zircon/kernel/lib/userabi/vdso.cc

// Copyright 2016 The Fuchsia Authors
//
// Use of this source code is governed by a MIT-style
// license that can be found in the LICENSE file or at
// https://opensource.org/licenses/MIT

#include <lib/affine/ratio.h>
#include <lib/boot-options/boot-options.h>
#include <lib/userabi/vdso-constants.h>
#include <lib/userabi/vdso.h>
#include <lib/version.h>
#include <platform.h>
#include <zircon/types.h>

#include <arch/quirks.h>
#include <fbl/alloc_checker.h>
#include <kernel/mp.h>
#include <ktl/array.h>
#include <object/handle.h>
#include <vm/pmm.h>
#include <vm/vm.h>
#include <vm/vm_aspace.h>
#include <vm/vm_object.h>

#include "vdso-code.h"

// This is defined in assembly via RODSO_IMAGE (see rodso-asm.h);
// vdso-code.h gives details about the image's size and layout.
extern "C" const char vdso_image[];

namespace {

class VDsoMutator {
 public:
  explicit VDsoMutator(const fbl::RefPtr<VmObject>& vmo) : vmo_(vmo) {}

  void RedirectSymbol(size_t idx1, size_t idx2, uintptr_t value) {
    auto [sym1, sym2] = ReadSymbol(idx1, idx2);

    // Just change the st_value of the symbol.
    sym1.value = sym2.value = value;
    WriteSymbol(idx1, sym1);
    WriteSymbol(idx2, sym2);
  }

  void BlockSymbol(size_t idx1, size_t idx2) {
    auto [sym1, sym2] = ReadSymbol(idx1, idx2);

    // First change the symbol to have local binding so it can't be resolved.
    // The high nybble is the STB_* bits; STB_LOCAL is 0.
    sym1.info &= 0xf;
    sym2.info &= 0xf;
    WriteSymbol(idx1, sym1);
    WriteSymbol(idx2, sym2);

    // Now fill the code region (a whole function) with safely invalid code.
    // This code should never be run, and any attempt to use it should crash.
    ASSERT(sym1.value >= VDSO_CODE_START);
    ASSERT(sym1.value + sym1.size < VDSO_CODE_END);
    zx_status_t status = vmo_->Write(GetTrapFill(sym1.size), sym1.value, sym1.size);
    ASSERT_MSG(status == ZX_OK, "vDSO VMO Write failed: %d", status);
  }

 private:
  struct ElfSym {
    uintptr_t info, value, size;
  };

#if ARCH_X86
  // Fill with the single-byte HLT instruction, so any place
  // user-mode jumps into this code, it gets a trap.
  using Insn = uint8_t;
  static constexpr Insn kTrapFill = 0xf4;  // hlt
#elif ARCH_ARM64
  // Fixed-size instructions.  Use 'brk #1' (what __builtin_trap() emits).
  using Insn = uint32_t;
  static constexpr Insn kTrapFill = 0xd4200020;
#else
#error what architecture?
#endif

  void* GetTrapFill(size_t fill_size) {
    ASSERT(fill_size % sizeof(Insn) == 0);
    fill_size /= sizeof(Insn);
    if (fill_size > trap_fill_size_) {
      fbl::AllocChecker ac;
      trap_fill_.reset(new (&ac) Insn[fill_size]);
      ASSERT(ac.check());
      trap_fill_size_ = fill_size;
      for (size_t i = 0; i < fill_size; ++i) {
        trap_fill_[i] = kTrapFill;
      }
    }
    return trap_fill_.get();
  }

  uintptr_t SymtabAddress(size_t idx) {
    ASSERT(idx < VDSO_DYNSYM_COUNT);
    return VDSO_DATA_START_dynsym + (idx * sizeof(ElfSym));
  }

