[analyzer] Remove untrusted buffer size warning in the TaintPropagation checker (#68607)

Before this commit the the checker alpha.security.taint.TaintPropagation always reported warnings when the size argument of a memcpy-like or malloc-like function was tainted. However, this produced false positive reports in situations where the size was tainted, but correctly performed bound checks guaranteed the safety of the call.
 
This commit removes the rough "always warn if the size argument is tainted" heuristic; but it would be good to add a more refined "warns if the size argument is tainted and can be too large" heuristic in follow-up commits. That logic would belong to CStringChecker and MallocChecker, because those are the checkers responsible for the more detailed modeling of memcpy-like and malloc-like functions. To mark this plan, TODO comments are added in those two checkers.
 
There were several test cases that used these sinks to test generic properties of taint tracking; those were adapted to use different logic.
 
As a minor unrelated change, this commit ensures that strcat (and its wide variant, wcsncat) propagates taint from the first argument to the first argument, i.e. a tainted string remains tainted if we concatenate it with another string. This change was required because the adapted variant of multipleTaintedArgs is relying on strncat to compose a value that combines taint from two different sources.

Author: dkrupp

clang/docs/analyzer/checkers.rst

@@ -3033,13 +3033,6 @@ Further examples of injection vulnerabilities this checker can find.
     sprintf(buf, s); // warn: untrusted data used as a format string
   }
 
-  void test() {
-    size_t ts;
-    scanf("%zd", &ts); // 'ts' marked as tainted
-    int *p = (int *)malloc(ts * sizeof(int));
-      // warn: untrusted data used as buffer size
-  }
-
 There are built-in sources, propagations and sinks even if no external taint
 configuration is provided.
 
@@ -3067,9 +3060,7 @@ Default propagations rules:
 
 Default sinks:
  ``printf``, ``setproctitle``, ``system``, ``popen``, ``execl``, ``execle``,
- ``execlp``, ``execv``, ``execvp``, ``execvP``, ``execve``, ``dlopen``,
- ``memcpy``, ``memmove``, ``strncpy``, ``strndup``, ``malloc``, ``calloc``,
- ``alloca``, ``memccpy``, ``realloc``, ``bcopy``
+ ``execlp``, ``execv``, ``execvp``, ``execvP``, ``execve``, ``dlopen``
 
 Please note that there are no built-in filter functions.
 

clang/lib/StaticAnalyzer/Checkers/CStringChecker.cpp

@@ -1338,6 +1338,9 @@ void CStringChecker::evalCopyCommon(CheckerContext &C, const CallEvent &Call,
 
   // If the size can be nonzero, we have to check the other arguments.
   if (stateNonZeroSize) {
+    // TODO: If Size is tainted and we cannot prove that it is smaller or equal
+    // to the size of the destination buffer, then emit a warning
+    // that an attacker may provoke a buffer overflow error.
     state = stateNonZeroSize;
 
     // Ensure the destination is not null. If it is NULL there will be a

clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp

@@ -59,13 +59,6 @@ constexpr llvm::StringLiteral MsgSanitizeSystemArgs =
     "Untrusted data is passed to a system call "
     "(CERT/STR02-C. Sanitize data passed to complex subsystems)";
 
-/// Check if tainted data is used as a buffer size in strn.. functions,
-/// and allocators.
-constexpr llvm::StringLiteral MsgTaintedBufferSize =
-    "Untrusted data is used to specify the buffer size "
-    "(CERT/STR31-C. Guarantee that storage for strings has sufficient space "
-    "for character data and the null terminator)";
-
 /// Check if tainted data is used as a custom sink's parameter.
 constexpr llvm::StringLiteral MsgCustomSink =
     "Untrusted data is passed to a user-defined sink";
@@ -298,14 +291,6 @@ class GenericTaintRule {
     return {{}, {}, std::move(SrcArgs), std::move(DstArgs)};
   }
 
