226 lines
8.6 KiB
C
226 lines
8.6 KiB
C
/*P:600 The x86 architecture has segments, which involve a table of descriptors
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* which can be used to do funky things with virtual address interpretation.
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* We originally used to use segments so the Guest couldn't alter the
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* Guest<->Host Switcher, and then we had to trim Guest segments, and restore
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* for userspace per-thread segments, but trim again for on userspace->kernel
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* transitions... This nightmarish creation was contained within this file,
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* where we knew not to tread without heavy armament and a change of underwear.
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*
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* In these modern times, the segment handling code consists of simple sanity
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* checks, and the worst you'll experience reading this code is butterfly-rash
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* from frolicking through its parklike serenity. :*/
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#include "lg.h"
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/*H:600
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* We've almost completed the Host; there's just one file to go!
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*
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* Segments & The Global Descriptor Table
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*
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* (That title sounds like a bad Nerdcore group. Not to suggest that there are
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* any good Nerdcore groups, but in high school a friend of mine had a band
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* called Joe Fish and the Chips, so there are definitely worse band names).
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*
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* To refresh: the GDT is a table of 8-byte values describing segments. Once
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* set up, these segments can be loaded into one of the 6 "segment registers".
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*
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* GDT entries are passed around as "struct desc_struct"s, which like IDT
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* entries are split into two 32-bit members, "a" and "b". One day, someone
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* will clean that up, and be declared a Hero. (No pressure, I'm just saying).
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*
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* Anyway, the GDT entry contains a base (the start address of the segment), a
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* limit (the size of the segment - 1), and some flags. Sounds simple, and it
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* would be, except those zany Intel engineers decided that it was too boring
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* to put the base at one end, the limit at the other, and the flags in
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* between. They decided to shotgun the bits at random throughout the 8 bytes,
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* like so:
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*
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* 0 16 40 48 52 56 63
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* [ limit part 1 ][ base part 1 ][ flags ][li][fl][base ]
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* mit ags part 2
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* part 2
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*
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* As a result, this file contains a certain amount of magic numeracy. Let's
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* begin.
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*/
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/* Is the descriptor the Guest wants us to put in OK?
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*
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* The flag which Intel says must be zero: must be zero. The descriptor must
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* be present, (this is actually checked earlier but is here for thorougness),
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* and the descriptor type must be 1 (a memory segment). */
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static int desc_ok(const struct desc_struct *gdt)
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{
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return ((gdt->b & 0x00209000) == 0x00009000);
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}
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/* Is the segment present? (Otherwise it can't be used by the Guest). */
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static int segment_present(const struct desc_struct *gdt)
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{
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return gdt->b & 0x8000;
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}
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/* There are several entries we don't let the Guest set. The TSS entry is the
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* "Task State Segment" which controls all kinds of delicate things. The
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* LGUEST_CS and LGUEST_DS entries are reserved for the Switcher, and the
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* the Guest can't be trusted to deal with double faults. */
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static int ignored_gdt(unsigned int num)
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{
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return (num == GDT_ENTRY_TSS
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|| num == GDT_ENTRY_LGUEST_CS
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|| num == GDT_ENTRY_LGUEST_DS
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|| num == GDT_ENTRY_DOUBLEFAULT_TSS);
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}
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/* If the Guest asks us to remove an entry from the GDT, we have to be careful.
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* If one of the segment registers is pointing at that entry the Switcher will
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* crash when it tries to reload the segment registers for the Guest.
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*
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* It doesn't make much sense for the Guest to try to remove its own code, data
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* or stack segments while they're in use: assume that's a Guest bug. If it's
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* one of the lesser segment registers using the removed entry, we simply set
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* that register to 0 (unusable). */
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static void check_segment_use(struct lguest *lg, unsigned int desc)
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{
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/* GDT entries are 8 bytes long, so we divide to get the index and
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* ignore the bottom bits. */
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if (lg->regs->gs / 8 == desc)
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lg->regs->gs = 0;
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if (lg->regs->fs / 8 == desc)
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lg->regs->fs = 0;
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if (lg->regs->es / 8 == desc)
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lg->regs->es = 0;
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if (lg->regs->ds / 8 == desc
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|| lg->regs->cs / 8 == desc
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|| lg->regs->ss / 8 == desc)
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kill_guest(lg, "Removed live GDT entry %u", desc);
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}
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/*H:610 Once the GDT has been changed, we look through the changed entries and
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* see if they're OK. If not, we'll call kill_guest() and the Guest will never
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* get to use the invalid entries. */
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static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end)
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{
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unsigned int i;
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for (i = start; i < end; i++) {
