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src: fix stack-buffer overflow for long exception lines #2404

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src: fix stack-buffer overflow for long exception lines #2404

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src: fix buffer overflow for long exception lines
skomski Sep 3, 2015
c154fde
src: use standard conform snprintf on windows
skomski Sep 3, 2015
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24 changes: 13 additions & 11 deletions src/node.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -1317,8 +1317,6 @@ void AppendExceptionLine(Environment* env,
err_obj->SetHiddenValue(env->processed_string(), True(env->isolate()));
}

char arrow[1024];

// Print (filename):(line number): (message).
node::Utf8Value filename(env->isolate(), message->GetScriptResourceName());
const char* filename_string = *filename;
Expand Down Expand Up @@ -1351,34 +1349,38 @@ void AppendExceptionLine(Environment* env,
int start = message->GetStartColumn();
int end = message->GetEndColumn();

char arrow[1024];
int max_off = sizeof(arrow) - 2;

int off = snprintf(arrow,
sizeof(arrow),
"%s:%i\n%s\n",
filename_string,
linenum,
sourceline_string);
CHECK_GE(off, 0);
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This can still cause an assertion to be raised when sourceline_string is really long and snprintf() tries to write past sizeof(arrow).

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If the total length of all of the strings being printed to arrow exceeds sizeof(arrow), snprintf() will truncate the output and return -1 to indicate that truncation has happened. When that happens (like in #2581), CHECK_GE(off, 0) will trigger and cause a C++ assertion.

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snprintf returns -1 only on an encoding error else it will return the number of characters that would have been written if if n had been sufficiently large. In #2581 it triggers at https://github.com/nodejs/node/blob/master/src/node.cc#L1379

Based on https://groups.google.com/d/msg/comp.std.c/lIvkxXr5_wE/aVuIpm52ToQJ for an encoding error you have to use mbrtowc() or wcrtomb(), directly or indirectly which you only do with "%lc" or "%ls" format specifiers. I think in that case an assert is enough.

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I'm testing on Windows where a custom snprintf() is used which internally uses _vsprintf_p() which MSDN says:

_vsprintf_p and _vswprintf_p return the number of characters written, not including the terminating null character, or a negative value if an output error occurs.

So on Windows, exceeding sizeof(arrow) must constitute an "output error" and that's why I'm seeing off == -1

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Yes the custom snprintf should be replaced with something like this:
The _TRUNCATE negates the non-standard -1 behaviour and VS 2015 has
snprintf.

#if _MSC_VER < 1900
int snprintf(char *buffer, size_t n, const char *format,...){
  va_list argp;
  int ret;
  va_start(argp, format);
  ret = _vscprintf(format, argp);
  vsnprintf_s(buffer, n, _TRUNCATE, format, argp);
  va_end(argp);
  return ret;
}
#endif

if (off > max_off) {
off = max_off;
}

// Print wavy underline (GetUnderline is deprecated).
for (int i = 0; i < start; i++) {
if (sourceline_string[i] == '\0' ||
static_cast<size_t>(off) >= sizeof(arrow)) {
if (sourceline_string[i] == '\0' || off >= max_off) {
break;
}
CHECK_LT(static_cast<size_t>(off), sizeof(arrow));
CHECK_LT(off, max_off);
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Isn't this redundant, as you have off >= max_off in the if condition above?

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Only replaced asserts from before

arrow[off++] = (sourceline_string[i] == '\t') ? '\t' : ' ';
}
for (int i = start; i < end; i++) {
if (sourceline_string[i] == '\0' ||
static_cast<size_t>(off) >= sizeof(arrow)) {
if (sourceline_string[i] == '\0' || off >= max_off) {
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off cannot be greater than max_off as the CHECK_LT in the previous loop ensures that, right?

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Previous loop is not always executed

break;
}
CHECK_LT(static_cast<size_t>(off), sizeof(arrow));
CHECK_LT(off, max_off);
arrow[off++] = '^';
}
CHECK_LE(static_cast<size_t>(off - 1), sizeof(arrow) - 1);
arrow[off++] = '\n';
arrow[off] = '\0';
CHECK_LE(off, max_off);
arrow[off] = '\n';
arrow[off + 1] = '\0';

Local<String> arrow_str = String::NewFromUtf8(env->isolate(), arrow);
Local<Value> msg;
Expand Down
18 changes: 10 additions & 8 deletions src/node_internals.h
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Original file line number Diff line number Diff line change
Expand Up @@ -95,17 +95,19 @@ void GetSockOrPeerName(const v8::FunctionCallbackInfo<v8::Value>& args) {
#ifdef _WIN32
// emulate snprintf() on windows, _snprintf() doesn't zero-terminate the buffer
// on overflow...
// VS 2015 added a standard conform snprintf
#if defined( _MSC_VER ) && (_MSC_VER < 1900)
#include <stdarg.h>
inline static int snprintf(char* buf, unsigned int len, const char* fmt, ...) {
va_list ap;
va_start(ap, fmt);
int n = _vsprintf_p(buf, len, fmt, ap);
if (len)
buf[len - 1] = '\0';
va_end(ap);
return n;
inline static int snprintf(char *buffer, size_t n, const char *format, ...) {
va_list argp;
va_start(argp, format);
int ret = _vscprintf(format, argp);
vsnprintf_s(buffer, n, _TRUNCATE, format, argp);
va_end(argp);
return ret;
}
#endif
#endif

#if defined(__x86_64__)
# define BITS_PER_LONG 64
Expand Down
1 change: 1 addition & 0 deletions test/fixtures/throws_error5.js
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1 change: 1 addition & 0 deletions test/fixtures/throws_error6.js
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11 changes: 10 additions & 1 deletion test/parallel/test-error-reporting.js
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Expand Up @@ -54,7 +54,16 @@ errExec('throws_error4.js', function(err, stdout, stderr) {
assert.ok(/SyntaxError/.test(stderr));
});

// Specific long exception line doesn't result in stack overflow
errExec('throws_error5.js', function(err, stdout, stderr) {
assert.ok(/SyntaxError/.test(stderr));
});

// Long exception line with length > errorBuffer doesn't result in assertion
errExec('throws_error6.js', function(err, stdout, stderr) {
assert.ok(/SyntaxError/.test(stderr));
});

process.on('exit', function() {
assert.equal(4, exits);
assert.equal(6, exits);
});