drivers/lguest/segments.c

   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. :*/
  12 #include "lg.h"
  13
  14 /*H:600
  15  * Segments & The Global Descriptor Table
  16  *
  17  * (That title sounds like a bad Nerdcore group.  Not to suggest that there are
  18  * any good Nerdcore groups, but in high school a friend of mine had a band
  19  * called Joe Fish and the Chips, so there are definitely worse band names).
  20  *
  21  * To refresh: the GDT is a table of 8-byte values describing segments.  Once
  22  * set up, these segments can be loaded into one of the 6 "segment registers".
  23  *
  24  * GDT entries are passed around as "struct desc_struct"s, which like IDT
  25  * entries are split into two 32-bit members, "a" and "b".  One day, someone
  26  * will clean that up, and be declared a Hero.  (No pressure, I'm just saying).
  27  *
  28  * Anyway, the GDT entry contains a base (the start address of the segment), a
  29  * limit (the size of the segment - 1), and some flags.  Sounds simple, and it
  30  * would be, except those zany Intel engineers decided that it was too boring
  31  * to put the base at one end, the limit at the other, and the flags in
  32  * between.  They decided to shotgun the bits at random throughout the 8 bytes,
  33  * like so:
  34  *
  35  * 0               16                     40       48  52  56     63
  36  * [ limit part 1 ][     base part 1     ][ flags ][li][fl][base ]
  37  *                                                  mit ags part 2
  38  *                                                part 2
  39  *
  40  * As a result, this file contains a certain amount of magic numeracy.  Let's
  41  * begin.
  42  */
  43
  44 /* There are several entries we don't let the Guest set.  The TSS entry is the
  45  * "Task State Segment" which controls all kinds of delicate things.  The
  46  * LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
  47  * the Guest can't be trusted to deal with double faults. */
  48 static int ignored_gdt(unsigned int num)
  49 {
  50         return (num == GDT_ENTRY_TSS
  51                 || num == GDT_ENTRY_LGUEST_CS
  52                 || num == GDT_ENTRY_LGUEST_DS
  53                 || num == GDT_ENTRY_DOUBLEFAULT_TSS);
  54 }
  55
  56 /*H:630 Once the Guest gave us new GDT entries, we fix them up a little.  We
  57  * don't care if they're invalid: the worst that can happen is a General
  58  * Protection Fault in the Switcher when it restores a Guest segment register
  59  * which tries to use that entry.  Then we kill the Guest for causing such a
  60  * mess: the message will be "unhandled trap 256". */
  61 static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
  62 {
  63         unsigned int i;
  64
  65         for (i = start; i < end; i++) {
  66                 /* We never copy these ones to real GDT, so we don't care what
  67                  * they say */
  68                 if (ignored_gdt(i))
  69                         continue;
  70
  71                 /* Segment descriptors contain a privilege level: the Guest is
  72                  * sometimes careless and leaves this as 0, even though it's
  73                  * running at privilege level 1.  If so, we fix it here. */
  74                 if ((lg->arch.gdt[i].b & 0x00006000) == 0)
  75                         lg->arch.gdt[i].b |= (GUEST_PL << 13);
  76
  77                 /* Each descriptor has an "accessed" bit.  If we don't set it
  78                  * now, the CPU will try to set it when the Guest first loads
  79                  * that entry into a segment register.  But the GDT isn't
  80                  * writable by the Guest, so bad things can happen. */
  81                 lg->arch.gdt[i].b |= 0x00000100;
  82         }
  83 }
  84
  85 /*H:610 Like the IDT, we never simply use the GDT the Guest gives us.  We keep
  86  * a GDT for each CPU, and copy across the Guest's entries each time we want to
  87  * run the Guest on that CPU.
