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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  */
get_free_idx(void)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 
tls_desc_okay(const struct user_desc * info)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 
set_tls_desc(struct task_struct * p,int idx,const struct user_desc * info,int n)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  */
do_set_thread_area(struct task_struct * p,int idx,struct user_desc __user * u_info,int can_allocate)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 
SYSCALL_DEFINE1(set_thread_area,struct user_desc __user *,u_info)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 
fill_user_desc(struct user_desc * info,int idx,const struct desc_struct * desc)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 
do_get_thread_area(struct task_struct * p,int idx,struct user_desc __user * u_info)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 
SYSCALL_DEFINE1(get_thread_area,struct user_desc __user *,u_info)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 
regset_tls_active(struct task_struct * target,const struct user_regset * regset)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 
regset_tls_get(struct task_struct * target,const struct user_regset * regset,unsigned int pos,unsigned int count,void * kbuf,void __user * ubuf)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 
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)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