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1 | /*P:600 The x86 architecture has segments, which involve a table of descriptors |
2 | * which can be used to do funky things with virtual address interpretation. | |
3 | * We originally used to use segments so the Guest couldn't alter the | |
4 | * Guest<->Host Switcher, and then we had to trim Guest segments, and restore | |
5 | * for userspace per-thread segments, but trim again for on userspace->kernel | |
6 | * transitions... This nightmarish creation was contained within this file, | |
7 | * where we knew not to tread without heavy armament and a change of underwear. | |
8 | * | |
9 | * In these modern times, the segment handling code consists of simple sanity | |
10 | * checks, and the worst you'll experience reading this code is butterfly-rash | |
11 | * from frolicking through its parklike serenity. :*/ | |
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12 | #include "lg.h" |
13 | ||
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14 | /*H:600 |
15 | * We've almost completed the Host; there's just one file to go! | |
16 | * | |
17 | * Segments & The Global Descriptor Table | |
18 | * | |
19 | * (That title sounds like a bad Nerdcore group. Not to suggest that there are | |
20 | * any good Nerdcore groups, but in high school a friend of mine had a band | |
21 | * called Joe Fish and the Chips, so there are definitely worse band names). | |
22 | * | |
23 | * To refresh: the GDT is a table of 8-byte values describing segments. Once | |
24 | * set up, these segments can be loaded into one of the 6 "segment registers". | |
25 | * | |
26 | * GDT entries are passed around as "struct desc_struct"s, which like IDT | |
27 | * entries are split into two 32-bit members, "a" and "b". One day, someone | |
28 | * will clean that up, and be declared a Hero. (No pressure, I'm just saying). | |
29 | * | |
30 | * Anyway, the GDT entry contains a base (the start address of the segment), a | |
31 | * limit (the size of the segment - 1), and some flags. Sounds simple, and it | |
32 | * would be, except those zany Intel engineers decided that it was too boring | |
33 | * to put the base at one end, the limit at the other, and the flags in | |
34 | * between. They decided to shotgun the bits at random throughout the 8 bytes, | |
35 | * like so: | |
36 | * | |
37 | * 0 16 40 48 52 56 63 | |
38 | * [ limit part 1 ][ base part 1 ][ flags ][li][fl][base ] | |
39 | * mit ags part 2 | |
40 | * part 2 | |
41 | * | |
42 | * As a result, this file contains a certain amount of magic numeracy. Let's | |
43 | * begin. | |
44 | */ | |
45 | ||
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46 | /* There are several entries we don't let the Guest set. The TSS entry is the |
47 | * "Task State Segment" which controls all kinds of delicate things. The | |
48 | * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the | |
49 | * the Guest can't be trusted to deal with double faults. */ | |
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50 | static int ignored_gdt(unsigned int num) |
51 | { | |
52 | return (num == GDT_ENTRY_TSS | |
53 | || num == GDT_ENTRY_LGUEST_CS | |
54 | || num == GDT_ENTRY_LGUEST_DS | |
55 | || num == GDT_ENTRY_DOUBLEFAULT_TSS); | |
56 | } | |
57 | ||
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58 | /*H:610 Once the GDT has been changed, we fix the new entries up a little. We |
59 | * don't care if they're invalid: the worst that can happen is a General | |
60 | * Protection Fault in the Switcher when it restores a Guest segment register | |
61 | * which tries to use that entry. Then we kill the Guest for causing such a | |
62 | * mess: the message will be "unhandled trap 256". */ | |
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63 | static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end) |
64 | { | |
65 | unsigned int i; | |
66 | ||
67 | for (i = start; i < end; i++) { | |
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68 | /* We never copy these ones to real GDT, so we don't care what |
69 | * they say */ | |
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70 | if (ignored_gdt(i)) |
71 | continue; | |
72 | ||
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73 | /* Segment descriptors contain a privilege level: the Guest is |
74 | * sometimes careless and leaves this as 0, even though it's | |
75 | * running at privilege level 1. If so, we fix it here. */ | |
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76 | if ((lg->gdt[i].b & 0x00006000) == 0) |
77 | lg->gdt[i].b |= (GUEST_PL << 13); | |
78 | ||
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79 | /* Each descriptor has an "accessed" bit. If we don't set it |
80 | * now, the CPU will try to set it when the Guest first loads | |
81 | * that entry into a segment register. But the GDT isn't | |
82 | * writable by the Guest, so bad things can happen. */ | |
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83 | lg->gdt[i].b |= 0x00000100; |
84 | } | |
85 | } | |
86 | ||
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87 | /* This routine is called at boot or modprobe time for each CPU to set up the |
