SchOS/kernel/arch/x86_64/ioperm.c

// The TSS I/O permission bitmap. See include/ioperm.h for why.

#include "types.h"
#include "defs.h"
#include "param.h"
#include "mmu.h"
#include "proc.h"
#include "ioperm.h"

// The 64-bit TSS, written out rather than indexed.
//
// The old code reached it as `uint *tss` and set `tss[16]` to put 0x68 in the
// I/O map base -- but the I/O map base is at offset 102, which is the high half
// of the dword at index 25, not 16. Index 16 is offset 64, in the middle of the
// interrupt stack table.
//
// That was not harmless. The field it meant to write stayed 0, and a base of 0
// with a limit of 0x67 does not mean "no bitmap" -- it means the bitmap starts
// at the top of the TSS, so the CPU checked ports 0-831 against the TSS's own
// bytes. Those bytes are RSP0: a kernel stack pointer, whose zero bits are
// *permission*. Some low ports were reachable from user mode and had been
// since the kernel was 64-bit, which nothing had any reason to notice.
//
// Naming the fields is what makes that visible, so they are named.
struct taskstate64 {
  uint32 res0;
  uint64 rsp0;
  uint64 rsp1;
  uint64 rsp2;
  uint64 res1;
  uint64 ist[7];
  uint64 res2;
  uint16 res3;
  uint16 iomb;              // offset from the base of the TSS
} __attribute__((packed));

#define IOMAP_BITS  65536
#define IOMAP_BYTES (IOMAP_BITS / 8)

// The TSS and its bitmap have to be one object: `iomb` is a 16-bit offset from
// the TSS base, and the TSS descriptor's limit is what cuts the bitmap off.
// So they are declared together and there is no way for them to drift apart.
//
// 8 KB per CPU of permanently resident kernel memory is the cost of being able
// to say "these ports and no others", and it is charged whether or not any
// process ever uses it. A bitmap sized to the highest port actually granted
// would be smaller and would have to be resized at runtime, under a descriptor
// the CPU is using; this is the version that is obviously correct.
struct tssblk {
  struct taskstate64 ts;
  uint8  map[IOMAP_BYTES];
  // Intel requires a byte of all-ones past the end, so an access that straddles
  // the final port still reads a deny bit rather than whatever follows.
  uint8  tail;
} __attribute__((packed, aligned(16)));

static struct tssblk tssblk[NCPU];

// What each CPU currently has open. Kept per CPU rather than read back from the
// process, because switching *away* has to close what was opened and the
// process that opened it may be part-way through exit() by then.
static struct {
  struct ioport_range r[NIOPORT];
  int n;
} installed[NCPU];

static void
map_set(uint8 *map, uint16 base, uint16 len, int allow)
{
  uint32 p, end = (uint32)base + len;

  if (end > IOMAP_BITS)
    end = IOMAP_BITS;
  for (p = base; p < end; p++) {
    if (allow)
      map[p >> 3] &= ~(1 << (p & 7));
    else
      map[p >> 3] |= (1 << (p & 7));
  }
}

void *
ioperm_tss(int cpu, uint64 *limit)
{
  struct tssblk *b = &tssblk[cpu];

  // Deny everything to start with. A zero bit is permission, so a zeroed
  // bitmap is an open machine -- this memset is not initialisation tidiness.
  memset(b->map, 0xFF, sizeof(b->map));
  b->tail = 0xFF;
  b->ts.iomb = (uint16)__builtin_offsetof(struct tssblk, map);
  installed[cpu].n = 0;

  // The limit is the offset of the last valid byte, and it has to include the
  // trailing 0xFF -- stopping one byte short is the difference between a
  // straddling access being denied and it reading the next object.
  *limit = (uint64)__builtin_offsetof(struct tssblk, tail);
  return b;
}

// The same block, without the reset. seginit() calls ioperm_tss() once per CPU
// to build it; everything after that just wants the pointer.
void *
ioperm_tss_of(int cpu)
{
  return &tssblk[cpu];
}

void
ioperm_set_rsp0(void *tss, addr_t stktop)
{
  ((struct taskstate64 *)tss)->rsp0 = (uint64)stktop;
}

void
ioperm_grant(struct proc *p, uint16 base, uint16 len)
{
  int i;

  if (len == 0)
    return;
  for (i = 0; i < p->nioport; i++)
    if (p->ioport[i].base == base && p->ioport[i].len == len)
      return;
  if (p->nioport >= NIOPORT)
    return;                     // refused quietly; the driver will fault loudly
  p->ioport[p->nioport].base = base;
  p->ioport[p->nioport].len  = len;
  p->nioport++;
}

void
ioperm_revoke_all(struct proc *p)
{
  p->nioport = 0;
  // The bitmap is not touched here. This can run on a CPU that is not the one
  // holding the process's ranges open, and the next switchuvm() on each CPU
  // closes what that CPU opened -- which is the only place the two can be made
  // to agree without an IPI.
}

void
ioperm_switch(struct proc *p)
{
  int c = cpunum();
  struct tssblk *b = &tssblk[c];
  int i;

  // Close whatever was open. Doing this unconditionally rather than diffing
  // against the incoming process costs a few hundred bit clears on a machine
  // where nobody uses ports at all -- which is `installed[c].n == 0`, and then
  // this loop does nothing.
  for (i = 0; i < installed[c].n; i++)
    map_set(b->map, installed[c].r[i].base, installed[c].r[i].len, 0);
  installed[c].n = 0;

  if (!p || p->nioport == 0)
    return;

  for (i = 0; i < p->nioport && i < NIOPORT; i++) {
    map_set(b->map, p->ioport[i].base, p->ioport[i].len, 1);
    installed[c].r[i] = p->ioport[i];
  }
  installed[c].n = i;
}