arch/x86/kernel/tls.c

   1 // SPDX-License-Identifier: GPL-2.0
   2 #include <linux/kernel.h>
   3 #include <linux/errno.h>
   4 #include <linux/sched.h>
   5 #include <linux/user.h>
   6 #include <linux/regset.h>
   7 #include <linux/syscalls.h>
   8 #include <linux/nospec.h>
   9
  10 #include <linux/uaccess.h>
  11 #include <asm/desc.h>
  12 #include <asm/ldt.h>
  13 #include <asm/processor.h>
  14 #include <asm/proto.h>
  15
  16 #include "tls.h"
  17
  18 /*
  19  * sys_alloc_thread_area: get a yet unused TLS descriptor index.
  20  */
  21 static int get_free_idx(void)
  22 {
  23         struct thread_struct *t = &current->thread;
  24         int idx;
  25
  26         for (idx = 0; idx < GDT_ENTRY_TLS_ENTRIES; idx++)
  27                 if (desc_empty(&t->tls_array[idx]))
  28                         return idx + GDT_ENTRY_TLS_MIN;
  29         return -ESRCH;
  30 }
  31
  32 static bool tls_desc_okay(const struct user_desc *info)
  33 {
  34         /*
  35          * For historical reasons (i.e. no one ever documented how any
  36          * of the segmentation APIs work), user programs can and do
  37          * assume that a struct user_desc that's all zeros except for
  38          * entry_number means "no segment at all".  This never actually
  39          * worked.  In fact, up to Linux 3.19, a struct user_desc like
  40          * this would create a 16-bit read-write segment with base and
  41          * limit both equal to zero.
  42          *
  43          * That was close enough to "no segment at all" until we
  44          * hardened this function to disallow 16-bit TLS segments.  Fix
  45          * it up by interpreting these zeroed segments the way that they
  46          * were almost certainly intended to be interpreted.
  47          *
  48          * The correct way to ask for "no segment at all" is to specify
  49          * a user_desc that satisfies LDT_empty.  To keep everything
  50          * working, we accept both.
  51          *
  52          * Note that there's a similar kludge in modify_ldt -- look at
  53          * the distinction between modes 1 and 0x11.
  54          */
  55         if (LDT_empty(info) || LDT_zero(info))
  56                 return true;
  57
  58         /*
  59          * espfix is required for 16-bit data segments, but espfix
  60          * only works for LDT segments.
  61          */
  62         if (!info->seg_32bit)
  63                 return false;
  64
  65         /* Only allow data segments in the TLS array. */
  66         if (info->contents > 1)
  67                 return false;
  68
  69         /*
  70          * Non-present segments with DPL 3 present an interesting attack
  71          * surface.  The kernel should handle such segments correctly,
  72          * but TLS is very difficult to protect in a sandbox, so prevent
  73          * such segments from being created.
  74          *
  75          * If userspace needs to remove a TLS entry, it can still delete
  76          * it outright.
  77          */
  78         if (info->seg_not_present)
  79                 return false;
  80
  81         return true;
  82 }
  83
  84 static void set_tls_desc(struct task_struct *p, int idx,
  85                          const struct user_desc *info, int n)
  86 {
  87         struct thread_struct *t = &p->thread;
  88         struct desc_struct *desc = &t->tls_array[idx - GDT_ENTRY_TLS_MIN];
  89         int cpu;
  90
  91         /*
  92          * We must not get preempted while modifying the TLS.
  93          */
  94         cpu = get_cpu();
  95
  96         while (n-- > 0) {
  97                 if (LDT_empty(info) || LDT_zero(info))
  98                         memset(desc, 0, sizeof(*desc));
  99                 else
 100                         fill_ldt(desc, info);
 101                 ++info;
 102                 ++desc;
 103         }
 104
 105         if (t == &current->thread)
 106                 load_TLS(t, cpu);
 107
 108         put_cpu();
 109 }
 110
 111 /*
 112  * Set a given TLS descriptor:
 113  */
 114 int do_set_thread_area(struct task_struct *p, int idx,
 115                        struct user_desc __user *u_info,
 116                        int can_allocate)
 117 {
 118         struct user_desc info;
 119         unsigned short __maybe_unused sel, modified_sel;
 120
 121         if (copy_from_user(&info, u_info, sizeof(info)))
 122                 return -EFAULT;
 123
 124         if (!tls_desc_okay(&info))
 125                 return -EINVAL;
 126
 127         if (idx == -1)
 128                 idx = info.entry_number;
 129
 130         /*
 131          * index -1 means the kernel should try to find and
 132          * allocate an empty descriptor:
 133          */
 134         if (idx == -1 && can_allocate) {
 135                 idx = get_free_idx();
 136                 if (idx < 0)
 137                         return idx;
 138                 if (put_user(idx, &u_info->entry_number))
 139                         return -EFAULT;
 140         }
 141
 142         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 143                 return -EINVAL;
 144
 145         set_tls_desc(p, idx, &info, 1);
 146
 147         /*
 148          * If DS, ES, FS, or GS points to the modified segment, forcibly
 149          * refresh it.  Only needed on x86_64 because x86_32 reloads them
 150          * on return to user mode.