-  /// Make a rule that taints all PropDstArgs if any of PropSrcArgs is tainted.
-  static GenericTaintRule
-  SinkProp(ArgSet &&SinkArgs, ArgSet &&SrcArgs, ArgSet &&DstArgs,
-           std::optional<StringRef> Msg = std::nullopt) {
-    return {
-        std::move(SinkArgs), {}, std::move(SrcArgs), std::move(DstArgs), Msg};
-  }
-
   /// Process a function which could either be a taint source, a taint sink, a
   /// taint filter or a taint propagator.
   void process(const GenericTaintChecker &Checker, const CallEvent &Call,
@@ -733,12 +718,21 @@ void GenericTaintChecker::initTaintRules(CheckerContext &C) const {
       {{CDM::CLibraryMaybeHardened, {{"stpcpy"}}},
        TR::Prop({{1}}, {{0, ReturnValueIndex}})},
       {{CDM::CLibraryMaybeHardened, {{"strcat"}}},
-       TR::Prop({{1}}, {{0, ReturnValueIndex}})},
+       TR::Prop({{0, 1}}, {{0, ReturnValueIndex}})},
       {{CDM::CLibraryMaybeHardened, {{"wcsncat"}}},
-       TR::Prop({{1}}, {{0, ReturnValueIndex}})},
+       TR::Prop({{0, 1}}, {{0, ReturnValueIndex}})},
       {{CDM::CLibrary, {{"strdup"}}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
       {{CDM::CLibrary, {{"strdupa"}}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
       {{CDM::CLibrary, {{"wcsdup"}}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
+      {{CDM::CLibrary, BI.getName(Builtin::BImemcpy)},
+       TR::Prop({{1, 2}}, {{0, ReturnValueIndex}})},
+      {{CDM::CLibrary, {BI.getName(Builtin::BImemmove)}},
+       TR::Prop({{1, 2}}, {{0, ReturnValueIndex}})},
+      {{CDM::CLibrary, {BI.getName(Builtin::BIstrncpy)}},
+       TR::Prop({{1, 2}}, {{0, ReturnValueIndex}})},
+      {{CDM::CLibrary, {BI.getName(Builtin::BIstrndup)}},
+       TR::Prop({{0, 1}}, {{ReturnValueIndex}})},
+      {{CDM::CLibrary, {"bcopy"}}, TR::Prop({{0, 2}}, {{1}})},
 
       // Sinks
       {{{"system"}}, TR::Sink({{0}}, MsgSanitizeSystemArgs)},
@@ -752,30 +746,15 @@ void GenericTaintChecker::initTaintRules(CheckerContext &C) const {
       {{{"execvp"}}, TR::Sink({{0, 1}}, MsgSanitizeSystemArgs)},
       {{{"execvpe"}}, TR::Sink({{0, 1, 2}}, MsgSanitizeSystemArgs)},
       {{{"dlopen"}}, TR::Sink({{0}}, MsgSanitizeSystemArgs)},
-      {{CDM::CLibrary, {{"malloc"}}}, TR::Sink({{0}}, MsgTaintedBufferSize)},
-      {{CDM::CLibrary, {{"calloc"}}}, TR::Sink({{0}}, MsgTaintedBufferSize)},
-      {{CDM::CLibrary, {{"alloca"}}}, TR::Sink({{0}}, MsgTaintedBufferSize)},
-      {{CDM::CLibrary, {{"memccpy"}}}, TR::Sink({{3}}, MsgTaintedBufferSize)},
-      {{CDM::CLibrary, {{"realloc"}}}, TR::Sink({{1}}, MsgTaintedBufferSize)},
+
+      // malloc, calloc, alloca, realloc, memccpy
+      // are intentionally not marked as taint sinks because unconditional
+      // reporting for these functions generates many false positives.
+      // These taint sinks should be implemented in other checkers with more
+      // sophisticated sanitation heuristics.
       {{{{"setproctitle"}}}, TR::Sink({{0}, 1}, MsgUncontrolledFormatString)},
       {{{{"setproctitle_fast"}}},
-       TR::Sink({{0}, 1}, MsgUncontrolledFormatString)},
-
-      // SinkProps
-      {{CDM::CLibraryMaybeHardened, BI.getName(Builtin::BImemcpy)},
-       TR::SinkProp({{2}}, {{1, 2}}, {{0, ReturnValueIndex}},
-                    MsgTaintedBufferSize)},
-      {{CDM::CLibraryMaybeHardened, {BI.getName(Builtin::BImemmove)}},
-       TR::SinkProp({{2}}, {{1, 2}}, {{0, ReturnValueIndex}},
-                    MsgTaintedBufferSize)},
-      {{CDM::CLibraryMaybeHardened, {BI.getName(Builtin::BIstrncpy)}},
-       TR::SinkProp({{2}}, {{1, 2}}, {{0, ReturnValueIndex}},
-                    MsgTaintedBufferSize)},
-      {{CDM::CLibrary, {BI.getName(Builtin::BIstrndup)}},
-       TR::SinkProp({{1}}, {{0, 1}}, {{ReturnValueIndex}},
-                    MsgTaintedBufferSize)},
-      {{CDM::CLibrary, {{"bcopy"}}},
-       TR::SinkProp({{2}}, {{0, 2}}, {{1}}, MsgTaintedBufferSize)}};
+       TR::Sink({{0}, 1}, MsgUncontrolledFormatString)}};
 
