projects/linux·blob master · refs

298 lines · 7.0 KiB

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