drivers/lguest/segments.c

   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  */
  52 static 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  */
  67 static 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].b & 0x00006000) == 0)
  85                         cpu->arch.gdt[i].b |= (GUEST_PL << 13);
  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].b |= 0x00000100;
  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  */
 106 void 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          * Forgive the magic flags: the 0x8900 means the entry is Present, it's
 118          * privilege level 0 Available 386 TSS system segment, and the 0x67
 119          * means Saturn is eclipsed by Mercury in the twelfth house.
 120          */
 121         gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
 122         gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
 123                 | ((tss >> 16) & 0x000000FF);
 124 }
 125
 126 /*
 127  * This routine sets up the initial Guest GDT for booting.  All entries start
 128  * as 0 (unusable).
 129  */
 130 void setup_guest_gdt(struct lg_cpu *cpu)
 131 {
 132         /*
 133          * Start with full 0-4G segments...except the Guest is allowed to use
 134          * them, so set the privilege level appropriately in the flags.
 135          */
 136         cpu->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
 137         cpu->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
 138         cpu->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
 139         cpu->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
 140 }
 141
 142 /*H:650
 143  * An optimization of copy_gdt(), for just the three "thead-local storage"
 144  * entries.
 145  */
 146 void copy_gdt_tls(const struct lg_cpu *cpu, struct desc_struct *gdt)
 147 {
 148         unsigned int i;
 149
 150         for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
 151                 gdt[i] = cpu->arch.gdt[i];
 152 }
 153
 154 /*H:640
 155  * When the Guest is run on a different CPU, or the GDT entries have changed,
 156  * copy_gdt() is called to copy the Guest's GDT entries across to this CPU's
 157  * GDT.
 158  */
 159 void copy_gdt(const struct lg_cpu *cpu, struct desc_struct *gdt)
 160 {
 161         unsigned int i;
 162
 163         /*
 164          * The default entries from setup_default_gdt_entries() are not
 165          * replaced.  See ignored_gdt() above.
 166          */
 167         for (i = 0; i < GDT_ENTRIES; i++)
 168                 if (!ignored_gdt(i))
 169                         gdt[i] = cpu->arch.gdt[i];
 170 }
 171
 172 /*H:620
 173  * This is where the Guest asks us to load a new GDT entry
 174  * (LHCALL_LOAD_GDT_ENTRY).  We tweak the entry and copy it in.
 175  */
 176 void load_guest_gdt_entry(struct lg_cpu *cpu, u32 num, u32 lo, u32 hi)
 177 {
 178         /*
 179          * We assume the Guest has the same number of GDT entries as the
 180          * Host, otherwise we'd have to dynamically allocate the Guest GDT.
 181          */
 182         if (num >= ARRAY_SIZE(cpu->arch.gdt))
 183                 kill_guest(cpu, "too many gdt entries %i", num);
 184
 185         /* Set it up, then fix it. */
 186         cpu->arch.gdt[num].a = lo;
 187         cpu->arch.gdt[num].b = hi;
 188         fixup_gdt_table(cpu, num, num+1);
 189         /*
 190          * Mark that the GDT changed so the core knows it has to copy it again,
 191          * even if the Guest is run on the same CPU.
 192          */
 193         cpu->changed |= CHANGED_GDT;
 194 }
 195
 196 /*
 197  * This is the fast-track version for just changing the three TLS entries.
 198  * Remember that this happens on every context switch, so it's worth
 199  * optimizing.  But wouldn't it be neater to have a single hypercall to cover
 200  * both cases?
 201  */
 202 void guest_load_tls(struct lg_cpu *cpu, unsigned long gtls)
 203 {
 204         struct desc_struct *tls = &cpu->arch.gdt[GDT_ENTRY_TLS_MIN];
 205
 206         __lgread(cpu, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
 207         fixup_gdt_table(cpu, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
 208         /* Note that just the TLS entries have changed. */
 209         cpu->changed |= CHANGED_GDT_TLS;
 210 }
 211
 212 /*H:660
 213  * With this, we have finished the Host.
 214  *
 215  * Five of the seven parts of our task are complete.  You have made it through
 216  * the Bit of Despair (I think that's somewhere in the page table code,
 217  * myself).
 218  *
 219  * Next, we examine "make Switcher".  It's short, but intense.
 220  */