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  */
get_free_idx(void)22 static 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 
tls_desc_okay(const struct user_desc * info)33 static 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 
set_tls_desc(struct task_struct * p,int idx,const struct user_desc * info,int n)85 static 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  */
do_set_thread_area(struct task_struct * p,int idx,struct user_desc __user * u_info,int can_allocate)115 int 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 __maybe_unused sel, 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 #ifdef CONFIG_X86_64
157 		savesegment(ds, sel);
158 		if (sel == modified_sel)
159 			loadsegment(ds, sel);
160 
161 		savesegment(es, sel);
162 		if (sel == modified_sel)
163 			loadsegment(es, sel);
164 
165 		savesegment(fs, sel);
166 		if (sel == modified_sel)
167 			loadsegment(fs, sel);
168 #endif
169 
170 		savesegment(gs, sel);
171 		if (sel == modified_sel)
172 			load_gs_index(sel);
173 	} else {
174 #ifdef CONFIG_X86_64
175 		if (p->thread.fsindex == modified_sel)
176 			p->thread.fsbase = info.base_addr;
177 
178 		if (p->thread.gsindex == modified_sel)
179 			p->thread.gsbase = info.base_addr;
180 #endif
181 	}
182 
183 	return 0;
184 }
185 
SYSCALL_DEFINE1(set_thread_area,struct user_desc __user *,u_info)186 SYSCALL_DEFINE1(set_thread_area, struct user_desc __user *, u_info)
187 {
188 	return do_set_thread_area(current, -1, u_info, 1);
189 }
190 
191 
192 /*
193  * Get the current Thread-Local Storage area:
194  */
195 
fill_user_desc(struct user_desc * info,int idx,const struct desc_struct * desc)196 static void fill_user_desc(struct user_desc *info, int idx,
197 			   const struct desc_struct *desc)
198 
199 {
200 	memset(info, 0, sizeof(*info));
201 	info->entry_number = idx;
202 	info->base_addr = get_desc_base(desc);
203 	info->limit = get_desc_limit(desc);
204 	info->seg_32bit = desc->d;
205 	info->contents = desc->type >> 2;
206 	info->read_exec_only = !(desc->type & 2);
207 	info->limit_in_pages = desc->g;
208 	info->seg_not_present = !desc->p;
209 	info->useable = desc->avl;
210 #ifdef CONFIG_X86_64
211 	info->lm = desc->l;
212 #endif
213 }
214 
do_get_thread_area(struct task_struct * p,int idx,struct user_desc __user * u_info)215 int do_get_thread_area(struct task_struct *p, int idx,
216 		       struct user_desc __user *u_info)
217 {
218 	struct user_desc info;
219 	int index;
220 
221 	if (idx == -1 && get_user(idx, &u_info->entry_number))
222 		return -EFAULT;
223 
224 	if (idx < GDT_ENTRY_TLS_MIN || idx > GDT_ENTRY_TLS_MAX)
225 		return -EINVAL;
226 
227 	index = idx - GDT_ENTRY_TLS_MIN;
228 	index = array_index_nospec(index,
229 			GDT_ENTRY_TLS_MAX - GDT_ENTRY_TLS_MIN + 1);
230 
231 	fill_user_desc(&info, idx, &p->thread.tls_array[index]);
232 
233 	if (copy_to_user(u_info, &info, sizeof(info)))
234 		return -EFAULT;
235 	return 0;
236 }
237 
SYSCALL_DEFINE1(get_thread_area,struct user_desc __user *,u_info)238 SYSCALL_DEFINE1(get_thread_area, struct user_desc __user *, u_info)
239 {
240 	return do_get_thread_area(current, -1, u_info);
241 }
242 
regset_tls_active(struct task_struct * target,const struct user_regset * regset)243 int regset_tls_active(struct task_struct *target,
244 		      const struct user_regset *regset)
245 {
246 	struct thread_struct *t = &target->thread;
247 	int n = GDT_ENTRY_TLS_ENTRIES;
248 	while (n > 0 && desc_empty(&t->tls_array[n - 1]))
249 		--n;
250 	return n;
251 }
252 
regset_tls_get(struct task_struct * target,const struct user_regset * regset,struct membuf to)253 int regset_tls_get(struct task_struct *target, const struct user_regset *regset,
254 		   struct membuf to)
255 {
256 	const struct desc_struct *tls;
257 	struct user_desc v;
258 	int pos;
259 
260 	for (pos = 0, tls = target->thread.tls_array; to.left; pos++, tls++) {
261 		fill_user_desc(&v, GDT_ENTRY_TLS_MIN + pos, tls);
262 		membuf_write(&to, &v, sizeof(v));
263 	}
264 	return 0;
265 }
266 
regset_tls_set(struct task_struct * target,const struct user_regset * regset,unsigned int pos,unsigned int count,const void * kbuf,const void __user * ubuf)267 int regset_tls_set(struct task_struct *target, const struct user_regset *regset,
268 		   unsigned int pos, unsigned int count,
269 		   const void *kbuf, const void __user *ubuf)
270 {
271 	struct user_desc infobuf[GDT_ENTRY_TLS_ENTRIES];
272 	const struct user_desc *info;
273 	int i;
274 
275 	if (pos >= GDT_ENTRY_TLS_ENTRIES * sizeof(struct user_desc) ||
276 	    (pos % sizeof(struct user_desc)) != 0 ||
277 	    (count % sizeof(struct user_desc)) != 0)
278 		return -EINVAL;
279 
280 	if (kbuf)
281 		info = kbuf;
282 	else if (__copy_from_user(infobuf, ubuf, count))
283 		return -EFAULT;
284 	else
285 		info = infobuf;
286 
287 	for (i = 0; i < count / sizeof(struct user_desc); i++)
288 		if (!tls_desc_okay(info + i))
289 			return -EINVAL;
290 
291 	set_tls_desc(target,
292 		     GDT_ENTRY_TLS_MIN + (pos / sizeof(struct user_desc)),
293 		     info, count / sizeof(struct user_desc));
294 
295 	return 0;
296 }
297