Kernel: Disable x86 RDTSC instruction in userspace

It's still possible to read the TSC via the read_tsc() syscall, but we will now clear some of the bottom bits for unprivileged users.
abyesilyurt · Jan 1, 2020 · 38f93ef · 38f93ef
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diff --git a/Base/usr/share/man/man1/crash.md b/Base/usr/share/man/man1/crash.md
@@ -34,6 +34,7 @@ kinds of crashes.
 * `-X`: Attempt to execute non-executable memory. (Not mapped with PROT\_EXEC.)
 * `-U`: Attempt to trigger an x86 User Mode Instruction Prevention fault.
 * `-I`: Use an x86 I/O instruction in userspace.
+* `-c`: Read the x86 TSC (Time Stamp Counter) directly.
 
 ## Examples
 

diff --git a/Kernel/Arch/i386/CPU.cpp b/Kernel/Arch/i386/CPU.cpp
@@ -526,6 +526,7 @@ bool g_cpu_supports_pae;
 bool g_cpu_supports_pge;
 bool g_cpu_supports_smep;
 bool g_cpu_supports_sse;
+bool g_cpu_supports_tsc;
 bool g_cpu_supports_umip;
 
 void detect_cpu_features()
@@ -534,6 +535,7 @@ void detect_cpu_features()
  g_cpu_supports_pae = (processor_info.edx() & (1 << 6));
  g_cpu_supports_pge = (processor_info.edx() & (1 << 13));
  g_cpu_supports_sse = (processor_info.edx() & (1 << 25));
+ g_cpu_supports_tsc = (processor_info.edx() & (1 << 4));
 
  CPUID extended_processor_info(0x80000001);
  g_cpu_supports_nx = (extended_processor_info.edx() & (1 << 20));

diff --git a/Kernel/Arch/i386/CPU.h b/Kernel/Arch/i386/CPU.h
@@ -513,4 +513,5 @@ extern bool g_cpu_supports_pae;
 extern bool g_cpu_supports_pge;
 extern bool g_cpu_supports_smep;
 extern bool g_cpu_supports_sse;
+extern bool g_cpu_supports_tsc;
 extern bool g_cpu_supports_umip;
diff --git a/Kernel/Process.cpp b/Kernel/Process.cpp
@@ -2452,6 +2452,8 @@ int Process::sys$read_tsc(u32* lsw, u32* msw)
  if (!validate_write_typed(msw))
  return -EFAULT;
  read_tsc(*lsw, *msw);
+ if (!is_superuser())
+ *lsw &= ~0xfff;
  return 0;
 }
 

diff --git a/Kernel/init.cpp b/Kernel/init.cpp
@@ -266,6 +266,14 @@ extern "C" [[noreturn]] void init(u32 physical_address_for_kernel_page_tables)
  kprintf("x86: UMIP support enabled\n");
  }
 
+ if (g_cpu_supports_tsc) {
+ asm volatile(
+ "mov %cr4, %eax\n"
+ "orl $0x4, %eax\n"
+ "mov %eax, %cr4\n");
+ kprintf("x86: RDTSC support restricted\n");
+ }
+
  RTC::initialize();
  PIC::initialize();
  gdt_init();

diff --git a/Userland/crash.cpp b/Userland/crash.cpp
@@ -11,7 +11,7 @@
 
 static void print_usage_and_exit()
 {
- printf("usage: crash -[AsdiamfMFTtSxyXUI]\n");
+ printf("usage: crash -[AsdiamfMFTtSxyXUIc]\n");
  exit(0);
 }
 
@@ -101,6 +101,7 @@ int main(int argc, char** argv)
  ExecuteNonExecutableMemory,
  TriggerUserModeInstructionPrevention,
  UseIOInstruction,
+ ReadTimestampCounter,
  };
  Mode mode = SegmentationViolation;
 
@@ -143,6 +144,8 @@ int main(int argc, char** argv)
  mode = TriggerUserModeInstructionPrevention;
  else if (String(argv[1]) == "-I")
  mode = UseIOInstruction;
+ else if (String(argv[1]) == "-c")
+ mode = ReadTimestampCounter;
  else
  print_usage_and_exit();
 
@@ -342,5 +345,12 @@ int main(int argc, char** argv)
  }).run(run_type);
  }
 
+ if (mode == ReadTimestampCounter || mode == TestAllCrashTypes) {
+ Crash("Read the CPU timestamp counter", [] {
+ asm volatile("rdtsc");
+ return Crash::Failure::DidNotCrash;
+ }).run(run_type);
+ }
+
  return 0;
 }