linux/drivers/lguest/segments.c
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   1/*P:600
   2 * The x86 architecture has segments, which involve a table of descriptors
   3 * which can be used to do funky things with virtual address interpretation.
   4 * We originally used to use segments so the Guest couldn't alter the
   5 * Guest<->Host Switcher, and then we had to trim Guest segments, and restore
   6 * for userspace per-thread segments, but trim again for on userspace->kernel
   7 * transitions...  This nightmarish creation was contained within this file,
   8 * where we knew not to tread without heavy armament and a change of underwear.
   9 *
  10 * In these modern times, the segment handling code consists of simple sanity
  11 * checks, and the worst you'll experience reading this code is butterfly-rash
  12 * from frolicking through its parklike serenity.
  13:*/
  14#include "lg.h"
  15
  16/*H:600
  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
  46/*
  47 * There are several entries we don't let the Guest set.  The TSS entry is the
  48 * "Task State Segment" which controls all kinds of delicate things.  The
  49 * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
  50 * the Guest can't be trusted to deal with double faults.
  51 */
  52static bool ignored_gdt(unsigned int num)
  53{
  54        return (num == GDT_ENTRY_TSS
  55                || num == GDT_ENTRY_LGUEST_CS
  56                || num == GDT_ENTRY_LGUEST_DS
  57                || num == GDT_ENTRY_DOUBLEFAULT_TSS);
  58}
  59
  60/*H:630
  61 * Once the Guest gave us new GDT entries, we fix them up a little.  We
  62 * don't care if they're invalid: the worst that can happen is a General
  63 * Protection Fault in the Switcher when it restores a Guest segment register
  64 * which tries to use that entry.  Then we kill the Guest for causing such a
  65 * mess: the message will be "unhandled trap 256".
  66 */
  67static void fixup_gdt_table(struct lg_cpu *cpu, unsigned start, unsigned end)
  68{
  69        unsigned int i;
  70
  71        for (i = start; i < end; i++) {
  72                /*
  73                 * We never copy these ones to real GDT, so we don't care what
  74                 * they say
  75                 */
  76                if (ignored_gdt(i))
  77                        continue;
  78
  79                /*
  80                 * Segment descriptors contain a privilege level: the Guest is
  81                 * sometimes careless and leaves this as 0, even though it's
  82                 * running at privilege level 1.  If so, we fix it here.
  83                 */
  84                if (cpu->arch.gdt[i].dpl == 0)
  85                        cpu->arch.gdt[i].dpl |= GUEST_PL;
  86
  87                /*
  88                 * Each descriptor has an "accessed" bit.  If we don't set it
  89                 * now, the CPU will try to set it when the Guest first loads
  90                 * that entry into a segment register.  But the GDT isn't
  91                 * writable by the Guest, so bad things can happen.
  92                 */
  93                cpu->arch.gdt[i].type |= 0x1;
  94        }
  95}
  96
  97/*H:610
  98 * Like the IDT, we never simply use the GDT the Guest gives us.  We keep
  99 * a GDT for each CPU, and copy across the Guest's entries each time we want to
 100 * run the Guest on that CPU.
 101 *
 102 * This routine is called at boot or modprobe time for each CPU to set up the
 103 * constant GDT entries: the ones which are the same no matter what Guest we're
 104 * running.
 105 */
 106void setup_default_gdt_entries(struct lguest_ro_state *state)
 107{
 108        struct desc_struct *gdt = state->guest_gdt;
 109        unsigned long tss = (unsigned long)&state->guest_tss;
 110
 111        /* The Switcher segments are full 0-4G segments, privilege level 0 */
 112        gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
 113        gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
 114
 115        /*
 116         * The TSS segment refers to the TSS entry for this particular CPU.
