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