 151          */
 152         modified_sel = (idx << 3) | 3;
 153
 154         if (p == current) {
 155 #ifdef CONFIG_X86_64
 156                 savesegment(ds, sel);
 157                 if (sel == modified_sel)
 158                         loadsegment(ds, sel);
 159
 160                 savesegment(es, sel);
 161                 if (sel == modified_sel)
 162                         loadsegment(es, sel);
 163
 164                 savesegment(fs, sel);
 165                 if (sel == modified_sel)
 166                         loadsegment(fs, sel);
 167
 168                 savesegment(gs, sel);
 169                 if (sel == modified_sel)
 170                         load_gs_index(sel);
 171 #endif
 172
 173 #ifdef CONFIG_X86_32_LAZY_GS
 174                 savesegment(gs, sel);
 175                 if (sel == modified_sel)
 176                         loadsegment(gs, sel);
 177 #endif
 178         } else {
 179 #ifdef CONFIG_X86_64
 180                 if (p->thread.fsindex == modified_sel)
 181                         p->thread.fsbase = info.base_addr;
 182
 183                 if (p->thread.gsindex == modified_sel)
 184                         p->thread.gsbase = info.base_addr;
 185 #endif
 186         }
 187
 188         return 0;
 189 }
 190
 191 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
 192 {
 193         return do_set_thread_area(current, -1, u_info, 1);
 194 }
 195
 196
 197 /*
 198  * Get the current Thread-Local Storage area:
 199  */
 200
 201 static void fill_user_desc(struct user_desc *info, int idx,
 202                            const struct desc_struct *desc)
 203
 204 {
 205         memset(info, 0, sizeof(*info));
 206         info->entry_number = idx;
 207         info->base_addr = get_desc_base(desc);
 208         info->limit = get_desc_limit(desc);
 209         info->seg_32bit = desc->d;
 210         info->contents = desc->type >> 2;
 211         info->read_exec_only = !(desc->type & 2);
 212         info->limit_in_pages = desc->g;
 213         info->seg_not_present = !desc->p;
 214         info->useable = desc->avl;
 215 #ifdef CONFIG_X86_64
 216         info->lm = desc->l;
 217 #endif
 218 }
 219
 220 int do_get_thread_area(struct task_struct *p, int idx,
 221                        struct user_desc __user *u_info)
 222 {
 223         struct user_desc info;
 224         int index;
 225
 226         if (idx == -1 && get_user(idx, &u_info->entry_number))
 227                 return -EFAULT;
 228
 229         if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
 230                 return -EINVAL;
 231
 232         index = idx - GDT_ENTRY_TLS_MIN;
 233         index = array_index_nospec(index,
 234                         GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN + 1);
 235
 236         fill_user_desc(&info, idx, &p->thread.tls_array[index]);
 237
 238         if (copy_to_user(u_info, &info, sizeof(info)))
 239                 return -EFAULT;
 240         return 0;
 241 }
 242
 243 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
 244 {
 245         return do_get_thread_area(current, -1, u_info);
 246 }
 247
 248 int regset_tls_active(struct task_struct *target,
 249                       const struct user_regset *regset)
 250 {
 251         struct thread_struct *t = &target->thread;
 252         int n = GDT_ENTRY_TLS_ENTRIES;
 253         while (n > 0 && desc_empty(&t->tls_array[n - 1]))
 254                 --n;
 255         return n;
 256 }
 257
 258 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
 259                    struct membuf to)
 260 {
 261         const struct desc_struct *tls;
 262         struct user_desc v;
 263         int pos;
 264
 265         for (pos = 0, tls = target->thread.tls_array; to.left; pos++, tls++) {
 266                 fill_user_desc(&v, GDT_ENTRY_TLS_MIN + pos, tls);
 267                 membuf_write(&to, &v, sizeof(v));
 268         }
 269         return 0;
 270 }
 271
 272 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
 273                    unsigned int pos, unsigned int count,
 274                    const void *kbuf, const void __user *ubuf)
 275 {
 276         struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
 277         const struct user_desc *info;
 278         int i;
 279
 280         if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
 281             (pos % sizeof(struct user_desc)) != 0 ||
 282             (count % sizeof(struct user_desc)) != 0)
 283                 return -EINVAL;
 284
 285         if (kbuf)
 286                 info = kbuf;
 287         else if (__copy_from_user(infobuf, ubuf, count))
 288                 return -EFAULT;
 289         else
 290                 info = infobuf;
 291
 292         for (i = 0; i < count / sizeof(struct user_desc); i++)
 293                 if (!tls_desc_okay(info + i))
 294                         return -EINVAL;
 295
 296         set_tls_desc(target,
 297                      GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
 298                      info, count / sizeof(struct user_desc));
 299
 300         return 0;
 301 }