 117         */
 118        gdt[GDT_ENTRY_TSS].a = 0;
 119        gdt[GDT_ENTRY_TSS].b = 0;
 120
 121        gdt[GDT_ENTRY_TSS].limit0 = 0x67;
 122        gdt[GDT_ENTRY_TSS].base0  = tss & 0xFFFF;
 123        gdt[GDT_ENTRY_TSS].base1  = (tss >> 16) & 0xFF;
 124        gdt[GDT_ENTRY_TSS].base2  = tss >> 24;
 125        gdt[GDT_ENTRY_TSS].type   = 0x9; /* 32-bit TSS (available) */
 126        gdt[GDT_ENTRY_TSS].p      = 0x1; /* Entry is present */
 127        gdt[GDT_ENTRY_TSS].dpl    = 0x0; /* Privilege level 0 */
 128        gdt[GDT_ENTRY_TSS].s      = 0x0; /* system segment */
 129
 130}
 131
 132/*
 133 * This routine sets up the initial Guest GDT for booting.  All entries start
 134 * as 0 (unusable).
 135 */
 136void setup_guest_gdt(struct lg_cpu *cpu)
 137{
 138        /*
 139         * Start with full 0-4G segments...except the Guest is allowed to use
 140         * them, so set the privilege level appropriately in the flags.
 141         */
 142        cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
 143        cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
 144        cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].dpl |= GUEST_PL;
 145        cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].dpl |= GUEST_PL;
 146}
 147
 148/*H:650
 149 * An optimization of copy_gdt(), for just the three "thead-local storage"
 150 * entries.
 151 */
 152void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt)
 153{
 154        unsigned int i;
 155
 156        for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
 157                gdt[i] = cpu->arch.gdt[i];
 158}
 159
 160/*H:640
 161 * When the Guest is run on a different CPU, or the GDT entries have changed,
 162 * copy_gdt() is called to copy the Guest's GDT entries across to this CPU's
 163 * GDT.
 164 */
 165void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt)
 166{
 167        unsigned int i;
 168
 169        /*
 170         * The default entries from setup_default_gdt_entries() are not
 171         * replaced.  See ignored_gdt() above.
 172         */
 173        for (i = 0; i < GDT_ENTRIES; i++)
 174                if (!ignored_gdt(i))
 175                        gdt[i] = cpu->arch.gdt[i];
 176}
 177
 178/*H:620
 179 * This is where the Guest asks us to load a new GDT entry
 180 * (LHCALL_LOAD_GDT_ENTRY).  We tweak the entry and copy it in.
 181 */
 182void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi)
 183{
 184        /*
 185         * We assume the Guest has the same number of GDT entries as the
 186         * Host, otherwise we'd have to dynamically allocate the Guest GDT.
 187         */
 188        if (num >= ARRAY_SIZE(cpu->arch.gdt)) {
 189                kill_guest(cpu, "too many gdt entries %i", num);
 190                return;
 191        }
 192
 193        /* Set it up, then fix it. */
 194        cpu->arch.gdt[num].a = lo;
 195        cpu->arch.gdt[num].b = hi;
 196        fixup_gdt_table(cpu, num, num+1);
 197        /*
 198         * Mark that the GDT changed so the core knows it has to copy it again,
 199         * even if the Guest is run on the same CPU.
 200         */
 201        cpu->changed |= CHANGED_GDT;
 202}
 203
 204/*
 205 * This is the fast-track version for just changing the three TLS entries.
 206 * Remember that this happens on every context switch, so it's worth
 207 * optimizing.  But wouldn't it be neater to have a single hypercall to cover
 208 * both cases?
 209 */
 210void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls)
 211{
 212        struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN];
 213
 214        __lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
 215        fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
 216        /* Note that just the TLS entries have changed. */
 217        cpu->changed |= CHANGED_GDT_TLS;
 218}
 219
 220/*H:660
 221 * With this, we have finished the Host.
 222 *
 223 * Five of the seven parts of our task are complete.  You have made it through
 224 * the Bit of Despair (I think that's somewhere in the page table code,
 225 * myself).
 226 *
 227 * Next, we examine "make Switcher".  It's short, but intense.
 228 */
 229
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