  88  *
  89  * This routine is called at boot or modprobe time for each CPU to set up the
  90  * constant GDT entries: the ones which are the same no matter what Guest we're
  91  * running. */
  92 void setup_default_gdt_entries(struct lguest_ro_state *state)
  93 {
  94         struct desc_struct *gdt = state->guest_gdt;
  95         unsigned long tss = (unsigned long)&state->guest_tss;
  96
  97         /* The Switcher segments are full 0-4G segments, privilege level 0 */
  98         gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
  99         gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
 100
 101         /* The TSS segment refers to the TSS entry for this particular CPU.
 102          * Forgive the magic flags: the 0x8900 means the entry is Present, it's
 103          * privilege level 0 Available 386 TSS system segment, and the 0x67
 104          * means Saturn is eclipsed by Mercury in the twelfth house. */
 105         gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
 106         gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
 107                 | ((tss >> 16) & 0x000000FF);
 108 }
 109
 110 /* This routine sets up the initial Guest GDT for booting.  All entries start
 111  * as 0 (unusable). */
 112 void setup_guest_gdt(struct lguest *lg)
 113 {
 114         /* Start with full 0-4G segments... */
 115         lg->arch.gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
 116         lg->arch.gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
 117         /* ...except the Guest is allowed to use them, so set the privilege
 118          * level appropriately in the flags. */
 119         lg->arch.gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
 120         lg->arch.gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
 121 }
 122
 123 /*H:650 An optimization of copy_gdt(), for just the three "thead-local storage"
 124  * entries. */
 125 void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
 126 {
 127         unsigned int i;
 128
 129         for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
 130                 gdt[i] = lg->arch.gdt[i];
 131 }
 132
 133 /*H:640 When the Guest is run on a different CPU, or the GDT entries have
 134  * changed, copy_gdt() is called to copy the Guest's GDT entries across to this
 135  * CPU's GDT. */
 136 void copy_gdt(const struct lguest *lg, struct desc_struct *gdt)
 137 {
 138         unsigned int i;
 139
 140         /* The default entries from setup_default_gdt_entries() are not
 141          * replaced.  See ignored_gdt() above. */
 142         for (i = 0; i < GDT_ENTRIES; i++)
 143                 if (!ignored_gdt(i))
 144                         gdt[i] = lg->arch.gdt[i];
 145 }
 146
 147 /*H:620 This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT).
 148  * We copy it from the Guest and tweak the entries. */
 149 void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num)
 150 {
 151         /* We assume the Guest has the same number of GDT entries as the
 152          * Host, otherwise we'd have to dynamically allocate the Guest GDT. */
 153         if (num > ARRAY_SIZE(lg->arch.gdt))
 154                 kill_guest(lg, "too many gdt entries %i", num);
 155
 156         /* We read the whole thing in, then fix it up. */
 157         __lgread(lg, lg->arch.gdt, table, num * sizeof(lg->arch.gdt[0]));
 158         fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->arch.gdt));
 159         /* Mark that the GDT changed so the core knows it has to copy it again,
 160          * even if the Guest is run on the same CPU. */
 161         lg->changed |= CHANGED_GDT;
 162 }
 163
 164 /* This is the fast-track version for just changing the three TLS entries.
 165  * Remember that this happens on every context switch, so it's worth
 166  * optimizing.  But wouldn't it be neater to have a single hypercall to cover
 167  * both cases? */
 168 void guest_load_tls(struct lguest *lg, unsigned long gtls)
 169 {
 170         struct desc_struct *tls = &lg->arch.gdt[GDT_ENTRY_TLS_MIN];
 171
 172         __lgread(lg, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
 173         fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
 174         /* Note that just the TLS entries have changed. */
 175         lg->changed |= CHANGED_GDT_TLS;
 176 }
 177 /*:*/
 178
 179 /*H:660
 180  * With this, we have finished the Host.
 181  *
 182  * Five of the seven parts of our task are complete.  You have made it through
 183  * the Bit of Despair (I think that's somewhere in the page table code,
 184  * myself).
 185  *
 186  * Next, we examine "make Switcher".  It's short, but intense.
 187  */