diff options
Diffstat (limited to 'drivers/lguest/segments.c')
| -rw-r--r-- | drivers/lguest/segments.c | 126 |
1 files changed, 115 insertions, 11 deletions
diff --git a/drivers/lguest/segments.c b/drivers/lguest/segments.c index 1b2cfe89dcd5..f675a41a80da 100644 --- a/drivers/lguest/segments.c +++ b/drivers/lguest/segments.c @@ -1,16 +1,68 @@ +/*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) { - /* MBZ=0, P=1, DT=1 */ 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 @@ -19,9 +71,18 @@ static int ignored_gdt(unsigned int num) || num == GDT_ENTRY_DOUBLEFAULT_TSS); } -/* We don't allow removal of CS, DS or SS; it doesn't make sense. */ +/* 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) @@ -33,13 +94,21 @@ static void check_segment_use(struct lguest *lg, unsigned int desc) || lg->regs->ss / 8 == desc) kill_guest(lg, "Removed live GDT entry %u", desc); } - +/*:*/ +/*M:009 We wouldn't need to check for removal of in-use segments if we handled + * faults in the Switcher. However, it's probably not a worthwhile + * optimization. :*/ + +/*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 */ + /* We never copy these ones to real GDT, so we don't care what + * they say */ if (ignored_gdt(i)) continue; @@ -53,41 +122,57 @@ static void fixup_gdt_table(struct lguest *lg, unsigned start, unsigned end) if (!desc_ok(&lg->gdt[i])) kill_guest(lg, "Bad GDT descriptor %i", i); - /* DPL 0 presumably means "for use by guest". */ + /* 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); - /* Set accessed bit, since gdt isn't writable. */ + /* 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; - /* Hypervisor segments. */ + /* 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; - /* This is the one which we *cannot* copy from guest, since tss - is depended on this lguest_ro_state, ie. this cpu. */ + /* 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); } -/* This is a fast version for the common case where only the three TLS entries - * have changed. */ +/* 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; @@ -96,22 +181,31 @@ void copy_gdt_tls(const struct lguest *lg, struct desc_struct *gdt) 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; } @@ -123,3 +217,13 @@ void guest_load_tls(struct lguest *lg, unsigned long gtls) 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. + */ |