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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  */
get_free_idx(void)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 
tls_desc_okay(const struct user_desc * info)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 
set_tls_desc(struct task_struct * p,int idx,const struct user_desc * info,int n)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  */
do_set_thread_area(struct task_struct * p,int idx,struct user_desc __user * u_info,int can_allocate)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 
SYSCALL_DEFINE1(set_thread_area,struct user_desc __user *,u_info)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 
fill_user_desc(struct user_desc * info,int idx,const struct desc_struct * desc)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 
do_get_thread_area(struct task_struct * p,int idx,struct user_desc __user * u_info)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 
SYSCALL_DEFINE1(get_thread_area,struct user_desc __user *,u_info)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 
regset_tls_active(struct task_struct * target,const struct user_regset * regset)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 
regset_tls_get(struct task_struct * target,const struct user_regset * regset,struct membuf to)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 
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)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 }
302