Blame view
drivers/lguest/segments.c
7.78 KB
2e04ef769 lguest: fix comme... |
1 2 |
/*P:600 * The x86 architecture has segments, which involve a table of descriptors |
f938d2c89 lguest: documenta... |
3 4 5 6 7 8 9 10 11 |
* which can be used to do funky things with virtual address interpretation. * We originally used to use segments so the Guest couldn't alter the * Guest<->Host Switcher, and then we had to trim Guest segments, and restore * for userspace per-thread segments, but trim again for on userspace->kernel * transitions... This nightmarish creation was contained within this file, * where we knew not to tread without heavy armament and a change of underwear. * * In these modern times, the segment handling code consists of simple sanity * checks, and the worst you'll experience reading this code is butterfly-rash |
2e04ef769 lguest: fix comme... |
12 13 |
* from frolicking through its parklike serenity. :*/ |
d7e28ffe6 lguest: the host ... |
14 |
#include "lg.h" |
bff672e63 lguest: documenta... |
15 |
/*H:600 |
bff672e63 lguest: documenta... |
16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 |
* Segments & The Global Descriptor Table * * (That title sounds like a bad Nerdcore group. Not to suggest that there are * any good Nerdcore groups, but in high school a friend of mine had a band * called Joe Fish and the Chips, so there are definitely worse band names). * * To refresh: the GDT is a table of 8-byte values describing segments. Once * set up, these segments can be loaded into one of the 6 "segment registers". * * GDT entries are passed around as "struct desc_struct"s, which like IDT * entries are split into two 32-bit members, "a" and "b". One day, someone * will clean that up, and be declared a Hero. (No pressure, I'm just saying). * * Anyway, the GDT entry contains a base (the start address of the segment), a * limit (the size of the segment - 1), and some flags. Sounds simple, and it * would be, except those zany Intel engineers decided that it was too boring * to put the base at one end, the limit at the other, and the flags in * between. They decided to shotgun the bits at random throughout the 8 bytes, * like so: * * 0 16 40 48 52 56 63 * [ limit part 1 ][ base part 1 ][ flags ][li][fl][base ] * mit ags part 2 * part 2 * * As a result, this file contains a certain amount of magic numeracy. Let's * begin. */ |
2e04ef769 lguest: fix comme... |
44 45 |
/* * There are several entries we don't let the Guest set. The TSS entry is the |
bff672e63 lguest: documenta... |
46 47 |
* "Task State Segment" which controls all kinds of delicate things. The * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the |
2e04ef769 lguest: fix comme... |
48 49 |
* the Guest can't be trusted to deal with double faults. */ |
df1693abc lguest: use bool ... |
50 |
static bool ignored_gdt(unsigned int num) |
d7e28ffe6 lguest: the host ... |
51 52 53 54 55 56 |
{ return (num == GDT_ENTRY_TSS || num == GDT_ENTRY_LGUEST_CS || num == GDT_ENTRY_LGUEST_DS || num == GDT_ENTRY_DOUBLEFAULT_TSS); } |
2e04ef769 lguest: fix comme... |
57 58 |
/*H:630 * Once the Guest gave us new GDT entries, we fix them up a little. We |
0d027c01c lguest: Fix Malic... |
59 60 61 |
* don't care if they're invalid: the worst that can happen is a General * Protection Fault in the Switcher when it restores a Guest segment register * which tries to use that entry. Then we kill the Guest for causing such a |
2e04ef769 lguest: fix comme... |
62 63 |
* mess: the message will be "unhandled trap 256". */ |
fc708b3e4 lguest: replace l... |
64 |
static void fixup_gdt_table(struct lg_cpu *cpu, unsigned start, unsigned end) |
d7e28ffe6 lguest: the host ... |
65 66 67 68 |
{ unsigned int i; for (i = start; i < end; i++) { |
2e04ef769 lguest: fix comme... |
69 70 71 72 |
