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
 100                         /*
 101                          * Always set the accessed bit so that the CPU
 102                          * doesn't try to write to the (read-only) GDT.
 103                          */
 104                         desc->type |= 1;
 105                 }
 106                 ++info;
 107                 ++desc;
 108         }
 109
 110         if (t == &current->thread)
 111                 load_TLS(t, cpu);
 112
 113         put_cpu();
 114 }
 115
 116 /*
 117  * Set a given TLS descriptor:
 118  */
 119 int do_set_thread_area(struct task_struct *p, int idx,
 120                        struct user_desc __user *u_info,
 121                        int can_allocate)
 122 {
 123         struct user_desc info;
 124         unsigned short __maybe_unused sel, modified_sel;
 125
 126         if (copy_from_user(&info, u_info, sizeof(info)))
 127                 return -EFAULT;
 128
 129         if (!tls_desc_okay(&info))
 130                 return -EINVAL;
 131
 132         if (idx == -1)
 133                 idx = info.entry_number;
 134
 135         /*
 136          * index -1 means the kernel should try to find and
 137          * allocate an empty descriptor:
 138          */
 139         if (idx == -1 && can_allocate) {
 140                 idx = get_free_idx();
 141                 if (idx < 0)
 142                         return idx;
 143                 if (put_user(idx, &u_info->entry_number))
 144                         return -EFAULT;
 145         }
 146
 147         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 148                 return -EINVAL;
 149
 150         set_tls_desc(p, idx, &info, 1);
 151
 152         /*
 153          * If DS, ES, FS, or GS points to the modified segment, forcibly
 154          * refresh it.  Only needed on x86_64 because x86_32 reloads them
 155          * on return to user mode.
 156          */
 157         modified_sel = (idx << 3) | 3;
 158
 159         if (p == current) {
 160 #ifdef CONFIG_X86_64
 161                 savesegment(ds, sel);
 162                 if (sel == modified_sel)
 163                         loadsegment(ds, sel);
 164
 165                 savesegment(es, sel);
 166                 if (sel == modified_sel)
 167                         loadsegment(es, sel);
 168
 169                 savesegment(fs, sel);
 170                 if (sel == modified_sel)
 171                         loadsegment(fs, sel);
 172
 173                 savesegment(gs, sel);
 174                 if (sel == modified_sel)
 175                         load_gs_index(sel);
 176 #endif
 177
 178 #ifdef CONFIG_X86_32_LAZY_GS
 179                 savesegment(gs, sel);
 180                 if (sel == modified_sel)
 181                         loadsegment(gs, sel);
 182 #endif
 183         } else {
 184 #ifdef CONFIG_X86_64
 185                 if (p->thread.fsindex == modified_sel)
 186                         p->thread.fsbase = info.base_addr;
 187
 188                 if (p->thread.gsindex == modified_sel)
 189                         p->thread.gsbase = info.base_addr;
 190 #endif
 191         }
 192
 193         return 0;
 194 }
 195
 196 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
 197 {
 198         return do_set_thread_area(current, -1, u_info, 1);
 199 }
 200
 201
 202 /*
 203  * Get the current Thread-Local Storage area:
 204  */
 205
 206 static void fill_user_desc(struct user_desc *info, int idx,
 207                            const struct desc_struct *desc)
 208
 209 {
 210         memset(info, 0, sizeof(*info));
 211         info->entry_number = idx;
 212         info->base_addr = get_desc_base(desc);
 213         info->limit = get_desc_limit(desc);
 214         info->seg_32bit = desc->d;
 215         info->contents = desc->type >> 2;
 216         info->read_exec_only = !(desc->type & 2);
 217         info->limit_in_pages = desc->g;
 218         info->seg_not_present = !desc->p;
 219         info->useable = desc->avl;
 220 #ifdef CONFIG_X86_64
 221         info->lm = desc->l;
 222 #endif
 223 }
 224
 225 int do_get_thread_area(struct task_struct *p, int idx,
 226                        struct user_desc __user *u_info)
 227 {
 228         struct user_desc info;
 229
 230         if (idx == -1 && get_user(idx, &u_info->entry_number))
 231                 return -EFAULT;
 232
 233         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 234                 return -EINVAL;
 235
 236         fill_user_desc(&info, idx,
 237                        &p->thread.tls_array[idx - GDT_ENTRY_TLS_MIN]);
 238
 239         if (copy_to_user(u_info, &info, sizeof(info)))
 240                 return -EFAULT;
 241         return 0;
 242 }
 243
 244 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
 245 {
 246         return do_get_thread_area(current, -1, u_info);
 247 }
 248
 249 int regset_tls_active(struct task_struct *target,
 250                       const struct user_regset *regset)
 251 {
 252         struct thread_struct *t = &target->thread;
 253         int n = GDT_ENTRY_TLS_ENTRIES;
 254         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 255                 --n;
 256         return n;
 257 }
 258
 259 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 260                    unsigned int pos, unsigned int count,
 261                    void *kbuf, void __user *ubuf)
 262 {
 263         const struct desc_struct *tls;
 264
 265         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 266             (pos % sizeof(struct user_desc)) != 0 ||
 267             (count % sizeof(struct user_desc)) != 0)
 268                 return -EINVAL;
 269
 270         pos /= sizeof(struct user_desc);
 271         count /= sizeof(struct user_desc);
 272
 273         tls = &target->thread.tls_array[pos];
 274
 275         if (kbuf) {
 276                 struct user_desc *info = kbuf;
 277                 while (count-- > 0)
 278                         fill_user_desc(info++, GDT_ENTRY_TLS_MIN + pos++,
 279                                        tls++);
 280         } else {
 281                 struct user_desc __user *u_info = ubuf;
 282                 while (count-- > 0) {
 283                         struct user_desc info;
 284                         fill_user_desc(&info, GDT_ENTRY_TLS_MIN + pos++, tls++);
 285                         if (__copy_to_user(u_info++, &info, sizeof(info)))
 286                                 return -EFAULT;
 287                 }
 288         }
 289
 290         return 0;
 291 }
 292
 293 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 294                    unsigned int pos, unsigned int count,
 295                    const void *kbuf, const void __user *ubuf)
 296 {
 297         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 298         const struct user_desc *info;
 299         int i;
 300
 301         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 302             (pos % sizeof(struct user_desc)) != 0 ||
 303             (count % sizeof(struct user_desc)) != 0)
 304                 return -EINVAL;
 305
 306         if (kbuf)
 307                 info = kbuf;
 308         else if (__copy_from_user(infobuf, ubuf, count))
 309                 return -EFAULT;
 310         else
 311                 info = infobuf;
 312
 313         for (i = 0; i < count / sizeof(struct user_desc); i++)
 314                 if (!tls_desc_okay(info + i))
 315                         return -EINVAL;
 316
 317         set_tls_desc(target,
 318                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 319                      info, count / sizeof(struct user_desc));
 320
 321         return 0;
 322 }