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.
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. :*/
15 * We've almost completed the Host; there's just one file to go!
17 * Segments & The Global Descriptor Table
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).
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".
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).
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,
38 * [ limit part 1 ][ base part 1 ][ flags ][li][fl][base ]
42 * As a result, this file contains a certain amount of magic numeracy. Let's
46 /* Is the descriptor the Guest wants us to put in OK?
48 * The flag which Intel says must be zero: must be zero. The descriptor must
49 * be present, (this is actually checked earlier but is here for thorougness),
50 * and the descriptor type must be 1 (a memory segment). */
51 static int desc_ok(const struct desc_struct *gdt)
53 return ((gdt->b & 0x00209000) == 0x00009000);
56 /* Is the segment present? (Otherwise it can't be used by the Guest). */
57 static int segment_present(const struct desc_struct *gdt)
59 return gdt->b & 0x8000;
62 /* There are several entries we don't let the Guest set. The TSS entry is the
63 * "Task State Segment" which controls all kinds of delicate things. The
64 * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
65 * the Guest can't be trusted to deal with double faults. */
66 static int ignored_gdt(unsigned int num)
68 return (num == GDT_ENTRY_TSS
69 || num == GDT_ENTRY_LGUEST_CS
70 || num == GDT_ENTRY_LGUEST_DS
71 || num == GDT_ENTRY_DOUBLEFAULT_TSS);
74 /* If the Guest asks us to remove an entry from the GDT, we have to be careful.
75 * If one of the segment registers is pointing at that entry the Switcher will
76 * crash when it tries to reload the segment registers for the Guest.
78 * It doesn't make much sense for the Guest to try to remove its own code, data
79 * or stack segments while they're in use: assume that's a Guest bug. If it's
80 * one of the lesser segment registers using the removed entry, we simply set
81 * that register to 0 (unusable). */
82 static void check_segment_use(struct lguest *lg, unsigned int desc)
84 /* GDT entries are 8 bytes long, so we divide to get the index and
85 * ignore the bottom bits. */
86 if (lg->regs->gs / 8 == desc)
88 if (lg->regs->fs / 8 == desc)
90 if (lg->regs->es / 8 == desc)
92 if (lg->regs->ds / 8 == desc
93 || lg->regs->cs / 8 == desc
94 || lg->regs->ss / 8 == desc)
95 kill_guest(lg, "Removed live GDT entry %u", desc);
98 /*M:009 We wouldn't need to check for removal of in-use segments if we handled
99 * faults in the Switcher. However, it's probably not a worthwhile
102 /*H:610 Once the GDT has been changed, we look through the changed entries and
103 * see if they're OK. If not, we'll call kill_guest() and the Guest will never
104 * get to use the invalid entries. */
105 static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
109 for (i = start; i < end; i++) {
110 /* We never copy these ones to real GDT, so we don't care what
115 /* We could fault in switch_to_guest if they are using
116 * a removed segment. */
117 if (!segment_present(&lg->gdt[i])) {
118 check_segment_use(lg, i);
122 if (!desc_ok(&lg->gdt[i]))
123 kill_guest(lg, "Bad GDT descriptor %i", i);
125 /* Segment descriptors contain a privilege level: the Guest is
126 * sometimes careless and leaves this as 0, even though it's
127 * running at privilege level 1. If so, we fix it here. */
128 if ((lg->gdt[i].b & 0x00006000) == 0)
129 lg->gdt[i].b |= (GUEST_PL << 13);
131 /* Each descriptor has an "accessed" bit. If we don't set it
132 * now, the CPU will try to set it when the Guest first loads
133 * that entry into a segment register. But the GDT isn't
134 * writable by the Guest, so bad things can happen. */
135 lg->gdt[i].b |= 0x00000100;
139 /* This routine is called at boot or modprobe time for each CPU to set up the
140 * "constant" GDT entries for Guests running on that CPU. */
141 void setup_default_gdt_entries(struct lguest_ro_state *state)
143 struct desc_struct *gdt = state->guest_gdt;
144 unsigned long tss = (unsigned long)&state->guest_tss;
146 /* The hypervisor segments are full 0-4G segments, privilege level 0 */
147 gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
148 gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
150 /* The TSS segment refers to the TSS entry for this CPU, so we cannot
151 * copy it from the Guest. Forgive the magic flags */
152 gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
153 gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
154 | ((tss >> 16) & 0x000000FF);
157 /* This routine is called before the Guest is run for the first time. */
158 void setup_guest_gdt(struct lguest *lg)
160 /* Start with full 0-4G segments... */
161 lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
162 lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
163 /* ...except the Guest is allowed to use them, so set the privilege
164 * level appropriately in the flags. */
165 lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
166 lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
169 /* Like the IDT, we never simply use the GDT the Guest gives us. We set up the
170 * GDTs for each CPU, then we copy across the entries each time we want to run
171 * a different Guest on that CPU. */
173 /* A partial GDT load, for the three "thead-local storage" entries. Otherwise
174 * it's just like load_guest_gdt(). So much, in fact, it would probably be
175 * neater to have a single hypercall to cover both. */
176 void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
180 for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
184 /* This is the full version */
185 void copy_gdt(const struct lguest *lg, struct desc_struct *gdt)
189 /* The default entries from setup_default_gdt_entries() are not
190 * replaced. See ignored_gdt() above. */
191 for (i = 0; i < GDT_ENTRIES; i++)
196 /* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */
197 void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num)
199 /* We assume the Guest has the same number of GDT entries as the
200 * Host, otherwise we'd have to dynamically allocate the Guest GDT. */
201 if (num > ARRAY_SIZE(lg->gdt))
202 kill_guest(lg, "too many gdt entries %i", num);
204 /* We read the whole thing in, then fix it up. */
205 lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0]));
206 fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt));
207 /* Mark that the GDT changed so the core knows it has to copy it again,
208 * even if the Guest is run on the same CPU. */
209 lg->changed |= CHANGED_GDT;
212 void guest_load_tls(struct lguest *lg, unsigned long gtls)
214 struct desc_struct *tls = &lg->gdt[GDT_ENTRY_TLS_MIN];
216 lgread(lg, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
217 fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
218 lg->changed |= CHANGED_GDT_TLS;
222 * With this, we have finished the Host.
224 * Five of the seven parts of our task are complete. You have made it through
225 * the Bit of Despair (I think that's somewhere in the page table code,
228 * Next, we examine "make Switcher". It's short, but intense.