/* * We never copy these ones to real GDT, so we don't care what * they say */ |
d7e28ffe6 lguest: the host ... |
73 74 |
if (ignored_gdt(i)) continue; |
2e04ef769 lguest: fix comme... |
75 76 |
/* * Segment descriptors contain a privilege level: the Guest is |
bff672e63 lguest: documenta... |
77 |
* sometimes careless and leaves this as 0, even though it's |
2e04ef769 lguest: fix comme... |
78 79 |
* running at privilege level 1. If so, we fix it here. */ |
39082f7e5 lguest: switch se... |
80 81 |
if (cpu->arch.gdt[i].dpl == 0) cpu->arch.gdt[i].dpl |= GUEST_PL; |
d7e28ffe6 lguest: the host ... |
82 |
|
2e04ef769 lguest: fix comme... |
83 84 |
/* * Each descriptor has an "accessed" bit. If we don't set it |
bff672e63 lguest: documenta... |
85 86 |
* now, the CPU will try to set it when the Guest first loads * that entry into a segment register. But the GDT isn't |
2e04ef769 lguest: fix comme... |
87 88 |
* writable by the Guest, so bad things can happen. */ |
39082f7e5 lguest: switch se... |
89 |
cpu->arch.gdt[i].type |= 0x1; |
d7e28ffe6 lguest: the host ... |
90 91 |
} } |
2e04ef769 lguest: fix comme... |
92 93 |
/*H:610 * Like the IDT, we never simply use the GDT the Guest gives us. We keep |
e1e72965e lguest: documenta... |
94 95 96 97 98 |
* a GDT for each CPU, and copy across the Guest's entries each time we want to * run the Guest on that CPU. * * This routine is called at boot or modprobe time for each CPU to set up the * constant GDT entries: the ones which are the same no matter what Guest we're |
2e04ef769 lguest: fix comme... |
99 100 |
* running. */ |
d7e28ffe6 lguest: the host ... |
101 102 103 104 |
void setup_default_gdt_entries(struct lguest_ro_state *state) { struct desc_struct *gdt = state->guest_gdt; unsigned long tss = (unsigned long)&state->guest_tss; |
e1e72965e lguest: documenta... |
105 |
/* The Switcher segments are full 0-4G segments, privilege level 0 */ |
d7e28ffe6 lguest: the host ... |
106 107 |
gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT; gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT; |
2e04ef769 lguest: fix comme... |
108 109 |
/* * The TSS segment refers to the TSS entry for this particular CPU. |
2e04ef769 lguest: fix comme... |
110 |
*/ |
39082f7e5 lguest: switch se... |
111 112 113 114 115 116 117 118 119 120 121 |
gdt[GDT_ENTRY_TSS].a = 0; gdt[GDT_ENTRY_TSS].b = 0; gdt[GDT_ENTRY_TSS].limit0 = 0x67; gdt[GDT_ENTRY_TSS].base0 = tss & 0xFFFF; gdt[GDT_ENTRY_TSS].base1 = (tss >> 16) & 0xFF; gdt[GDT_ENTRY_TSS].base2 = tss >> 24; gdt[GDT_ENTRY_TSS].type = 0x9; /* 32-bit TSS (available) */ gdt[GDT_ENTRY_TSS].p = 0x1; /* Entry is present */ gdt[GDT_ENTRY_TSS].dpl = 0x0; /* Privilege level 0 */ gdt[GDT_ENTRY_TSS].s = 0x0; /* system segment */ |
d7e28ffe6 lguest: the host ... |
122 |
} |
2e04ef769 lguest: fix comme... |
123 124 125 126 |
/* * This routine sets up the initial Guest GDT for booting. All entries start * as 0 (unusable). */ |
fc708b3e4 lguest: replace l... |
127 |
void setup_guest_gdt(struct lg_cpu *cpu) |
d7e28ffe6 lguest: the host ... |
128 |
{ |
2e04ef769 lguest: fix comme... |
129 130 131 132 |
/* * Start with full 0-4G segments...except the Guest is allowed to use * them, so set the privilege level appropriately in the flags. */ |
fc708b3e4 lguest: replace l... |
133 134 |
cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT; cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT; |
39082f7e5 lguest: switch se... |
135 136 |
cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].dpl |= GUEST_PL; cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].dpl |= GUEST_PL; |
d7e28ffe6 lguest: the host ... |
137 |
} |
2e04ef769 lguest: fix comme... |
138 139 140 141 |
/*H:650 * An optimization of copy_gdt(), for just the three "thead-local storage" * entries. */ |
fc708b3e4 lguest: replace l... |
