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Linux/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 }
274 

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