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
   7 #include <asm/uaccess.h>
   8 #include <asm/desc.h>
   9 #include <asm/system.h>
  10 #include <asm/ldt.h>
  11 #include <asm/processor.h>
  12 #include <asm/proto.h>
  13 #include <asm/syscalls.h>
  14
  15 #include "tls.h"
  16
  17 /*
  18  * sys_alloc_thread_area: get a yet unused TLS descriptor index.
  19  */
  20 static int get_free_idx(void)
  21 {
  22         struct thread_struct *t = &current->thread;
  23         int idx;
  24
  25         for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
  26                 if (desc_empty(&t->tls_array[idx]))
  27                         return idx + GDT_ENTRY_TLS_MIN;
  28         return -ESRCH;
  29 }
  30
  31 static bool tls_desc_okay(const struct user_desc *info)
  32 {
  33         if (LDT_empty(info))
  34                 return true;
  35
  36         /*
  37          * espfix is required for 16-bit data segments, but espfix
  38          * only works for LDT segments.
  39          */
  40         if (!info->seg_32bit)
  41                 return false;
  42
  43         /* Only allow data segments in the TLS array. */
  44         if (info->contents > 1)
  45                 return false;
  46
  47         /*
  48          * Non-present segments with DPL 3 present an interesting attack
  49          * surface.  The kernel should handle such segments correctly,
  50          * but TLS is very difficult to protect in a sandbox, so prevent
  51          * such segments from being created.
  52          *
  53          * If userspace needs to remove a TLS entry, it can still delete
  54          * it outright.
  55          */
  56         if (info->seg_not_present)
  57                 return false;
  58
  59 #ifdef CONFIG_X86_64
  60         /* The L bit makes no sense for data. */
  61         if (info->lm)
  62                 return false;
  63 #endif
  64
  65         return true;
  66 }
  67
  68 static void set_tls_desc(struct task_struct *p, int idx,
  69                          const struct user_desc *info, int n)
  70 {
  71         struct thread_struct *t = &p->thread;
  72         struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
  73         int cpu;
  74
  75         /*
  76          * We must not get preempted while modifying the TLS.
  77          */
  78         cpu = get_cpu();
  79
  80         while (n-- > 0) {
  81                 if (LDT_empty(info))
  82                         desc->a = desc->b = 0;
  83                 else
  84                         fill_ldt(desc, info);
  85                 ++info;
  86                 ++desc;
  87         }
  88
  89         if (t == &current->thread)
  90                 load_TLS(t, cpu);
  91
  92         put_cpu();
  93 }
  94
  95 /*
  96  * Set a given TLS descriptor:
  97  */
  98 int do_set_thread_area(struct task_struct *p, int idx,
  99                        struct user_desc __user *u_info,
 100                        int can_allocate)
 101 {
 102         struct user_desc info;
 103
 104         if (copy_from_user(&info, u_info, sizeof(info)))
 105                 return -EFAULT;
 106
 107         if (!tls_desc_okay(&info))
 108                 return -EINVAL;
 109
 110         if (idx == -1)
 111                 idx = info.entry_number;
 112
 113         /*
 114          * index -1 means the kernel should try to find and
 115          * allocate an empty descriptor:
 116          */
 117         if (idx == -1 && can_allocate) {
 118                 idx = get_free_idx();
 119                 if (idx < 0)
 120                         return idx;
 121                 if (put_user(idx, &u_info->entry_number))
 122                         return -EFAULT;
 123         }
 124
 125         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 126                 return -EINVAL;
 127
 128         set_tls_desc(p, idx, &info, 1);
 129
 130         return 0;
 131 }
 132
 133 asmlinkage int sys_set_thread_area(struct user_desc __user *u_info)
 134 {
 135         int ret = do_set_thread_area(current, -1, u_info, 1);
 136         asmlinkage_protect(1, ret, u_info);
 137         return ret;
 138 }
 139
 140
 141 /*
 142  * Get the current Thread-Local Storage area:
 143  */
 144
 145 static void fill_user_desc(struct user_desc *info, int idx,
 146                            const struct desc_struct *desc)
 147
 148 {
 149         memset(info, 0, sizeof(*info));
 150         info->entry_number = idx;
 151         info->base_addr = get_desc_base(desc);
 152         info->limit = get_desc_limit(desc);
 153         info->seg_32bit = desc->d;
 154         info->contents = desc->type >> 2;
 155         info->read_exec_only = !(desc->type & 2);
 156         info->limit_in_pages = desc->g;
 157         info->seg_not_present = !desc->p;
 158         info->useable = desc->avl;
 159 #ifdef CONFIG_X86_64
 160         info->lm = desc->l;
 161 #endif
 162 }
 163
 164 int do_get_thread_area(struct task_struct *p, int idx,
 165                        struct user_desc __user *u_info)
 166 {
 167         struct user_desc info;
 168
 169         if (idx == -1 && get_user(idx, &u_info->entry_number))
 170                 return -EFAULT;
 171
 172         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 173                 return -EINVAL;
 174
 175         fill_user_desc(&info, idx,
 176                        &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
 177
 178         if (copy_to_user(u_info, &info, sizeof(info)))
 179                 return -EFAULT;
 180         return 0;
 181 }
 182
 183 asmlinkage int sys_get_thread_area(struct user_desc __user *u_info)
 184 {
 185         int ret = do_get_thread_area(current, -1, u_info);
 186         asmlinkage_protect(1, ret, u_info);
 187         return ret;
 188 }
 189
 190 int regset_tls_active(struct task_struct *target,
 191                       const struct user_regset *regset)
 192 {
 193         struct thread_struct *t = &target->thread;
 194         int n = GDT_ENTRY_TLS_ENTRIES;
 195         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 196                 --n;
 197         return n;
 198 }
 199
 200 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 201                    unsigned int pos, unsigned int count,
 202                    void *kbuf, void __user *ubuf)
 203 {
 204         const struct desc_struct *tls;
 205
 206         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 207             (pos % sizeof(struct user_desc)) != 0 ||
 208             (count % sizeof(struct user_desc)) != 0)
 209                 return -EINVAL;
 210
 211         pos /= sizeof(struct user_desc);
 212         count /= sizeof(struct user_desc);
 213
 214         tls = &target->thread.tls_array[pos];
 215
 216         if (kbuf) {
 217                 struct user_desc *info = kbuf;
 218                 while (count-- > 0)
 219                         fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
 220                                        tls++);
 221         } else {
 222                 struct user_desc __user *u_info = ubuf;
 223                 while (count-- > 0) {
 224                         struct user_desc info;
 225                         fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
 226                         if (__copy_to_user(u_info++, &info, sizeof(info)))
 227                                 return -EFAULT;
 228                 }
 229         }
 230
 231         return 0;
 232 }
 233
 234 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 235                    unsigned int pos, unsigned int count,
 236                    const void *kbuf, const void __user *ubuf)
 237 {
 238         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 239         const struct user_desc *info;
 240         int i;
 241
 242         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 243             (pos % sizeof(struct user_desc)) != 0 ||
 244             (count % sizeof(struct user_desc)) != 0)
 245                 return -EINVAL;
 246
 247         if (kbuf)
 248                 info = kbuf;
 249         else if (__copy_from_user(infobuf, ubuf, count))
 250                 return -EFAULT;
 251         else
 252                 info = infobuf;
 253
 254         for (i = 0; i < count / sizeof(struct user_desc); i++)
 255                 if (!tls_desc_okay(info + i))
 256                         return -EINVAL;
 257
 258         set_tls_desc(target,
 259                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 260                      info, count / sizeof(struct user_desc));
 261
 262         return 0;
 263 }