  ElfSym ReadSymbol(size_t idx) {
    ElfSym sym;
    zx_status_t status = vmo_->Read(&sym, SymtabAddress(idx), sizeof(sym));
    ASSERT_MSG(status == ZX_OK, "vDSO VMO Read failed: %d", status);
    return sym;
  }

  ktl::array<ElfSym, 2> ReadSymbol(size_t idx1, size_t idx2) {
    ElfSym sym1 = ReadSymbol(idx1);
    ElfSym sym2 = ReadSymbol(idx2);
    ASSERT_MSG(sym1.value == sym2.value, "dynsym %zu vs %zu value %#lx vs %#lx", idx2, idx2,
               sym1.value, sym2.value);
    ASSERT_MSG(sym1.size == sym2.size, "dynsym %zu vs %zu size %#lx vs %#lx", idx2, idx2, sym1.size,
               sym2.size);
    return {sym1, sym2};
  }

  void WriteSymbol(size_t idx, const ElfSym& sym) {
    zx_status_t status = vmo_->Write(&sym, SymtabAddress(idx), sizeof(sym));
    ASSERT_MSG(status == ZX_OK, "vDSO VMO Write failed: %d", status);
  }

  const fbl::RefPtr<VmObject>& vmo_;
  ktl::unique_ptr<Insn[]> trap_fill_;
  size_t trap_fill_size_ = 0;
};

#define PASTE(a, b, c) PASTE_1(a, b, c)
#define PASTE_1(a, b, c) a##b##c

#define REDIRECT_SYSCALL(mutator, symbol, target)                                       \
  mutator.RedirectSymbol(PASTE(VDSO_DYNSYM_, symbol, ), PASTE(VDSO_DYNSYM__, symbol, ), \
                         PASTE(VDSO_CODE_, target, ))

// Block the named zx_* function.  The symbol table entry will
// become invisible to runtime symbol resolution, and the code of
// the function will be clobbered with trapping instructions.
#define BLOCK_SYSCALL(mutator, symbol) \
  mutator.BlockSymbol(PASTE(VDSO_DYNSYM_, symbol, ), PASTE(VDSO_DYNSYM__, symbol, ))

// Random attributes in kazoo fidl files become "categories" of syscalls.
// For each category, define a function block_<category> to block all the
// syscalls in that category.  These functions can be used in
// VDso::CreateVariant (below) to block a category of syscalls for a particular
// variant vDSO.
#define SYSCALL_CATEGORY_BEGIN(category) \
  [[maybe_unused]] void block_##category##_syscalls(VDsoMutator& mutator) {
#define SYSCALL_IN_CATEGORY(syscall) BLOCK_SYSCALL(mutator, zx_##syscall);
#define SYSCALL_CATEGORY_END(category) }
#include <lib/syscalls/category.inc>
#undef SYSCALL_CATEGORY_BEGIN
#undef SYSCALL_IN_CATEGORY_END
#undef SYSCALL_CATEGORY_END

// This is extracted from the vDSO image at build time.
using VdsoBuildIdNote = ktl::array<uint8_t, VDSO_BUILD_ID_NOTE_SIZE>;
constexpr VdsoBuildIdNote kVdsoBuildIdNote = VDSO_BUILD_ID_NOTE_BYTES;

// That should exactly match the note read from the vDSO image at runtime.
void CheckBuildId(const fbl::RefPtr<VmObject>& vmo) {
  VdsoBuildIdNote note;
  zx_status_t status = vmo->Read(&note, VDSO_BUILD_ID_NOTE_ADDRESS, sizeof(note));
  ASSERT_MSG(status == ZX_OK, "vDSO VMO Read failed: %d", status);
  ASSERT(note == kVdsoBuildIdNote);
}

// Fill out the contents of the vdso_constants struct.
void SetConstants(const fbl::RefPtr<VmObject>& vmo) {
  zx_ticks_t per_second = ticks_per_second();

  // Grab a copy of the ticks to mono ratio; we need this to initialize the
  // constants window.
  affine::Ratio ticks_to_mono_ratio = platform_get_ticks_to_time_ratio();

  // At this point in time, we absolutely must know the rate that our tick
  // counter is ticking at.  If we don't, then something has gone horribly
  // wrong.
  ASSERT(per_second != 0);
  ASSERT(ticks_to_mono_ratio.numerator() != 0);
  ASSERT(ticks_to_mono_ratio.denominator() != 0);

