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         /*
  33          * For historical reasons (i.e. no one ever documented how any
  34          * of the segmentation APIs work), user programs can and do
  35          * assume that a struct user_desc that's all zeros except for
  36          * entry_number means "no segment at all".  This never actually
  37          * worked.  In fact, up to Linux 3.19, a struct user_desc like
  38          * this would create a 16-bit read-write segment with base and
  39          * limit both equal to zero.
  40          *
  41          * That was close enough to "no segment at all" until we
  42          * hardened this function to disallow 16-bit TLS segments.  Fix
  43          * it up by interpreting these zeroed segments the way that they
  44          * were almost certainly intended to be interpreted.
  45          *
  46          * The correct way to ask for "no segment at all" is to specify
  47          * a user_desc that satisfies LDT_empty.  To keep everything
  48          * working, we accept both.
  49          *
  50          * Note that there's a similar kludge in modify_ldt -- look at
  51          * the distinction between modes 1 and 0x11.
  52          */
  53         if (LDT_empty(info) || LDT_zero(info))
  54                 return true;
  55
  56         /*
  57          * espfix is required for 16-bit data segments, but espfix
  58          * only works for LDT segments.
  59          */
  60         if (!info->seg_32bit)
  61                 return false;
  62
  63         /* Only allow data segments in the TLS array. */
  64         if (info->contents > 1)
  65                 return false;
  66
  67         /*
  68          * Non-present segments with DPL 3 present an interesting attack
  69          * surface.  The kernel should handle such segments correctly,
  70          * but TLS is very difficult to protect in a sandbox, so prevent
  71          * such segments from being created.
  72          *
  73          * If userspace needs to remove a TLS entry, it can still delete
  74          * it outright.
  75          */
  76         if (info->seg_not_present)
  77                 return false;
  78
  79         return true;
  80 }
  81
  82 static void set_tls_desc(struct task_struct *p, int idx,
  83                          const struct user_desc *info, int n)
  84 {
  85         struct thread_struct *t = &p->thread;
  86         struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
  87         int cpu;
  88
  89         /*
  90          * We must not get preempted while modifying the TLS.
  91          */
  92         cpu = get_cpu();
  93
  94         while (n-- > 0) {
  95                 if (LDT_empty(info) || LDT_zero(info))
  96                         desc->a = desc->b = 0;
  97                 else
  98                         fill_ldt(desc, info);
  99                 ++info;
 100                 ++desc;
 101         }
 102
 103         if (t == &current->thread)
 104                 load_TLS(t, cpu);
 105
 106         put_cpu();
 107 }
 108
 109 /*
 110  * Set a given TLS descriptor:
 111  */
 112 int do_set_thread_area(struct task_struct *p, int idx,
 113                        struct user_desc __user *u_info,
 114                        int can_allocate)
 115 {
 116         struct user_desc info;
 117
 118         if (copy_from_user(&info, u_info, sizeof(info)))
 119                 return -EFAULT;
 120
 121         if (!tls_desc_okay(&info))
 122                 return -EINVAL;
 123
 124         if (idx == -1)
 125                 idx = info.entry_number;
 126
 127         /*
 128          * index -1 means the kernel should try to find and
 129          * allocate an empty descriptor:
 130          */
 131         if (idx == -1 && can_allocate) {
 132                 idx = get_free_idx();
 133                 if (idx < 0)
 134                         return idx;
 135                 if (put_user(idx, &u_info->entry_number))
 136                         return -EFAULT;
 137         }
 138
 139         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 140                 return -EINVAL;
 141
 142         set_tls_desc(p, idx, &info, 1);
 143
 144         return 0;
 145 }
 146
 147 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
 148 {
 149         return do_set_thread_area(current, -1, u_info, 1);
 150 }
 151
 152
 153 /*
 154  * Get the current Thread-Local Storage area:
 155  */
 156
 157 static void fill_user_desc(struct user_desc *info, int idx,
 158                            const struct desc_struct *desc)
 159
 160 {
 161         memset(info, 0, sizeof(*info));
 162         info->entry_number = idx;
 163         info->base_addr = get_desc_base(desc);
 164         info->limit = get_desc_limit(desc);
 165         info->seg_32bit = desc->d;
 166         info->contents = desc->type >> 2;
 167         info->read_exec_only = !(desc->type & 2);
 168         info->limit_in_pages = desc->g;
 169         info->seg_not_present = !desc->p;
 170         info->useable = desc->avl;
 171 #ifdef CONFIG_X86_64
 172         info->lm = desc->l;
 173 #endif
 174 }
 175
 176 int do_get_thread_area(struct task_struct *p, int idx,
 177                        struct user_desc __user *u_info)
 178 {
 179         struct user_desc info;
 180
 181         if (idx == -1 && get_user(idx, &u_info->entry_number))
 182                 return -EFAULT;
 183
 184         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 185                 return -EINVAL;
 186
 187         fill_user_desc(&info, idx,
 188                        &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
 189
 190         if (copy_to_user(u_info, &info, sizeof(info)))
 191                 return -EFAULT;
 192         return 0;
 193 }
 194
 195 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
 196 {
 197         return do_get_thread_area(current, -1, u_info);
 198 }
 199
 200 int regset_tls_active(struct task_struct *target,
 201                       const struct user_regset *regset)
 202 {
 203         struct thread_struct *t = &target->thread;
 204         int n = GDT_ENTRY_TLS_ENTRIES;
 205         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 206                 --n;
 207         return n;
 208 }
 209
 210 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 211                    unsigned int pos, unsigned int count,
 212                    void *kbuf, void __user *ubuf)
 213 {
 214         const struct desc_struct *tls;
 215
 216         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 217             (pos % sizeof(struct user_desc)) != 0 ||
 218             (count % sizeof(struct user_desc)) != 0)
 219                 return -EINVAL;
 220
 221         pos /= sizeof(struct user_desc);
 222         count /= sizeof(struct user_desc);
 223
 224         tls = &target->thread.tls_array[pos];
 225
 226         if (kbuf) {
 227                 struct user_desc *info = kbuf;
 228                 while (count-- > 0)
 229                         fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
 230                                        tls++);
 231         } else {
 232                 struct user_desc __user *u_info = ubuf;
 233                 while (count-- > 0) {
 234                         struct user_desc info;
 235                         fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
 236                         if (__copy_to_user(u_info++, &info, sizeof(info)))
 237                                 return -EFAULT;
 238                 }
 239         }
 240
 241         return 0;
 242 }
 243
 244 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 245                    unsigned int pos, unsigned int count,
 246                    const void *kbuf, const void __user *ubuf)
 247 {
 248         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 249         const struct user_desc *info;
 250         int i;
 251
 252         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 253             (pos % sizeof(struct user_desc)) != 0 ||
 254             (count % sizeof(struct user_desc)) != 0)
 255                 return -EINVAL;
 256
 257         if (kbuf)
 258                 info = kbuf;
 259         else if (__copy_from_user(infobuf, ubuf, count))
 260                 return -EFAULT;
 261         else
 262                 info = infobuf;
 263
 264         for (i = 0; i < count / sizeof(struct user_desc); i++)
 265                 if (!tls_desc_okay(info + i))
 266                         return -EINVAL;
 267
 268         set_tls_desc(target,
 269                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 270                      info, count / sizeof(struct user_desc));
 271
 272         return 0;
 273 }