142 |
void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt) |
d7e28ffe6 lguest: the host ... |
143 144 145 146 |
{ unsigned int i; for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++) |
fc708b3e4 lguest: replace l... |
147 |
gdt[i] = cpu->arch.gdt[i]; |
d7e28ffe6 lguest: the host ... |
148 |
} |
2e04ef769 lguest: fix comme... |
149 150 151 152 153 |
/*H:640 * When the Guest is run on a different CPU, or the GDT entries have changed, * copy_gdt() is called to copy the Guest's GDT entries across to this CPU's * GDT. */ |
fc708b3e4 lguest: replace l... |
154 |
void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt) |
d7e28ffe6 lguest: the host ... |
155 156 |
{ unsigned int i; |
2e04ef769 lguest: fix comme... |
157 158 159 160 |
/* * The default entries from setup_default_gdt_entries() are not * replaced. See ignored_gdt() above. */ |
d7e28ffe6 lguest: the host ... |
161 162 |
for (i = 0; i < GDT_ENTRIES; i++) if (!ignored_gdt(i)) |
fc708b3e4 lguest: replace l... |
163 |
gdt[i] = cpu->arch.gdt[i]; |
d7e28ffe6 lguest: the host ... |
164 |
} |
2e04ef769 lguest: fix comme... |
165 166 167 168 |
/*H:620 * This is where the Guest asks us to load a new GDT entry * (LHCALL_LOAD_GDT_ENTRY). We tweak the entry and copy it in. */ |
a489f0b55 lguest: fix guest... |
169 |
void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi) |
d7e28ffe6 lguest: the host ... |
170 |
{ |
2e04ef769 lguest: fix comme... |
171 172 173 174 |
/* * We assume the Guest has the same number of GDT entries as the * Host, otherwise we'd have to dynamically allocate the Guest GDT. */ |
3e27249c8 lguest: fix bug i... |
175 |
if (num >= ARRAY_SIZE(cpu->arch.gdt)) { |
382ac6b3f lguest: get rid o... |
176 |
kill_guest(cpu, "too many gdt entries %i", num); |
3e27249c8 lguest: fix bug i... |
177 178 |
return; } |
d7e28ffe6 lguest: the host ... |
179 |
|
a489f0b55 lguest: fix guest... |
180 181 182 183 |
/* Set it up, then fix it. */ cpu->arch.gdt[num].a = lo; cpu->arch.gdt[num].b = hi; fixup_gdt_table(cpu, num, num+1); |
2e04ef769 lguest: fix comme... |
184 185 186 187 |
/* * Mark that the GDT changed so the core knows it has to copy it again, * even if the Guest is run on the same CPU. */ |
ae3749dcd lguest: move chan... |
188 |
cpu->changed |= CHANGED_GDT; |
d7e28ffe6 lguest: the host ... |
189 |
} |
2e04ef769 lguest: fix comme... |
190 191 |
/* * This is the fast-track version for just changing the three TLS entries. |
e1e72965e lguest: documenta... |
192 193 |
* Remember that this happens on every context switch, so it's worth * optimizing. But wouldn't it be neater to have a single hypercall to cover |
2e04ef769 lguest: fix comme... |
194 195 |
* both cases? */ |
fc708b3e4 lguest: replace l... |
196 |
void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls) |
d7e28ffe6 lguest: the host ... |
197 |
{ |
fc708b3e4 lguest: replace l... |
198 |
struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN]; |
d7e28ffe6 lguest: the host ... |
199 |
|
382ac6b3f lguest: get rid o... |
200 |
__lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES); |
fc708b3e4 lguest: replace l... |
201 |
fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1); |
e1e72965e lguest: documenta... |
202 |
/* Note that just the TLS entries have changed. */ |
ae3749dcd lguest: move chan... |
203 |
cpu->changed |= CHANGED_GDT_TLS; |
d7e28ffe6 lguest: the host ... |
204 |
} |
bff672e63 lguest: documenta... |
205 |
|
e1e72965e lguest: documenta... |
206 |
/*H:660 |
bff672e63 lguest: documenta... |
207 208 209 210 211 212 213 214 |
* With this, we have finished the Host. * * Five of the seven parts of our task are complete. You have made it through * the Bit of Despair (I think that's somewhere in the page table code, * myself). * * Next, we examine "make Switcher". It's short, but intense. */ |