Fix unreachable heap corruption in base64_decode()

Give size_mul_check() external linkage and use it in base64_decode() to
avoid a potential integer wrap.

Closes #19222

3 files changed, 3 insertions, 14 deletions

diff --git a/src/common/util.c b/src/common/util.c
index 9d134c1bb2..d02eb664d7 100644
--- a/src/common/util.c
+++ b/src/common/util.c
@@ -188,7 +188,7 @@ tor_malloc_zero_(size_t size DMALLOC_PARAMS)
 #define SQRT_SIZE_MAX_P1 (((size_t)1) << (sizeof(size_t)*4))
 
 /** Return non-zero if and only if the product of the arguments is exact. */
-static inline int
+inline int
 size_mul_check(const size_t x, const size_t y)
 {
   /* This first check is equivalent to
@@ -202,15 +202,6 @@ size_mul_check(const size_t x, const size_t y)
           x <= SIZE_MAX / y);
 }
 
-#ifdef TOR_UNIT_TESTS
-/** Exposed for unit tests only */
-int
-size_mul_check__(const size_t x, const size_t y)
-{
-  return size_mul_check(x,y);
-}
-#endif
-
 /** Allocate a chunk of <b>nmemb</b>*<b>size</b> bytes of memory, fill
  * the memory with zero bytes, and return a pointer to the result.
  * Log and terminate the process on error.  (Same as
diff --git a/src/common/util.h b/src/common/util.h
index 2b3e48ea8e..ff566151cb 100644
--- a/src/common/util.h
+++ b/src/common/util.h
@@ -553,9 +553,7 @@ STATIC int format_helper_exit_status(unsigned char child_state,
 
 #endif
 
-#ifdef TOR_UNIT_TESTS
-int size_mul_check__(const size_t x, const size_t y);
-#endif
+int size_mul_check(const size_t x, const size_t y);
 
 #define ARRAY_LENGTH(x) ((sizeof(x)) / sizeof(x[0]))
 
diff --git a/src/common/util_format.c b/src/common/util_format.c
index aef9db85c8..6e0a04586a 100644
--- a/src/common/util_format.c
+++ b/src/common/util_format.c
@@ -398,7 +398,7 @@ base64_decode(char *dest, size_t destlen, const char *src, size_t srclen)
    * Number of bytes required to hold all bits == (srclen*6)/8.
    * Yes, we want to round down: anything that hangs over the end of a
    * byte is padding. */
-  if (destlen < (srclen*3)/4)
+  if (!size_mul_check(srclen, 3) || destlen < (srclen*3)/4)
     return -1;
   if (destlen > SIZE_T_CEILING)
     return -1;