provides the address of the missing or invalid * entry via BNDSTATUS, so we don't have to go look it up. */ bd_entry = bndcsr->bndstatus & MPX_BNDSTA_ADDR_MASK; /* * Make sure the directory entry is within where we think * the directory is. */ if ((bd_entry < bd_base) || (bd_entry >= bd_base + MPX_BD_SIZE_BYTES)) return -EINVAL; return allocate_bt((long __user *)bd_entry); } int mpx_handle_bd_fault(void) { /* * Userspace never asked us to manage the bounds tables, * so refuse to help. */ if (!kernel_managing_mpx_tables(current->mm)) return -EINVAL; if (do_mpx_bt_fault()) { force_sig(SIGSEGV, current); /* * The force_sig() is essentially "handling" this * exception, so we do not pass up the error * from do_mpx_bt_fault(). */ } return 0; } /* * A thin wrapper around get_user_pages(). Returns 0 if the * fault was resolved or -errno if not. */ static int mpx_resolve_fault(long __user *addr, int write) { long gup_ret; int nr_pages = 1; int force = 0; gup_ret = get_user_pages(current, current->mm, (unsigned long)addr, nr_pages, write, force, NULL, NULL); /* * get_user_pages() returns number of pages gotten. * 0 means we failed to fault in and get anything, * probably because 'addr' is bad. */ if (!gup_ret) return -EFAULT; /* Other error, return it */ if (gup_ret < 0) return gup_ret; /* must have gup'd a page and gup_ret>0, success */ return 0; } /* * Get the base of bounds tables pointed by specific bounds * directory entry. */ static int get_bt_addr(struct mm_struct *mm, long __user *bd_entry, unsigned long *bt_addr) { int ret; int valid_bit; if (!access_ok(VERIFY_READ, (bd_entry), sizeof(*bd_entry))) return -EFAULT; while (1) { int need_write = 0; pagefault_disable(); ret = get_user(*bt_addr, bd_entry); pagefault_enable(); if (!ret) break; if (ret == -EFAULT) ret = mpx_resolve_fault(bd_entry, need_write); /* * If we could not resolve the fault, consider it * userspace's fault and error out. */ if (ret) return ret; } valid_bit = *bt_addr & MPX_BD_ENTRY_VALID_FLAG; *bt_addr &= MPX_BT_ADDR_MASK; /* * When the kernel is managing bounds tables, a bounds directory * entry will either have a valid address (plus the valid bit) * *OR* be completely empty. If we see a !valid entry *and* some * data in the address field, we know something is wrong. This * -EINVAL return will cause a SIGSEGV. */ if (!valid_bit && *bt_addr) return -EINVAL; /* * Do we have an completely zeroed bt entry? That is OK. It * just means there was no bounds table for this memory. Make * sure to distinguish this from -EINVAL, which will cause * a SEGV. */ if (!valid_bit) return -ENOENT; return 0; } /* * Free the backing physical pages of bounds table 'bt_addr'. * Assume start...end is within that bounds table. */ static int zap_bt_entries(struct mm_struct *mm, unsigned long bt_addr, unsigned long start, unsigned long end) { struct vm_area_struct *vma; unsigned long addr, len; /* * Find the first overlapping vma. If vma->vm_start > start, there * will be a hole in the bounds table. This -EINVAL return will * cause a SIGSEGV. */ vma = find_vma(mm, start); if (!vma || vma->vm_start > start) return -EINVAL; /* * A NUMA policy on a VM_MPX VMA could cause this bouds table to * be split. So we need to look across the entire 'start -> end' * range of this bounds table, find all of the VM_MPX VMAs, and * zap only those. */ addr = start; while (vma && vma->vm_start < end) { /* * We followed a bounds directory entry down * here. If we find a non-MPX VMA, that's bad, * so stop immediately and return an error. This * probably results in a SIGSEGV. */ if (!is_mpx_vma(vma)) return -EINVAL; len = min(vma->vm_end, end) - addr; zap_page_range(vma, addr, len, NULL); trace_mpx_unmap_zap(addr, addr+len); vma = vma->vm_next; addr = vma->vm_start; } return 0; } static int unmap_single_bt(struct mm_struct *mm, long __user *bd_entry, unsigned long bt_addr) { unsigned long expected_old_val = bt_addr | MPX_BD_ENTRY_VALID_FLAG; unsigned long actual_old_val = 0; int ret; while (1) { int need_write = 1; pagefault_disable(); ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry, expected_old_val, 0); pagefault_enable(); if (!ret) break; if (ret == -EFAULT) ret = mpx_resolve_fault(bd_entry, need_write); /* * If we could not resolve the fault, consider it * userspace's fault and error out. */ if (ret) return ret; } /* * The cmpxchg was performed, check the results. */ if (actual_old_val != expected_old_val) { /* * Someone else raced with us to unmap the table. * There was no bounds table pointed to by the * directory, so declare success. Somebody freed * it. */ if (!actual_old_val) return 0; /* * Something messed with the bounds directory * entry. We hold mmap_sem for read or write * here, so it could not be a _new_ bounds table * that someone just allocated. Something is * wrong, so pass up the error and SIGSEGV. */ return -EINVAL; } /* * Note, we are likely being called under do_munmap() already. To * avoid recursion, do_munmap() will check whether it comes * from one bounds table through VM_MPX flag. */ return do_munmap(mm, bt_addr, MPX_BT_SIZE_BYTES); } /* * If the bounds table pointed by bounds directory 'bd_entry' is * not shared, unmap this whole bounds table. Otherwise, only free * those backing physical pages of bounds table entries covered * in this virtual address region start...end. */ static int unmap_shared_bt(struct