   // `getenv` returns taint only in untrusted environments.
   if (TR::UntrustedEnv(C)) {

clang/lib/StaticAnalyzer/Checkers/MallocChecker.cpp

@@ -1819,6 +1819,10 @@ ProgramStateRef MallocChecker::MallocMemAux(CheckerContext &C,
   if (Size.isUndef())
     Size = UnknownVal();
 
+  // TODO: If Size is tainted and we cannot prove that it is within
+  // reasonable bounds, emit a warning that an attacker may
+  // provoke a memory exhaustion error.
+
   // Set the region's extent.
   State = setDynamicExtent(State, RetVal.getAsRegion(),
                            Size.castAs<DefinedOrUnknownSVal>(), SVB);

clang/test/Analysis/taint-diagnostic-visitor.c

@@ -10,7 +10,8 @@ int scanf(const char *restrict format, ...);
 int system(const char *command);
 char* getenv( const char* env_var );
 size_t strlen( const char* str );
-int atoi( const char* str );
+char *strcat( char *dest, const char *src );
+char* strcpy( char* dest, const char* src );
 void *malloc(size_t size );
 void free( void *ptr );
 char *fgets(char *str, int n, FILE *stream);
@@ -53,34 +54,32 @@ void taintDiagnosticVLA(void) {
 
 // Tests if the originated note is correctly placed even if the path is
 // propagating through variables and expressions
-char *taintDiagnosticPropagation(){
-  char *pathbuf;
-  char *size=getenv("SIZE"); // expected-note {{Taint originated here}}
-                                 // expected-note@-1 {{Taint propagated to the return value}}
-  if (size){ // expected-note {{Assuming 'size' is non-null}}
-	               // expected-note@-1 {{Taking true branch}}
-    pathbuf=(char*) malloc(atoi(size)); // expected-warning{{Untrusted data is used to specify the buffer size}}
-                                                // expected-note@-1{{Untrusted data is used to specify the buffer size}}
-                                                // expected-note@-2 {{Taint propagated to the return value}}
-    return pathbuf;
+int taintDiagnosticPropagation(){
+  int res;
+  char *cmd=getenv("CMD"); // expected-note {{Taint originated here}}
+                           // expected-note@-1 {{Taint propagated to the return value}}
+  if (cmd){ // expected-note {{Assuming 'cmd' is non-null}}
+            // expected-note@-1 {{Taking true branch}}
+    res = system(cmd); // expected-warning{{Untrusted data is passed to a system call}}
+                       // expected-note@-1{{Untrusted data is passed to a system call}}
+    return res;
   }
-  return 0;
+  return -1;
 }
 
