py/vstr: Raise a RuntimeError if fixed vstr buffer overflows.

Current users of fixed vstr buffers (building file paths) assume that there
is no overflow and do not check for overflow after building the vstr.  This
has the potential to lead to NULL pointer dereferences
(when vstr_null_terminated_str returns NULL because it can't allocate RAM
for the terminating byte) and stat'ing and loading invalid path names (due
to the path being truncated).  The safest and simplest thing to do in these
cases is just raise an exception if a write goes beyond the end of a fixed
vstr buffer, which is what this patch does.  It also simplifies the vstr
code.

ports/unix/coverage.c

@@ -181,8 +181,21 @@ STATIC mp_obj_t extra_coverage(void) {
         mp_printf(&mp_plat_print, "%.*s\n", (int)vstr->len, vstr->buf);
 
         VSTR_FIXED(fix, 4);
-        vstr_add_str(&fix, "large");
-        mp_printf(&mp_plat_print, "%.*s\n", (int)fix.len, fix.buf);
+        nlr_buf_t nlr;
+        if (nlr_push(&nlr) == 0) {
+            vstr_add_str(&fix, "large");
+            nlr_pop();
+        } else {
+            mp_obj_print_exception(&mp_plat_print, MP_OBJ_FROM_PTR(nlr.ret_val));
+        }
+
+        fix.len = fix.alloc;
+        if (nlr_push(&nlr) == 0) {
+            vstr_null_terminated_str(&fix);
+            nlr_pop();
+        } else {
+            mp_obj_print_exception(&mp_plat_print, MP_OBJ_FROM_PTR(nlr.ret_val));
+        }
     }
 
     // repl autocomplete

py/vstr.c

@@ -30,7 +30,7 @@
 #include <assert.h>
 
 #include "py/mpconfig.h"
-#include "py/misc.h"
+#include "py/runtime.h"
 #include "py/mpprint.h"
 
 // returned value is always at least 1 greater than argument
@@ -92,7 +92,9 @@ void vstr_free(vstr_t *vstr) {
 // Extend vstr strictly by requested size, return pointer to newly added chunk.
 char *vstr_extend(vstr_t *vstr, size_t size) {
     if (vstr->fixed_buf) {
-        return NULL;
+        // We can't reallocate, and the caller is expecting the space to
+        // be there, so the only safe option is to raise an exception.
+        mp_raise_msg(&mp_type_RuntimeError, NULL);
     }
     char *new_buf = m_renew(char, vstr->buf, vstr->alloc, vstr->alloc + size);
     char *p = new_buf + vstr->alloc;
@@ -101,17 +103,18 @@ char *vstr_extend(vstr_t *vstr, size_t size) {
     return p;
 }
 
-STATIC bool vstr_ensure_extra(vstr_t *vstr, size_t size) {
+STATIC void vstr_ensure_extra(vstr_t *vstr, size_t size) {
     if (vstr->len + size > vstr->alloc) {
         if (vstr->fixed_buf) {
-            return false;
+            // We can't reallocate, and the caller is expecting the space to
+            // be there, so the only safe option is to raise an exception.
+            mp_raise_msg(&mp_type_RuntimeError, NULL);
         }
         size_t new_alloc = ROUND_ALLOC((vstr->len + size) + 16);
         char *new_buf = m_renew(char, vstr->buf, vstr->alloc, new_alloc);
         vstr->alloc = new_alloc;
         vstr->buf = new_buf;
     }
-    return true;
 }
 
 void vstr_hint_size(vstr_t *vstr, size_t size) {
@@ -119,9 +122,7 @@ void vstr_hint_size(vstr_t *vstr, size_t size) {
 }
 
 char *vstr_add_len(vstr_t *vstr, size_t len) {
-    if (!vstr_ensure_extra(vstr, len)) {
-        return NULL;
-    }
+    vstr_ensure_extra(vstr, len);
     char *buf = vstr->buf + vstr->len;
     vstr->len += len;
     return buf;
@@ -131,9 +132,7 @@ char *vstr_add_len(vstr_t *vstr, size_t len) {
 char *vstr_null_terminated_str(vstr_t *vstr) {
     // If there's no more room, add single byte
     if (vstr->alloc == vstr->len) {
-        if (vstr_extend(vstr, 1) == NULL) {
-            return NULL;
-        }
+        vstr_extend(vstr, 1);
     }
     vstr->buf[vstr->len] = '\0';
     return vstr->buf;
@@ -141,9 +140,6 @@ char *vstr_null_terminated_str(vstr_t *vstr) {
 
 void vstr_add_byte(vstr_t *vstr, byte b) {
     byte *buf = (byte*)vstr_add_len(vstr, 1);
-    if (buf == NULL) {
-        return;
-    }
     buf[0] = b;
 }
 