  // Initialize the constants that should be visible to the vDSO.
  // Rather than assigning each member individually, do this with
  // struct assignment and a compound literal so that the compiler
  // can warn if the initializer list omits any member.
  vdso_constants constants = {
      arch_max_num_cpus(),
      {
          arch_cpu_features(),
          arch_get_hw_breakpoint_count(),
          arch_get_hw_watchpoint_count(),
      },
      arch_dcache_line_size(),
      arch_icache_line_size(),
      PAGE_SIZE,
      0,
      per_second,
      ticks_to_mono_ratio.numerator(),
      ticks_to_mono_ratio.denominator(),
      pmm_count_total_bytes(),
      strlen(version_string()),
      "",
  };
  ASSERT(constants.version_string_len < sizeof(constants.version_string));
  memcpy(constants.version_string, version_string(), constants.version_string_len);

  static_assert(sizeof(constants) == VDSO_DATA_CONSTANTS_SIZE, "gen-rodso-code.sh is suspect");
  zx_status_t status = vmo->Write(&constants, VDSO_DATA_CONSTANTS, sizeof(constants));
  ASSERT_MSG(status == ZX_OK, "vDSO VMO Write failed: %d", status);
}

// Conditionally patch some of the entry points related to time based on
// platform details which get determined at runtime.
void PatchTimeSyscalls(VDsoMutator mutator) {
  // If user mode cannot access the tick counter registers, or kernel command
  // line arguments demand that we access the tick counter via a syscall
  // instead of direct observation, then we need to make sure to redirect
  // symbol in the vDSO such that we always syscall in order to query ticks.
  //
  // Since this can effect how clock monotonic is calculated as well, we may
  // need to redirect zx_clock_get_monotonic as well.
  const bool need_syscall_for_ticks =
      !platform_usermode_can_access_tick_registers() || gBootOptions->vdso_ticks_get_force_syscall;

  if (need_syscall_for_ticks) {
    REDIRECT_SYSCALL(mutator, zx_ticks_get, SYSCALL_zx_ticks_get_via_kernel);
  }
#if ARCH_ARM64
  else {
    // Wait for a _really_ long time for all of the CPUs to have started up, so
    // we know whether or not to deploy the ARM A73 timer read mitigation or
    // not.  If we timeout from this, then something is extremely wrong.  In
    // that situation, go ahead and install the mitigation anyway. It is slower,
    // but at least it will read correctly on all cores.
    //
    // see arch/quirks.h for details about the quirk itself.
    zx_status_t status = mp_wait_for_all_cpus_started(Deadline::after(ZX_SEC(30)));
    if ((status != ZX_OK) || arch_quirks_needs_arm_erratum_858921_mitigation()) {
      if (status != ZX_OK) {
        KERNEL_OOPS(
            "WARNING: Timed out waiting for all CPUs to start.  Installing A73 quirks for "
            "zx_ticks_get in VDSO as a defensive measure.\n");
      } else {
        dprintf(INFO, "Installing A73 quirks for zx_ticks_get in VDSO\n");
      }
      REDIRECT_SYSCALL(mutator, zx_ticks_get, ticks_get_arm_a73);
    }
  }
#endif

  if (gBootOptions->vdso_clock_get_monotonic_force_syscall) {
    // Force a syscall for zx_clock_get_monotonic if instructed to do so by the
    // kernel command line arguments.  Make sure to swap out the implementation
    // of zx_deadline_after as well.
    REDIRECT_SYSCALL(mutator, zx_clock_get_monotonic, SYSCALL_zx_clock_get_monotonic_via_kernel);
    REDIRECT_SYSCALL(mutator, zx_deadline_after, deadline_after_via_kernel_mono);
  } else if (need_syscall_for_ticks) {
    // If ticks must be accessed via syscall, then choose the alternate form
    // for clock_get_monotonic which performs the scaling in user mode, but
    // thunks into the kernel to read the ticks register.
    REDIRECT_SYSCALL(mutator, zx_clock_get_monotonic, clock_get_monotonic_via_kernel_ticks);
    REDIRECT_SYSCALL(mutator, zx_deadline_after, deadline_after_via_kernel_ticks);
  }
}

