[analyzer] TaintPropagation checker strlen() should not propagate (#66086)

strlen(..) call should not propagate taintedness,
because it brings in many false positive findings. It is a common
pattern to copy user provided input to another buffer. In these cases we
always
get warnings about tainted data used as the malloc parameter:

buf = malloc(strlen(tainted_txt) + 1); // false warning

This pattern can lead to a denial of service attack only, when the
attacker can directly specify the size of the allocated area as an
arbitrary large number (e.g. the value is converted from a user provided
string).

Later, we could reintroduce strlen() as a taint propagating function
with the consideration not to emit warnings when the tainted value
cannot be "arbitrarily large" (such as the size of an already allocated
memory block).

The change has been evaluated on the following open source projects:

- memcached: [1 lost false
positive](https://codechecker-demo.eastus.cloudapp.azure.com/Default/reports?run=memcached_1.6.8_ednikru_taint_nostrlen_baseline&newcheck=memcached_1.6.8_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved)

- tmux: 0 lost reports
- twin [3 lost false
positives](https://codechecker-demo.eastus.cloudapp.azure.com/Default/reports?run=twin_v0.8.1_ednikru_taint_nostrlen_baseline&newcheck=twin_v0.8.1_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved)
- vim [1 lost false
positive](https://codechecker-demo.eastus.cloudapp.azure.com/Default/reports?run=vim_v8.2.1920_ednikru_taint_nostrlen_baseline&newcheck=vim_v8.2.1920_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved)
- openssl 0 lost reports
- sqliste [2 lost false
positives](https://codechecker-demo.eastus.cloudapp.azure.com/Default/reports?run=sqlite_version-3.33.0_ednikru_taint_nostrlen_baseline&newcheck=sqlite_version-3.33.0_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved)
- ffmpeg 0 lost repots
- postgresql [3 lost false
positives](https://codechecker-demo.eastus.cloudapp.azure.com/Default/reports?run=postgres_REL_13_0_ednikru_taint_nostrlen_baseline&newcheck=postgres_REL_13_0_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved)
- tinyxml 0 lost reports
- libwebm 0 lost reports
- xerces 0 lost reports

In all cases the lost reports are originating from copying untrusted
environment variables into another buffer.

There are 2 types of lost false positive reports:
1) [Where the warning is emitted at the malloc call by the
TaintPropagation Checker
](https://codechecker-demo.eastus.cloudapp.azure.com/Default/report-detail?run=memcached_1.6.8_ednikru_taint_nostrlen_baseline&newcheck=memcached_1.6.8_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved&report-id=2648506&report-hash=2079221954026f17e1ecb614f5f054db&report-filepath=%2amemcached.c)
`
            len = strlen(portnumber_filename)+4+1;
            temp_portnumber_filename = malloc(len);
`

2) When pointers are set based on the length of the tainted string by
the ArrayOutofBoundsv2 checker.
For example [this
](https://codechecker-demo.eastus.cloudapp.azure.com/Default/report-detail?run=vim_v8.2.1920_ednikru_taint_nostrlen_baseline&newcheck=vim_v8.2.1920_ednikru_taint_nostrlen_new&is-unique=on&diff-type=Resolved&report-id=2649310&report-hash=79dc8522d2cd34ca8e1b2dc2db64b2df&report-filepath=%2aos_unix.c)case.

clang/docs/ReleaseNotes.rst

@@ -450,6 +450,13 @@ Static Analyzer
   bitwise shift operators produce undefined behavior (because some operand is
   negative or too large).
 
+- The ``alpha.security.taint.TaintPropagation`` checker no longer propagates
+  taint on ``strlen`` and ``strnlen`` calls, unless these are marked
+  explicitly propagators in the user-provided taint configuration file.
+  This removal empirically reduces the number of false positive reports.
+  Read the PR for the details.
+  (`#66086 <https://github.com/llvm/llvm-project/pull/66086>`_)
+
 .. _release-notes-sanitizers:
 
 Sanitizers

clang/docs/analyzer/checkers.rst

@@ -2599,8 +2599,8 @@ Default propagations rules:
  ``memcpy``, ``memmem``, ``memmove``, ``mbtowc``, ``pread``, ``qsort``,
  ``qsort_r``, ``rawmemchr``, ``read``, ``recv``, ``recvfrom``, ``rindex``,
  ``strcasestr``, ``strchr``, ``strchrnul``, ``strcasecmp``, ``strcmp``,
- ``strcspn``, ``strlen``, ``strncasecmp``, ``strncmp``, ``strndup``,
- ``strndupa``, ``strnlen``, ``strpbrk``, ``strrchr``, ``strsep``, ``strspn``,
+ ``strcspn``, ``strncasecmp``, ``strncmp``, ``strndup``,
+ ``strndupa``, ``strpbrk``, ``strrchr``, ``strsep``, ``strspn``,
  ``strstr``, ``strtol``, ``strtoll``, ``strtoul``, ``strtoull``, ``tolower``,
  ``toupper``, ``ttyname``, ``ttyname_r``, ``wctomb``, ``wcwidth``
 

clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp

@@ -695,9 +695,10 @@ void GenericTaintChecker::initTaintRules(CheckerContext &C) const {
       {{{"strpbrk"}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
       {{{"strndup"}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
       {{{"strndupa"}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
-      {{{"strlen"}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
-      {{{"wcslen"}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
-      {{{"strnlen"}}, TR::Prop({{0}}, {{ReturnValueIndex}})},
+
+      // strlen, wcslen, strnlen and alike intentionally don't propagate taint.
+      // See the details here: https://github.com/llvm/llvm-project/pull/66086
+
       {{{"strtol"}}, TR::Prop({{0}}, {{1, ReturnValueIndex}})},
       {{{"strtoll"}}, TR::Prop({{0}}, {{1, ReturnValueIndex}})},
       {{{"strtoul"}}, TR::Prop({{0}}, {{1, ReturnValueIndex}})},

