[yaml2obj][MachO] Fix crash from integer underflow with invalid cmdsize (llvm#165924)

yaml2obj would crash when processing Mach-O load commands with cmdsize
smaller than the actual structure size e.g. LC_SEGMENT_64 with
cmdsize=56 instead of 72. The crash occurred due to integer underflow
when calculating padding: cmdsize - BytesWritten wraps to a large value
when negative, causing a massive allocation attempt.

Author: rjmansfield

llvm/lib/ObjectYAML/MachOEmitter.cpp

@@ -19,14 +19,26 @@
 #include "llvm/Support/Error.h"
 #include "llvm/Support/FormatVariadic.h"
 #include "llvm/Support/LEB128.h"
-#include "llvm/Support/YAMLTraits.h"
 #include "llvm/Support/SystemZ/zOSSupport.h"
+#include "llvm/Support/WithColor.h"
+#include "llvm/Support/YAMLTraits.h"
 #include "llvm/Support/raw_ostream.h"
 
 using namespace llvm;
 
 namespace {
 
+static const char *getLoadCommandName(uint32_t cmd) {
+  switch (cmd) {
+#define HANDLE_LOAD_COMMAND(LCName, LCValue, LCStruct)                         \
+  case MachO::LCName:                                                          \
+    return #LCName;
+#include "llvm/BinaryFormat/MachO.def"
+  default:
+    return nullptr;
+  }
+}
+
 class MachOWriter {
 public:
   MachOWriter(MachOYAML::Object &Obj) : Obj(Obj), fileStart(0) {
@@ -244,7 +256,8 @@ void MachOWriter::ZeroToOffset(raw_ostream &OS, size_t Offset) {
 }
 
 void MachOWriter::writeLoadCommands(raw_ostream &OS) {
-  for (auto &LC : Obj.LoadCommands) {
+  for (size_t i = 0; i < Obj.LoadCommands.size(); ++i) {
+    auto &LC = Obj.LoadCommands[i];
     size_t BytesWritten = 0;
     llvm::MachO::macho_load_command Data = LC.Data;
 
@@ -285,7 +298,25 @@ void MachOWriter::writeLoadCommands(raw_ostream &OS) {
 
     // Fill remaining bytes with 0. This will only get hit in partially
     // specified test cases.
-    auto BytesRemaining = LC.Data.load_command_data.cmdsize - BytesWritten;
+    // Prevent integer underflow if BytesWritten exceeds cmdsize.
+    if (BytesWritten > LC.Data.load_command_data.cmdsize) {
+      std::string Name;
+      const char *NameCStr = getLoadCommandName(LC.Data.load_command_data.cmd);
+      if (NameCStr)
+        Name = NameCStr;
+      else
+        Name = ("(0x" + Twine::utohexstr(LC.Data.load_command_data.cmd) + ")")
+                   .str();
+
+      WithColor::warning() << "load command " << i << " " << Name
+                           << " cmdsize too small ("
+                           << LC.Data.load_command_data.cmdsize
+                           << " bytes) for actual size (" << BytesWritten
+                           << " bytes)\n";
+    }
+    auto BytesRemaining = (BytesWritten < LC.Data.load_command_data.cmdsize)
+                              ? LC.Data.load_command_data.cmdsize - BytesWritten
+                              : 0;
     if (BytesRemaining > 0) {
       ZeroFillBytes(OS, BytesRemaining);
     }

llvm/test/ObjectYAML/MachO/load-cmdsize-too-small.yaml

@@ -0,0 +1,55 @@
+## Test that yaml2obj handles load commands with cmdsize smaller than the
+## actual structure size without crashing (due to integer underflow).
+
+## Test with a known load command (LC_SEGMENT_64).
+# RUN: yaml2obj %s --docnum=1 -o %t1 2>&1 | FileCheck %s --check-prefix=WARNING-KNOWN
+# RUN: not llvm-readobj --file-headers %t1 2>&1 | FileCheck %s --check-prefix=MALFORMED
+
+# WARNING-KNOWN: warning: load command 0 LC_SEGMENT_64 cmdsize too small (56 bytes) for actual size (72 bytes)
+
+# MALFORMED: error: {{.*}}: truncated or malformed object (load command 0 LC_SEGMENT_64 cmdsize too small)
+
+--- !mach-o
+FileHeader:
+  magic:           0xFEEDFACF
+  cputype:         0x01000007
+  cpusubtype:      0x00000003
+  filetype:        0x00000001
+  ncmds:           1
+  sizeofcmds:      56
+  flags:           0x00002000
+  reserved:        0x00000000
+LoadCommands:
+  - cmd:             LC_SEGMENT_64
+    cmdsize:         56          ## Should be 72 for LC_SEGMENT_64
+    segname:         '__TEXT'
+    vmaddr:          0x1000
+    vmsize:          0x10
+    fileoff:         0
+    filesize:        0
+    maxprot:         7
+    initprot:        5
+    nsects:          0
+    flags:           0
+...
+
+## Test with an unknown load command value.
+# RUN: yaml2obj %s --docnum=2 -o %t2 2>&1 | FileCheck %s --check-prefix=WARNING-UNKNOWN
+
+# WARNING-UNKNOWN: warning: load command 0 (0xdeadbeef) cmdsize too small (8 bytes) for actual size (20 bytes)
+
+--- !mach-o
+FileHeader:
+  magic:           0xFEEDFACF
+  cputype:         0x01000007
+  cpusubtype:      0x00000003
+  filetype:        0x00000001
+  ncmds:           1
+  sizeofcmds:      20
+  flags:           0x00002000
+  reserved:        0x00000000
+LoadCommands:
+  - cmd:             0xDEADBEEF
+    cmdsize:         8
+    PayloadBytes:    [0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x07, 0x08, 0x09, 0x0A, 0x0B, 0x0C]
+...