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/* We never copy these ones to real GDT, so we don't care what
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* they say */
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if (ignored_gdt(i))
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continue;
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/* We could fault in switch_to_guest if they are using
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* a removed segment. */
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if (!segment_present(&lg->gdt[i])) {
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check_segment_use(lg, i);
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continue;
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}
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if (!desc_ok(&lg->gdt[i]))
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kill_guest(lg, "Bad GDT descriptor %i", i);
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/* Segment descriptors contain a privilege level: the Guest is
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* sometimes careless and leaves this as 0, even though it's
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* running at privilege level 1. If so, we fix it here. */
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if ((lg->gdt[i].b & 0x00006000) == 0)
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lg->gdt[i].b |= (GUEST_PL << 13);
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/* Each descriptor has an "accessed" bit. If we don't set it
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* now, the CPU will try to set it when the Guest first loads
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* that entry into a segment register. But the GDT isn't
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* writable by the Guest, so bad things can happen. */
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lg->gdt[i].b |= 0x00000100;
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}
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}
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/* This routine is called at boot or modprobe time for each CPU to set up the
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* "constant" GDT entries for Guests running on that CPU. */
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void setup_default_gdt_entries(struct lguest_ro_state *state)
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{
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struct desc_struct *gdt = state->guest_gdt;
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unsigned long tss = (unsigned long)&state->guest_tss;
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/* The hypervisor segments are full 0-4G segments, privilege level 0 */
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gdt[GDT_ENTRY_LGUEST_CS] = FULL_EXEC_SEGMENT;
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gdt[GDT_ENTRY_LGUEST_DS] = FULL_SEGMENT;
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/* The TSS segment refers to the TSS entry for this CPU, so we cannot
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* copy it from the Guest. Forgive the magic flags */
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gdt[GDT_ENTRY_TSS].a = 0x00000067 | (tss << 16);
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gdt[GDT_ENTRY_TSS].b = 0x00008900 | (tss & 0xFF000000)
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| ((tss >> 16) & 0x000000FF);
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}
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/* This routine is called before the Guest is run for the first time. */
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void setup_guest_gdt(struct lguest *lg)
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{
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/* Start with full 0-4G segments... */
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lg->gdt[GDT_ENTRY_KERNEL_CS] = FULL_EXEC_SEGMENT;
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lg->gdt[GDT_ENTRY_KERNEL_DS] = FULL_SEGMENT;
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/* ...except the Guest is allowed to use them, so set the privilege
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* level appropriately in the flags. */
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lg->gdt[GDT_ENTRY_KERNEL_CS].b |= (GUEST_PL << 13);
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lg->gdt[GDT_ENTRY_KERNEL_DS].b |= (GUEST_PL << 13);
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}
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/* Like the IDT, we never simply use the GDT the Guest gives us. We set up the
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* GDTs for each CPU, then we copy across the entries each time we want to run
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* a different Guest on that CPU. */
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/* A partial GDT load, for the three "thead-local storage" entries. Otherwise
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* it's just like load_guest_gdt(). So much, in fact, it would probably be
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* neater to have a single hypercall to cover both. */
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void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt)
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{
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unsigned int i;
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for (i = GDT_ENTRY_TLS_MIN; i <= GDT_ENTRY_TLS_MAX; i++)
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gdt[i] = lg->gdt[i];
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}
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/* This is the full version */
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void copy_gdt(const struct lguest *lg, struct desc_struct *gdt)
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{
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unsigned int i;
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/* The default entries from setup_default_gdt_entries() are not
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* replaced. See ignored_gdt() above. */
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for (i = 0; i < GDT_ENTRIES; i++)
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if (!ignored_gdt(i))
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gdt[i] = lg->gdt[i];
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}
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/* This is where the Guest asks us to load a new GDT (LHCALL_LOAD_GDT). */
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void load_guest_gdt(struct lguest *lg, unsigned long table, u32 num)
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{
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/* We assume the Guest has the same number of GDT entries as the
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* Host, otherwise we'd have to dynamically allocate the Guest GDT. */
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if (num > ARRAY_SIZE(lg->gdt))
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kill_guest(lg, "too many gdt entries %i", num);
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/* We read the whole thing in, then fix it up. */
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lgread(lg, lg->gdt, table, num * sizeof(lg->gdt[0]));
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fixup_gdt_table(lg, 0, ARRAY_SIZE(lg->gdt));
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/* Mark that the GDT changed so the core knows it has to copy it again,
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* even if the Guest is run on the same CPU. */
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lg->changed |= CHANGED_GDT;
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}
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void guest_load_tls(struct lguest *lg, unsigned long gtls)
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{
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struct desc_struct *tls = &lg->gdt[GDT_ENTRY_TLS_MIN];
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lgread(lg, tls, gtls, sizeof(*tls)*GDT_ENTRY_TLS_ENTRIES);
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fixup_gdt_table(lg, GDT_ENTRY_TLS_MIN, GDT_ENTRY_TLS_MAX+1);
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lg->changed |= CHANGED_GDT_TLS;
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}
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/*
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* With this, we have finished the Host.
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*
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* Five of the seven parts of our task are complete. You have made it through
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* the Bit of Despair (I think that's somewhere in the page table code,
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* myself).
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*
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* Next, we examine "make Switcher". It's short, but intense.
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*/
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