 // Taint origin should be marked correctly even if there are multiple taint
 // sources in the function
-char *taintDiagnosticPropagation2(){
-  char *pathbuf;
+int taintDiagnosticPropagation2(){
+  int res;
   char *user_env2=getenv("USER_ENV_VAR2");//unrelated taint source
-  char *size=getenv("SIZE"); // expected-note {{Taint originated here}}
-                                 // expected-note@-1 {{Taint propagated to the return value}}
+  char *cmd=getenv("CMD"); // expected-note {{Taint originated here}}
+                           // expected-note@-1 {{Taint propagated to the return value}}
   char *user_env=getenv("USER_ENV_VAR");//unrelated taint source
-  if (size){ // expected-note {{Assuming 'size' is non-null}}
-	               // expected-note@-1 {{Taking true branch}}
-    pathbuf=(char*) malloc(atoi(size)+1); // expected-warning{{Untrusted data is used to specify the buffer size}}
-                                                // expected-note@-1{{Untrusted data is used to specify the buffer size}}
-                                                // expected-note@-2 {{Taint propagated to the return value}}
-    return pathbuf;
+  if (cmd){ // expected-note {{Assuming 'cmd' is non-null}}
+	          // expected-note@-1 {{Taking true branch}}
+    res = system(cmd); // expected-warning{{Untrusted data is passed to a system call}}
+                       // expected-note@-1{{Untrusted data is passed to a system call}}
+    return res;
   }
   return 0;
 }
@@ -95,22 +94,24 @@ void testReadStdIn(){
 }
 
 void multipleTaintSources(void) {
-  int x,y,z;
-  scanf("%d", &x); // expected-note {{Taint originated here}}
+  char cmd[2048], file[1024];
+  scanf ("%1022[^\n] ", cmd); // expected-note {{Taint originated here}}
                    // expected-note@-1 {{Taint propagated to the 2nd argument}}
-  scanf("%d", &y); // expected-note {{Taint originated here}}
+  scanf ("%1023[^\n]", file); // expected-note {{Taint originated here}}
                    // expected-note@-1 {{Taint propagated to the 2nd argument}}
-  scanf("%d", &z);
-  int* ptr = (int*) malloc(y + x); // expected-warning {{Untrusted data is used to specify the buffer size}}
-                                   // expected-note@-1{{Untrusted data is used to specify the buffer size}}
-  free (ptr);
+  strcat(cmd, file); // expected-note {{Taint propagated to the 1st argument}}
+  strcat(cmd, " "); // expected-note {{Taint propagated to the 1st argument}}
+  system(cmd); // expected-warning {{Untrusted data is passed to a system call}}
+               // expected-note@-1{{Untrusted data is passed to a system call}}
 }
 
 void multipleTaintedArgs(void) {
-  int x,y;
-  scanf("%d %d", &x, &y); // expected-note {{Taint originated here}}
+  char cmd[1024], file[1024], buf[2048];
+  scanf("%1022s %1023s", cmd, file); // expected-note {{Taint originated here}}
                           // expected-note@-1 {{Taint propagated to the 2nd argument, 3rd argument}}
-  int* ptr = (int*) malloc(x + y); // expected-warning {{Untrusted data is used to specify the buffer size}}
-                                   // expected-note@-1{{Untrusted data is used to specify the buffer size}}
-  free (ptr);
+  strcpy(buf, cmd);// expected-note {{Taint propagated to the 1st argument}}
+  strcat(buf, " ");// expected-note {{Taint propagated to the 1st argument}}
+  strcat(buf, file);// expected-note {{Taint propagated to the 1st argument}}
+  system(buf); // expected-warning {{Untrusted data is passed to a system call}}
+               // expected-note@-1{{Untrusted data is passed to a system call}}
 }