mm_struct *mm, long __user *bd_entry, unsigned long start, unsigned long end, bool prev_shared, bool next_shared) { unsigned long bt_addr; int ret; ret = get_bt_addr(mm, bd_entry, &bt_addr); /* * We could see an "error" ret for not-present bounds * tables (not really an error), or actual errors, but * stop unmapping either way. */ if (ret) return ret; if (prev_shared && next_shared) ret = zap_bt_entries(mm, bt_addr, bt_addr+MPX_GET_BT_ENTRY_OFFSET(start), bt_addr+MPX_GET_BT_ENTRY_OFFSET(end)); else if (prev_shared) ret = zap_bt_entries(mm, bt_addr, bt_addr+MPX_GET_BT_ENTRY_OFFSET(start), bt_addr+MPX_BT_SIZE_BYTES); else if (next_shared) ret = zap_bt_entries(mm, bt_addr, bt_addr, bt_addr+MPX_GET_BT_ENTRY_OFFSET(end)); else ret = unmap_single_bt(mm, bd_entry, bt_addr); return ret; } /* * A virtual address region being munmap()ed might share bounds table * with adjacent VMAs. We only need to free the backing physical * memory of these shared bounds tables entries covered in this virtual * address region. */ static int unmap_edge_bts(struct mm_struct *mm, unsigned long start, unsigned long end) { int ret; long __user *bde_start, *bde_end; struct vm_area_struct *prev, *next; bool prev_shared = false, next_shared = false; bde_start = mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(start); bde_end = mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(end-1); /* * Check whether bde_start and bde_end are shared with adjacent * VMAs. * * We already unliked the VMAs from the mm's rbtree so 'start' * is guaranteed to be in a hole. This gets us the first VMA * before the hole in to 'prev' and the next VMA after the hole * in to 'next'. */ next = find_vma_prev(mm, start, &prev); if (prev && (mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(prev->vm_end-1)) == bde_start) prev_shared = true; if (next && (mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(next->vm_start)) == bde_end) next_shared = true; /* * This virtual address region being munmap()ed is only * covered by one bounds table. * * In this case, if this table is also shared with adjacent * VMAs, only part of the backing physical memory of the bounds * table need be freeed. Otherwise the whole bounds table need * be unmapped. */ if (bde_start == bde_end) { return unmap_shared_bt(mm, bde_start, start, end, prev_shared, next_shared); } /* * If more than one bounds tables are covered in this virtual * address region being munmap()ed, we need to separately check * whether bde_start and bde_end are shared with adjacent VMAs. */ ret = unmap_shared_bt(mm, bde_start, start, end, prev_shared, false); if (ret) return ret; ret = unmap_shared_bt(mm, bde_end, start, end, false, next_shared); if (ret) return ret; return 0; } static int mpx_unmap_tables(struct mm_struct *mm, unsigned long start, unsigned long end) { int ret; long __user *bd_entry, *bde_start, *bde_end; unsigned long bt_addr; trace_mpx_unmap_search(start, end); /* * "Edge" bounds tables are those which are being used by the region * (start -> end), but that may be shared with adjacent areas. If they * turn out to be completely unshared, they will be freed. If they are * shared, we will free the backing store (like an MADV_DONTNEED) for * areas used by this region. */ ret = unmap_edge_bts(mm, start, end); switch (ret) { /* non-present tables are OK */ case 0: case -ENOENT: /* Success, or no tables to unmap */ break; case -EINVAL: case -EFAULT: default: return ret; } /* * Only unmap the bounds table that are * 1. fully covered * 2. not at the edges of the mapping, even if full aligned */ bde_start = mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(start); bde_end = mm->bd_addr + MPX_GET_BD_ENTRY_OFFSET(end-1); for (bd_entry = bde_start + 1; bd_entry < bde_end; bd_entry++) { ret = get_bt_addr(mm, bd_entry, &bt_addr); switch (ret) { case 0: break; case -ENOENT: /* No table here, try the next one */ continue; case -EINVAL: case -EFAULT: default: /* * Note: we are being strict here. * Any time we run in to an issue * unmapping tables, we stop and * SIGSEGV. */ return ret; } ret = unmap_single_bt(mm, bd_entry, bt_addr); if (ret) return ret; } return 0; } /* * Free unused bounds tables covered in a virtual address region being * munmap()ed. Assume end > start. * * This function will be called by do_munmap(), and the VMAs covering * the virtual address region start...end have already been split if * necessary, and the 'vma' is the first vma in this range (start -> end). */ void mpx_notify_unmap(struct mm_struct *mm, struct vm_area_struct *vma, unsigned long start, unsigned long end) { int ret; /* * Refuse to do anything unless userspace has asked * the kernel to help manage the bounds tables, */ if (!kernel_managing_mpx_tables(current->mm)) return; /* * This will look across the entire 'start -> end' range, * and find all of the non-VM_MPX VMAs. * * To avoid recursion, if a VM_MPX vma is found in the range * (start->end), we will not continue follow-up work. This * recursion represents having bounds tables for bounds tables, * which should not occur normally. Being strict about it here * helps ensure that we do not have an exploitable stack overflow. */ do { if (vma->vm_flags & VM_MPX) return; vma = vma->vm_next; } while (vma && vma->vm_start < end); ret = mpx_unmap_tables(mm, start, end); if (ret) force_sig(SIGSEGV, current); }