Add basic "security rules" (#4298)

Author: lrstewart

stuffer/s2n_stuffer.h

@@ -16,6 +16,7 @@
 #pragma once
 
 #include <limits.h>
+#include <stdarg.h>
 #include <stdint.h>
 #include <stdlib.h>
 #include <sys/uio.h>
@@ -165,6 +166,16 @@ int s2n_stuffer_skip_read_until(struct s2n_stuffer *stuffer, const char *target)
 int s2n_stuffer_alloc_ro_from_string(struct s2n_stuffer *stuffer, const char *str);
 int s2n_stuffer_init_ro_from_string(struct s2n_stuffer *stuffer, uint8_t *data, uint32_t length);
 
+/* Stuffer versions of sprintf methods, except:
+ * - They write bytes, not strings. They do not write a final '\0'. Unfortunately,
+ *   they do still require enough space for a final '\0'-- we'd have to reimplement
+ *   sprintf to avoid that.
+ * - vprintf does not consume the vargs. It calls va_copy before using
+ *   the varg argument, so can be called repeatedly with the same vargs.
+ */
+int s2n_stuffer_printf(struct s2n_stuffer *stuffer, const char *format, ...);
+int s2n_stuffer_vprintf(struct s2n_stuffer *stuffer, const char *format, va_list vargs);
+
 /* Read a private key from a PEM encoded stuffer to an ASN1/DER encoded one */
 int s2n_stuffer_private_key_from_pem(struct s2n_stuffer *pem, struct s2n_stuffer *asn1, int *type);
 

stuffer/s2n_stuffer_text.c

@@ -14,6 +14,7 @@
  */
 
 #include <string.h>
+#include <sys/param.h>
 
 #include "stuffer/s2n_stuffer.h"
 #include "utils/s2n_mem.h"
@@ -206,3 +207,73 @@ int s2n_stuffer_init_ro_from_string(struct s2n_stuffer *stuffer, uint8_t *data,
 
     return S2N_SUCCESS;
 }
+
+/* If we call va_start or va_copy there MUST be a matching call to va_end,
+ * so we should use DEFER_CLEANUP with our va_lists.
+ * Unfortunately, some environments implement va_list in ways that don't
+ * act as expected when passed by reference. For example, because va_end is
+ * a macro it may expect va_list to be an array (maybe to call sizeof),
+ * but passing va_list by reference will cause it to decay to a pointer instead.
+ * To avoid any surprises, just wrap the va_list in our own struct.
+ */
+struct s2n_va_list {
+    va_list va_list;
+};
+
+static void s2n_va_list_cleanup(struct s2n_va_list *list)
+{
+    if (list) {
+        va_end(list->va_list);
+    }
+}
+
+int s2n_stuffer_vprintf(struct s2n_stuffer *stuffer, const char *format, va_list vargs_in)
+{
+    POSIX_PRECONDITION(s2n_stuffer_validate(stuffer));
+    POSIX_ENSURE_REF(format);
+
+    /* vsnprintf consumes the va_list, so copy it first */
+    DEFER_CLEANUP(struct s2n_va_list vargs_1 = { 0 }, s2n_va_list_cleanup);
+    va_copy(vargs_1.va_list, vargs_in);
+
+    /* The first call to vsnprintf calculates the size of the formatted string.
+     * str_len does not include the one byte vsnprintf requires for a trailing '\0',
+     * so we need one more byte.
+     */
+    int str_len = vsnprintf(NULL, 0, format, vargs_1.va_list);
+    POSIX_ENSURE_GTE(str_len, 0);
+    int mem_size = str_len + 1;
+
+    /* 'tainted' indicates that pointers to the contents of the stuffer exist,
+     * so resizing / reallocated the stuffer will invalidate those pointers.
+     * However, we do no resize the stuffer in this method after creating `str`
+     * and `str` does not live beyond this method, so ignore `str` for the
+     * purposes of tracking 'tainted'.
+     */
+    bool previously_tainted = stuffer->tainted;
+    char *str = s2n_stuffer_raw_write(stuffer, mem_size);
+    stuffer->tainted = previously_tainted;
+    POSIX_GUARD_PTR(str);
+
+    /* vsnprintf again consumes the va_list, so copy it first */
+    DEFER_CLEANUP(struct s2n_va_list vargs_2 = { 0 }, s2n_va_list_cleanup);
+    va_copy(vargs_2.va_list, vargs_in);
+
+    /* This time, vsnprintf actually writes the formatted string */
+    int written = vsnprintf(str, mem_size, format, vargs_2.va_list);
+    POSIX_ENSURE_GTE(written, 0);
+
+    /* We don't actually use c-strings, so erase the final '\0' */
+    POSIX_GUARD(s2n_stuffer_wipe_n(stuffer, 1));
+
+    POSIX_POSTCONDITION(s2n_stuffer_validate(stuffer));
+    return S2N_SUCCESS;
+}
+
+int s2n_stuffer_printf(struct s2n_stuffer *stuffer, const char *format, ...)
+{
+    DEFER_CLEANUP(struct s2n_va_list vargs = { 0 }, s2n_va_list_cleanup);
+    va_start(vargs.va_list, format);
+    POSIX_GUARD(s2n_stuffer_vprintf(stuffer, format, vargs.va_list));
+    return S2N_SUCCESS;
+}