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

Daniel Krupp via cfe-commits cfe-commits at lists.llvm.org
Sat Sep 16 02:22:17 PDT 2023


https://github.com/dkrupp updated https://github.com/llvm/llvm-project/pull/66086

>From f8997b16c74543eb57b272c4dd4abca1a10d9ac7 Mon Sep 17 00:00:00 2001
From: Daniel Krupp <daniel.krupp at ericsson.com>
Date: Fri, 8 Sep 2023 16:57:49 +0200
Subject: [PATCH] [analyzer] TaintPropagation checker strlen() should not
 propagate

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).
---
 clang/docs/analyzer/checkers.rst                    |  4 ++--
 .../StaticAnalyzer/Checkers/GenericTaintChecker.cpp |  3 ---
 clang/test/Analysis/taint-diagnostic-visitor.c      | 13 +++++++------
 clang/test/Analysis/taint-generic.c                 | 10 +++++-----
 4 files changed, 14 insertions(+), 16 deletions(-)

diff --git a/clang/docs/analyzer/checkers.rst b/clang/docs/analyzer/checkers.rst
index 54ea49e7426cc86..dbd6d7787823530 100644
--- a/clang/docs/analyzer/checkers.rst
+++ b/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``
 
diff --git a/clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp
index 5da0f34b3d0464f..95a759c251ca490 100644
--- a/clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp
@@ -695,9 +695,6 @@ 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}})},
       {{{"strtol"}}, TR::Prop({{0}}, {{1, ReturnValueIndex}})},
       {{{"strtoll"}}, TR::Prop({{0}}, {{1, ReturnValueIndex}})},
       {{{"strtoul"}}, TR::Prop({{0}}, {{1, ReturnValueIndex}})},
diff --git a/clang/test/Analysis/taint-diagnostic-visitor.c b/clang/test/Analysis/taint-diagnostic-visitor.c
index f1b9ceebdd9a6b8..8a7510177f3e444 100644
--- a/clang/test/Analysis/taint-diagnostic-visitor.c
+++ b/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 at -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 at -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 at -1{{Untrusted data is used to specify the buffer size}}
                                                 // expected-note at -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 at -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 at -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 at -1{{Untrusted data is used to specify the buffer size}}
                                                 // expected-note at -2 {{Taint propagated to the return value}}
     return pathbuf;
diff --git a/clang/test/Analysis/taint-generic.c b/clang/test/Analysis/taint-generic.c
index c674a03cfaec6c8..359a7a264c8c810 100644
--- a/clang/test/Analysis/taint-generic.c
+++ b/clang/test/Analysis/taint-generic.c
@@ -450,7 +450,7 @@ void test_wchar_apis_propagate(const char *path) {
   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 +458,7 @@ 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, ...);
@@ -953,16 +953,16 @@ void testStrlen() {
   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) {
   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);



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