88 | * "constant" GDT entries for Guests running on that CPU. */ | |
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89 | void setup_default_gdt_entries(struct lguest_ro_state *state) |
90 | { | |
91 | struct desc_struct *gdt = state->guest_gdt; | |
92 | unsigned long tss = (unsigned long)&state->guest_tss; | |
93 | ||
bff672e6 | 94 | /* The hypervisor segments are full 0-4G segments, privilege level 0 */ |
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95 | gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT; |
96 | gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT; | |
97 | ||
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98 | /* The TSS segment refers to the TSS entry for this CPU, so we cannot |
99 | * copy it from the Guest. Forgive the magic flags */ | |
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100 | gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16); |
101 | gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000) | |
102 | | ((tss >> 16) & 0x000000FF); | |
103 | } | |
104 | ||
bff672e6 | 105 | /* This routine is called before the Guest is run for the first time. */ |
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106 | void setup_guest_gdt(struct lguest *lg) |
107 | { | |
bff672e6 | 108 | /* Start with full 0-4G segments... */ |
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109 | lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT; |
110 | lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT; | |
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111 | /* ...except the Guest is allowed to use them, so set the privilege |
112 | * level appropriately in the flags. */ | |
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113 | lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13); |
114 | lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13); | |
115 | } | |
116 | ||
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117 | /* Like the IDT, we never simply use the GDT the Guest gives us. We set up the |
118 | * GDTs for each CPU, then we copy across the entries each time we want to run | |
119 | * a different Guest on that CPU. */ | |
120 | ||
121 | /* A partial GDT load, for the three "thead-local storage" entries. Otherwise | |
122 | * it's just like load_guest_gdt(). So much, in fact, it would probably be | |
123 | * neater to have a single hypercall to cover both. */ | |
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124 | void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt) |
125 | { | |
126 | unsigned int i; | |
127 | ||
128 | for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++) | |
129 | gdt[i] = lg->gdt[i]; | |
130 | } | |
131 | ||
bff672e6 | 132 | /* This is the full version */ |
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133 | void copy_gdt(const struct lguest *lg, struct desc_struct *gdt) |
134 | { | |
135 | unsigned int i; | |
136 | ||
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137 | /* The default entries from setup_default_gdt_entries() are not |
138 | * replaced. See ignored_gdt() above. */ | |
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139 | for (i = 0; i < GDT_ENTRIES; i++) |
140 | if (!ignored_gdt(i)) | |
141 | gdt[i] = lg->gdt[i]; | |
142 | } | |
143 | ||
bff672e6 | 144 | /* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */ |
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145 | void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num) |
146 | { | |
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147 | /* We assume the Guest has the same number of GDT entries as the |
148 | * Host, otherwise we'd have to dynamically allocate the Guest GDT. */ | |
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149 | if (num > ARRAY_SIZE(lg->gdt)) |
150 | kill_guest(lg, "too many gdt entries %i", num); | |
151 | ||
bff672e6 | 152 | /* We read the whole thing in, then fix it up. */ |
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153 | lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0])); |
154 | fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt)); | |
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155 | /* Mark that the GDT changed so the core knows it has to copy it again, |
156 | * even if the Guest is run on the same CPU. */ | |
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157 | lg->changed |= CHANGED_GDT; |
158 | } | |
159 | ||
160 | void guest_load_tls(struct lguest *lg, unsigned long gtls) | |
161 | { | |
162 | struct desc_struct *tls = &lg->gdt[GDT_ENTRY_TLS_MIN]; | |
163 | ||
164 | lgread(lg, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES); | |
165 | fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1); | |
166 | lg->changed |= CHANGED_GDT_TLS; | |
167 | } | |
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168 | |
169 | /* | |
170 | * With this, we have finished the Host. | |
171 | * | |
172 | * Five of the seven parts of our task are complete. You have made it through | |
173 | * the Bit of Despair (I think that's somewhere in the page table code, | |
174 | * myself). | |
175 | * | |
176 | * Next, we examine "make Switcher". It's short, but intense. | |
177 | */ |