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 <linux/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         unsigned short __maybe_unused sel, modified_sel;
 118
 119         if (copy_from_user(&info, u_info, sizeof(info)))
 120                 return -EFAULT;
 121
 122         if (!tls_desc_okay(&info))
 123                 return -EINVAL;
 124
 125         if (idx == -1)
 126                 idx = info.entry_number;
 127
 128         /*
 129          * index -1 means the kernel should try to find and
 130          * allocate an empty descriptor:
 131          */
 132         if (idx == -1 && can_allocate) {
 133                 idx = get_free_idx();
 134                 if (idx < 0)
 135                         return idx;
 136                 if (put_user(idx, &u_info->entry_number))
 137                         return -EFAULT;
 138         }
 139
 140         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 141                 return -EINVAL;
 142
 143         set_tls_desc(p, idx, &info, 1);
 144
 145         /*
 146          * If DS, ES, FS, or GS points to the modified segment, forcibly
 147          * refresh it.  Only needed on x86_64 because x86_32 reloads them
 148          * on return to user mode.
 149          */
 150         modified_sel = (idx << 3) | 3;
 151
 152         if (p == current) {
 153 #ifdef CONFIG_X86_64
 154                 savesegment(ds, sel);
 155                 if (sel == modified_sel)
 156                         loadsegment(ds, sel);
 157
 158                 savesegment(es, sel);
 159                 if (sel == modified_sel)
 160                         loadsegment(es, sel);
 161
 162                 savesegment(fs, sel);
 163                 if (sel == modified_sel)
 164                         loadsegment(fs, sel);
 165
 166                 savesegment(gs, sel);
 167                 if (sel == modified_sel)
 168                         load_gs_index(sel);
 169 #endif
 170
 171 #ifdef CONFIG_X86_32_LAZY_GS
 172                 savesegment(gs, sel);
 173                 if (sel == modified_sel)
 174                         loadsegment(gs, sel);
 175 #endif
 176         } else {
 177 #ifdef CONFIG_X86_64
 178                 if (p->thread.fsindex == modified_sel)
 179                         p->thread.fsbase = info.base_addr;
 180
 181                 if (p->thread.gsindex == modified_sel)
 182                         p->thread.gsbase = info.base_addr;
 183 #endif
 184         }
 185
 186         return 0;
 187 }
 188
 189 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
 190 {
 191         return do_set_thread_area(current, -1, u_info, 1);
 192 }
 193
 194
 195 /*
 196  * Get the current Thread-Local Storage area:
 197  */
 198
 199 static void fill_user_desc(struct user_desc *info, int idx,
 200                            const struct desc_struct *desc)
 201
 202 {
 203         memset(info, 0, sizeof(*info));
 204         info->entry_number = idx;
 205         info->base_addr = get_desc_base(desc);
 206         info->limit = get_desc_limit(desc);
 207         info->seg_32bit = desc->d;
 208         info->contents = desc->type >> 2;
 209         info->read_exec_only = !(desc->type & 2);
 210         info->limit_in_pages = desc->g;
 211         info->seg_not_present = !desc->p;
 212         info->useable = desc->avl;
 213 #ifdef CONFIG_X86_64
 214         info->lm = desc->l;
 215 #endif
 216 }
 217
 218 int do_get_thread_area(struct task_struct *p, int idx,
 219                        struct user_desc __user *u_info)
 220 {
 221         struct user_desc info;
 222
 223         if (idx == -1 && get_user(idx, &u_info->entry_number))
 224                 return -EFAULT;
 225
 226         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 227                 return -EINVAL;
 228
 229         fill_user_desc(&info, idx,
 230                        &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
 231
 232         if (copy_to_user(u_info, &info, sizeof(info)))
 233                 return -EFAULT;
 234         return 0;
 235 }
 236
 237 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
 238 {
 239         return do_get_thread_area(current, -1, u_info);
 240 }
 241
 242 int regset_tls_active(struct task_struct *target,
 243                       const struct user_regset *regset)
 244 {
 245         struct thread_struct *t = &target->thread;
 246         int n = GDT_ENTRY_TLS_ENTRIES;
 247         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 248                 --n;
 249         return n;
 250 }
 251
 252 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 253                    unsigned int pos, unsigned int count,
 254                    void *kbuf, void __user *ubuf)
 255 {
 256         const struct desc_struct *tls;
 257
 258         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 259             (pos % sizeof(struct user_desc)) != 0 ||
 260             (count % sizeof(struct user_desc)) != 0)
 261                 return -EINVAL;
 262
 263         pos /= sizeof(struct user_desc);
 264         count /= sizeof(struct user_desc);
 265
 266         tls = &target->thread.tls_array[pos];
 267
 268         if (kbuf) {
 269                 struct user_desc *info = kbuf;
 270                 while (count-- > 0)
 271                         fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
 272                                        tls++);
 273         } else {
 274                 struct user_desc __user *u_info = ubuf;
 275                 while (count-- > 0) {
 276                         struct user_desc info;
 277                         fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
 278                         if (__copy_to_user(u_info++, &info, sizeof(info)))
 279                                 return -EFAULT;
 280                 }
 281         }
 282
 283         return 0;
 284 }
 285
 286 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 287                    unsigned int pos, unsigned int count,
 288                    const void *kbuf, const void __user *ubuf)
 289 {
 290         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 291         const struct user_desc *info;
 292         int i;
 293
 294         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 295             (pos % sizeof(struct user_desc)) != 0 ||
 296             (count % sizeof(struct user_desc)) != 0)
 297                 return -EINVAL;
 298
 299         if (kbuf)
 300                 info = kbuf;
 301         else if (__copy_from_user(infobuf, ubuf, count))
 302                 return -EFAULT;
 303         else
 304                 info = infobuf;
 305
 306         for (i = 0; i < count / sizeof(struct user_desc); i++)
 307                 if (!tls_desc_okay(info + i))
 308                         return -EINVAL;
 309
 310         set_tls_desc(target,
 311                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 312                      info, count / sizeof(struct user_desc));
 313
 314         return 0;
 315 }