@@ -153,31 +149,19 @@ void vstr_add_char(vstr_t *vstr, unichar c) {
     // Is it worth just calling vstr_add_len(vstr, 4)?
     if (c < 0x80) {
         byte *buf = (byte*)vstr_add_len(vstr, 1);
-        if (buf == NULL) {
-            return;
-        }
         *buf = (byte)c;
     } else if (c < 0x800) {
         byte *buf = (byte*)vstr_add_len(vstr, 2);
-        if (buf == NULL) {
-            return;
-        }
         buf[0] = (c >> 6) | 0xC0;
         buf[1] = (c & 0x3F) | 0x80;
     } else if (c < 0x10000) {
         byte *buf = (byte*)vstr_add_len(vstr, 3);
-        if (buf == NULL) {
-            return;
-        }
         buf[0] = (c >> 12) | 0xE0;
         buf[1] = ((c >> 6) & 0x3F) | 0x80;
         buf[2] = (c & 0x3F) | 0x80;
     } else {
         assert(c < 0x110000);
         byte *buf = (byte*)vstr_add_len(vstr, 4);
-        if (buf == NULL) {
-            return;
-        }
         buf[0] = (c >> 18) | 0xF0;
         buf[1] = ((c >> 12) & 0x3F) | 0x80;
         buf[2] = ((c >> 6) & 0x3F) | 0x80;
@@ -193,16 +177,7 @@ void vstr_add_str(vstr_t *vstr, const char *str) {
 }
 
 void vstr_add_strn(vstr_t *vstr, const char *str, size_t len) {
-    if (!vstr_ensure_extra(vstr, len)) {
-        // if buf is fixed, we got here because there isn't enough room left
-        // so just try to copy as much as we can, with room for a possible null byte
-        if (vstr->fixed_buf && vstr->len < vstr->alloc) {
-            len = vstr->alloc - vstr->len;
-            goto copy;
-        }
-        return;
-    }
-copy:
+    vstr_ensure_extra(vstr, len);
     memmove(vstr->buf + vstr->len, str, len);
     vstr->len += len;
 }
@@ -214,9 +189,7 @@ STATIC char *vstr_ins_blank_bytes(vstr_t *vstr, size_t byte_pos, size_t byte_len
     }
     if (byte_len > 0) {
         // ensure room for the new bytes
-        if (!vstr_ensure_extra(vstr, byte_len)) {
-            return NULL;
-        }
+        vstr_ensure_extra(vstr, byte_len);
         // copy up the string to make room for the new bytes
         memmove(vstr->buf + byte_pos + byte_len, vstr->buf + byte_pos, l - byte_pos);
         // increase the length
@@ -227,17 +200,13 @@ STATIC char *vstr_ins_blank_bytes(vstr_t *vstr, size_t byte_pos, size_t byte_len
 
 void vstr_ins_byte(vstr_t *vstr, size_t byte_pos, byte b) {
     char *s = vstr_ins_blank_bytes(vstr, byte_pos, 1);
-    if (s != NULL) {
-        *s = b;
-    }
+    *s = b;
 }
 
 void vstr_ins_char(vstr_t *vstr, size_t char_pos, unichar chr) {
     // TODO UNICODE
     char *s = vstr_ins_blank_bytes(vstr, char_pos, 1);
-    if (s != NULL) {
-        *s = chr;
-    }
+    *s = chr;
 }
 
 void vstr_cut_head_bytes(vstr_t *vstr, size_t bytes_to_cut) {

tests/unix/extra_coverage.py.exp

@@ -18,7 +18,8 @@ sts
 
 test
 tes
-larg
+RuntimeError: 
+RuntimeError: 
 # repl
 ame__