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

@@ -10,6 +10,7 @@ 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 );
 void *malloc(size_t size );
 void free( void *ptr );
 char *fgets(char *str, int n, FILE *stream);
@@ -54,11 +55,11 @@ void taintDiagnosticVLA(void) {
 // propagating through variables and expressions
 char *taintDiagnosticPropagation(){
   char *pathbuf;
-  char *pathlist=getenv("PATH"); // expected-note {{Taint originated here}}
+  char *size=getenv("SIZE"); // expected-note {{Taint originated here}}
                                  // expected-note@-1 {{Taint propagated to the return value}}
-  if (pathlist){ // expected-note {{Assuming 'pathlist' is non-null}}
+  if (size){ // expected-note {{Assuming 'size' is non-null}}
 	               // expected-note@-1 {{Taking true branch}}
-    pathbuf=(char*) malloc(strlen(pathlist)+1); // expected-warning{{Untrusted data is used to specify the buffer size}}
+    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;
@@ -71,12 +72,12 @@ char *taintDiagnosticPropagation(){
 char *taintDiagnosticPropagation2(){
   char *pathbuf;
   char *user_env2=getenv("USER_ENV_VAR2");//unrelated taint source
-  char *pathlist=getenv("PATH"); // expected-note {{Taint originated here}}
+  char *size=getenv("SIZE"); // 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 (pathlist){ // expected-note {{Assuming 'pathlist' is non-null}}
+  if (size){ // expected-note {{Assuming 'size' is non-null}}
 	               // expected-note@-1 {{Taking true branch}}
-    pathbuf=(char*) malloc(strlen(pathlist)+1); // expected-warning{{Untrusted data is used to specify the buffer size}}
+    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;

clang/test/Analysis/taint-generic.c

@@ -443,14 +443,18 @@ int testSprintf_propagates_taint(char *buf, char *msg) {
   return 1 / x;                    // expected-warning {{Division by a tainted value, possibly zero}}
 }
 
-void test_wchar_apis_propagate(const char *path) {
+void test_wchar_apis_dont_propagate(const char *path) {
+  // strlen, wcslen, strnlen and alike intentionally don't propagate taint.
+  // See the details here: https://github.com/llvm/llvm-project/pull/66086
+  // This isn't ideal, but this is only what we have now.
+
   FILE *f = fopen(path, "r");
   clang_analyzer_isTainted_charp((char*)f);  // expected-warning {{YES}}
   wchar_t wbuf[10];
   fgetws(wbuf, sizeof(wbuf)/sizeof(*wbuf), f);
   clang_analyzer_isTainted_wchar(*wbuf); // expected-warning {{YES}}
   int n = wcslen(wbuf);
-  clang_analyzer_isTainted_int(n); // expected-warning {{YES}}
+  clang_analyzer_isTainted_int(n); // expected-warning {{NO}}
 
   wchar_t dst[100] = L"ABC";
   clang_analyzer_isTainted_wchar(*dst); // expected-warning {{NO}}
@@ -458,7 +462,7 @@ void test_wchar_apis_propagate(const char *path) {
   clang_analyzer_isTainted_wchar(*dst); // expected-warning {{YES}}
 
   int m = wcslen(dst);
-  clang_analyzer_isTainted_int(m); // expected-warning {{YES}}
+  clang_analyzer_isTainted_int(m); // expected-warning {{NO}}
 }
 
 int scanf_s(const char *format, ...);
@@ -948,21 +952,27 @@ void testStrndupa(size_t n) {
 }
 
 size_t strlen(const char *s);
-void testStrlen() {
+void testStrlen_dont_propagate() {
+  // strlen, wcslen, strnlen and alike intentionally don't propagate taint.
+  // See the details here: https://github.com/llvm/llvm-project/pull/66086
+  // This isn't ideal, but this is only what we have now.
   char s[10];
   scanf("%9s", s);
 
   size_t result = strlen(s);
-  clang_analyzer_isTainted_int(result); // expected-warning {{YES}}
+  // strlen propagating taint would bring in many false positives
+  clang_analyzer_isTainted_int(result); // expected-warning {{NO}}
 }
 
 size_t strnlen(const char *s, size_t maxlen);
-void testStrnlen(size_t maxlen) {
+void testStrnlen_dont_propagate(size_t maxlen) {
+  // strlen, wcslen, strnlen and alike intentionally don't propagate taint.
+  // See the details here: https://github.com/llvm/llvm-project/pull/66086
+  // This isn't ideal, but this is only what we have now.
   char s[10];
   scanf("%9s", s);
-
   size_t result = strnlen(s, maxlen);
-  clang_analyzer_isTainted_int(result); // expected-warning {{YES}}
+  clang_analyzer_isTainted_int(result); // expected-warning {{NO}}
 }
 
 long strtol(const char *restrict nptr, char **restrict endptr, int base);