}  // anonymous namespace

const VDso* VDso::instance_ = NULL;

// Private constructor, can only be called by Create (below).
VDso::VDso(KernelHandle<VmObjectDispatcher>* vmo_kernel_handle)
    : RoDso("vdso/full", vdso_image, VDSO_CODE_END, VDSO_CODE_START, vmo_kernel_handle) {}

// This is called exactly once, at boot time.
const VDso* VDso::Create(KernelHandle<VmObjectDispatcher>* vmo_kernel_handles) {
  ASSERT(!instance_);

  fbl::AllocChecker ac;
  VDso* vdso = new (&ac) VDso(&vmo_kernel_handles[0]);
  ASSERT(ac.check());

  // Sanity-check that it's the exact vDSO image the kernel was compiled for.
  CheckBuildId(vdso->vmo()->vmo());

  // Fill out the contents of the vdso_constants struct.
  SetConstants(vdso->vmo()->vmo());

  PatchTimeSyscalls(VDsoMutator{vdso->vmo()->vmo()});

  DEBUG_ASSERT(!(vdso->vmo_rights() & ZX_RIGHT_WRITE));
  for (size_t v = static_cast<size_t>(Variant::FULL) + 1; v < static_cast<size_t>(Variant::COUNT);
       ++v)
    vdso->CreateVariant(static_cast<Variant>(v), &vmo_kernel_handles[v]);

  instance_ = vdso;
  return instance_;
}

uintptr_t VDso::base_address(const fbl::RefPtr<VmMapping>& code_mapping) {
  return code_mapping ? code_mapping->base() - VDSO_CODE_START : 0;
}

// Each vDSO variant VMO is made via a COW clone of the main/default vDSO
// VMO.  A variant can block some system calls, by syscall category.
// This works by modifying the symbol table entries to make the symbols
// invisible to dynamic linking (STB_LOCAL) and then clobbering the code
// with trapping instructions.  In this way, all the code locations are the
// same across variants and the syscall entry enforcement doesn't have to
// care which variant is in use.  The places where the blocked
// syscalls' syscall entry instructions would be no longer have the syscall
// instructions, so a process using the variant can never get into syscall
// entry with that PC value and hence can never pass the vDSO enforcement
// test.
void VDso::CreateVariant(Variant variant, KernelHandle<VmObjectDispatcher>* vmo_kernel_handle) {
  DEBUG_ASSERT(variant > Variant::FULL);
  DEBUG_ASSERT(variant < Variant::COUNT);
  DEBUG_ASSERT(!variant_vmo_[variant_index(variant)]);

  fbl::RefPtr<VmObject> new_vmo;
  zx_status_t status = vmo()->CreateChild(ZX_VMO_CHILD_COPY_ON_WRITE, 0, size(), false, &new_vmo);
  ASSERT(status == ZX_OK);

  VDsoMutator mutator{new_vmo};

  const char* name = nullptr;
  switch (variant) {
    case Variant::TEST1:
      name = "vdso/test1";
      block_test_category1_syscalls(mutator);
      break;

    case Variant::TEST2:
      name = "vdso/test2";
      block_test_category2_syscalls(mutator);
      break;

    // No default case so the compiler will warn about new enum entries.
    case Variant::FULL:
    case Variant::COUNT:
      PANIC("VDso::CreateVariant called with bad variant");
  }

  zx_rights_t rights;
  status = VmObjectDispatcher::Create(ktl::move(new_vmo), size(),
                                      VmObjectDispatcher::InitialMutability::kMutable,
                                      vmo_kernel_handle, &rights);
  ASSERT(status == ZX_OK);

  status = vmo_kernel_handle->dispatcher()->set_name(name, strlen(name));
  ASSERT(status == ZX_OK);

  variant_vmo_[variant_index(variant)] = vmo_kernel_handle->dispatcher();
}