clang/test/Analysis/taint-generic.c

@@ -305,15 +305,21 @@ void testGets_s(void) {
 
 void testTaintedBufferSize(void) {
   size_t ts;
+  // The functions malloc, calloc, bcopy and memcpy are not taint sinks in the
+  // default config of GenericTaintChecker (because that would cause too many
+  // false positives).
+  // FIXME: We should generate warnings when a value passed to these functions
+  // is tainted and _can be very large_ (because that's exploitable). This
+  // functionality probably belongs to the checkers that do more detailed
+  // modeling of these functions (MallocChecker and CStringChecker).
   scanf("%zd", &ts);
-
-  int *buf1 = (int*)malloc(ts*sizeof(int)); // expected-warning {{Untrusted data is used to specify the buffer size}}
-  char *dst = (char*)calloc(ts, sizeof(char)); //expected-warning {{Untrusted data is used to specify the buffer size}}
-  bcopy(buf1, dst, ts); // expected-warning {{Untrusted data is used to specify the buffer size}}
-  __builtin_memcpy(dst, buf1, (ts + 4)*sizeof(char)); // expected-warning {{Untrusted data is used to specify the buffer size}}
+  int *buf1 = (int*)malloc(ts*sizeof(int)); // warn here, ts is unbounded and tainted
+  char *dst = (char*)calloc(ts, sizeof(char)); // warn here, ts is unbounded tainted
+  bcopy(buf1, dst, ts); // no warning here, since the size of buf1, dst equals ts. Cannot overflow.
+  __builtin_memcpy(dst, buf1, (ts + 4)*sizeof(char)); // warn here, dst overflows (whatever the value of ts)
 
   // If both buffers are trusted, do not issue a warning.
-  char *dst2 = (char*)malloc(ts*sizeof(char)); // expected-warning {{Untrusted data is used to specify the buffer size}}
+  char *dst2 = (char*)malloc(ts*sizeof(char)); // warn here, ts in unbounded
   strncat(dst2, dst, ts); // no-warning
 }
 
@@ -353,7 +359,7 @@ void testStruct(void) {
 
   sock = socket(AF_INET, SOCK_STREAM, 0);
   read(sock, &tainted, sizeof(tainted));
-  __builtin_memcpy(buffer, tainted.buf, tainted.length); // expected-warning {{Untrusted data is used to specify the buffer size}}
+  clang_analyzer_isTainted_int(tainted.length); // expected-warning {{YES }}
 }
 
 void testStructArray(void) {
@@ -368,17 +374,17 @@ void testStructArray(void) {
   __builtin_memset(srcbuf, 0, sizeof(srcbuf));
 
   read(sock, &tainted[0], sizeof(tainted));
-  __builtin_memcpy(dstbuf, srcbuf, tainted[0].length); // expected-warning {{Untrusted data is used to specify the buffer size}}
+  clang_analyzer_isTainted_int(tainted[0].length); // expected-warning {{YES}}
 
   __builtin_memset(&tainted, 0, sizeof(tainted));
   read(sock, &tainted, sizeof(tainted));
-  __builtin_memcpy(dstbuf, srcbuf, tainted[0].length); // expected-warning {{Untrusted data is used to specify the buffer size}}
+  clang_analyzer_isTainted_int(tainted[0].length); // expected-warning {{YES}}
 
   __builtin_memset(&tainted, 0, sizeof(tainted));
   // If we taint element 1, we should not raise an alert on taint for element 0 or element 2
   read(sock, &tainted[1], sizeof(tainted));
-  __builtin_memcpy(dstbuf, srcbuf, tainted[0].length); // no-warning
-  __builtin_memcpy(dstbuf, srcbuf, tainted[2].length); // no-warning
+  clang_analyzer_isTainted_int(tainted[0].length); // expected-warning {{NO}}
+  clang_analyzer_isTainted_int(tainted[2].length); // expected-warning {{NO}}
 }
 
 void testUnion(void) {