arch/x86/kernel/tls.c

   1 #include <linux/kernel.h>
   2 #include <linux/errno.h>
   3 #include <linux/sched.h>
   4 #include <linux/user.h>
   5 #include <linux/regset.h>
   6 #include <linux/syscalls.h>
   7
   8 #include <asm/uaccess.h>
   9 #include <asm/desc.h>
  10 #include <asm/ldt.h>
  11 #include <asm/processor.h>
  12 #include <asm/proto.h>
  13
  14 #include "tls.h"
  15
  16 /*
  17  * sys_alloc_thread_area: get a yet unused TLS descriptor index.
  18  */
  19 static int get_free_idx(void)
  20 {
  21         struct thread_struct *t = &current->thread;
  22         int idx;
  23
  24         for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
  25                 if (desc_empty(&t->tls_array[idx]))
  26                         return idx + GDT_ENTRY_TLS_MIN;
  27         return -ESRCH;
  28 }
  29
  30 static bool tls_desc_okay(const struct user_desc *info)
  31 {
  32         if (LDT_empty(info))
  33                 return true;
  34
  35         /*
  36          * espfix is required for 16-bit data segments, but espfix
  37          * only works for LDT segments.
  38          */
  39         if (!info->seg_32bit)
  40                 return false;
  41
  42         /* Only allow data segments in the TLS array. */
  43         if (info->contents > 1)
  44                 return false;
  45
  46         /*
  47          * Non-present segments with DPL 3 present an interesting attack
  48          * surface.  The kernel should handle such segments correctly,
  49          * but TLS is very difficult to protect in a sandbox, so prevent
  50          * such segments from being created.
  51          *
  52          * If userspace needs to remove a TLS entry, it can still delete
  53          * it outright.
  54          */
  55         if (info->seg_not_present)
  56                 return false;
  57
  58 #ifdef CONFIG_X86_64
  59         /* The L bit makes no sense for data. */
  60         if (info->lm)
  61                 return false;
  62 #endif
  63
  64         return true;
  65 }
  66
  67 static void set_tls_desc(struct task_struct *p, int idx,
  68                          const struct user_desc *info, int n)
  69 {
  70         struct thread_struct *t = &p->thread;
  71         struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
  72         int cpu;
  73
  74         /*
  75          * We must not get preempted while modifying the TLS.
  76          */
  77         cpu = get_cpu();
  78
  79         while (n-- > 0) {
  80                 if (LDT_empty(info))
  81                         desc->a = desc->b = 0;
  82                 else
  83                         fill_ldt(desc, info);
  84                 ++info;
  85                 ++desc;
  86         }
  87
  88         if (t == &current->thread)
  89                 load_TLS(t, cpu);
  90
  91         put_cpu();
  92 }
  93
  94 /*
  95  * Set a given TLS descriptor:
  96  */
  97 int do_set_thread_area(struct task_struct *p, int idx,
  98                        struct user_desc __user *u_info,
  99                        int can_allocate)
 100 {
 101         struct user_desc info;
 102
 103         if (copy_from_user(&info, u_info, sizeof(info)))
 104                 return -EFAULT;
 105
 106         if (!tls_desc_okay(&info))
 107                 return -EINVAL;
 108
 109         if (idx == -1)
 110                 idx = info.entry_number;
 111
 112         /*
 113          * index -1 means the kernel should try to find and
 114          * allocate an empty descriptor:
 115          */
 116         if (idx == -1 && can_allocate) {
 117                 idx = get_free_idx();
 118                 if (idx < 0)
 119                         return idx;
 120                 if (put_user(idx, &u_info->entry_number))
 121                         return -EFAULT;
 122         }
 123
 124         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 125                 return -EINVAL;
 126
 127         set_tls_desc(p, idx, &info, 1);
 128
 129         return 0;
 130 }
 131
 132 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
 133 {
 134         return do_set_thread_area(current, -1, u_info, 1);
 135 }
 136
 137
 138 /*
 139  * Get the current Thread-Local Storage area:
 140  */
 141
 142 static void fill_user_desc(struct user_desc *info, int idx,
 143                            const struct desc_struct *desc)
 144
 145 {
 146         memset(info, 0, sizeof(*info));
 147         info->entry_number = idx;
 148         info->base_addr = get_desc_base(desc);
 149         info->limit = get_desc_limit(desc);
 150         info->seg_32bit = desc->d;
 151         info->contents = desc->type >> 2;
 152         info->read_exec_only = !(desc->type & 2);
 153         info->limit_in_pages = desc->g;
 154         info->seg_not_present = !desc->p;
 155         info->useable = desc->avl;
 156 #ifdef CONFIG_X86_64
 157         info->lm = desc->l;
 158 #endif
 159 }
 160
 161 int do_get_thread_area(struct task_struct *p, int idx,
 162                        struct user_desc __user *u_info)
 163 {
 164         struct user_desc info;
 165
 166         if (idx == -1 && get_user(idx, &u_info->entry_number))
 167                 return -EFAULT;
 168
 169         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 170                 return -EINVAL;
 171
 172         fill_user_desc(&info, idx,
 173                        &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
 174
 175         if (copy_to_user(u_info, &info, sizeof(info)))
 176                 return -EFAULT;
 177         return 0;
 178 }
 179
 180 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
 181 {
 182         return do_get_thread_area(current, -1, u_info);
 183 }
 184
 185 int regset_tls_active(struct task_struct *target,
 186                       const struct user_regset *regset)
 187 {
 188         struct thread_struct *t = &target->thread;
 189         int n = GDT_ENTRY_TLS_ENTRIES;
 190         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 191                 --n;
 192         return n;
 193 }
 194
 195 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 196                    unsigned int pos, unsigned int count,
 197                    void *kbuf, void __user *ubuf)
 198 {
 199         const struct desc_struct *tls;
 200
 201         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 202             (pos % sizeof(struct user_desc)) != 0 ||
 203             (count % sizeof(struct user_desc)) != 0)
 204                 return -EINVAL;
 205
 206         pos /= sizeof(struct user_desc);
 207         count /= sizeof(struct user_desc);
 208
 209         tls = &target->thread.tls_array[pos];
 210
 211         if (kbuf) {
 212                 struct user_desc *info = kbuf;
 213                 while (count-- > 0)
 214                         fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
 215                                        tls++);
 216         } else {
 217                 struct user_desc __user *u_info = ubuf;
 218                 while (count-- > 0) {
 219                         struct user_desc info;
 220                         fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
 221                         if (__copy_to_user(u_info++, &info, sizeof(info)))
 222                                 return -EFAULT;
 223                 }
 224         }
 225
 226         return 0;
 227 }
 228
 229 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 230                    unsigned int pos, unsigned int count,
 231                    const void *kbuf, const void __user *ubuf)
 232 {
 233         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 234         const struct user_desc *info;
 235         int i;
 236
 237         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 238             (pos % sizeof(struct user_desc)) != 0 ||
 239             (count % sizeof(struct user_desc)) != 0)
 240                 return -EINVAL;
 241
 242         if (kbuf)
 243                 info = kbuf;
 244         else if (__copy_from_user(infobuf, ubuf, count))
 245                 return -EFAULT;
 246         else
 247                 info = infobuf;
 248
 249         for (i = 0; i < count / sizeof(struct user_desc); i++)
 250                 if (!tls_desc_okay(info + i))
 251                         return -EINVAL;
 252
 253         set_tls_desc(target,
 254                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 255                      info, count / sizeof(struct user_desc));
 256
 257         return 0;
 258 }