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src: fix out-of-bounds write in ucs2 encode of unaligned buffer #64697

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src: fix out-of-bounds write in ucs2 encode of unaligned buffer #64697

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7 changes: 5 additions & 2 deletions src/string_bytes.cc
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Original file line number Diff line number Diff line change
Expand Up @@ -723,8 +723,11 @@ MaybeLocal<Value> StringBytes::Encode(Isolate* isolate,
}
if (reinterpret_cast<uintptr_t>(buf) % 2 != 0) {
return EncodeTwoByteString(
isolate, str_len, [buf, buflen](uint16_t* dst) {
memcpy(dst, buf, buflen);
isolate, str_len, [buf, str_len](uint16_t* dst) {
// Copy whole code units only. buflen may be odd, but the
// destination holds str_len (== buflen / 2) uint16_t units, so
// copying buflen bytes would write one byte past it.
memcpy(dst, buf, str_len * sizeof(uint16_t));
});
}
return ExternTwoByteString::NewFromCopy(
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17 changes: 17 additions & 0 deletions test/parallel/test-buffer-tostring-range.js
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Expand Up @@ -118,6 +118,23 @@ assert.strictEqual(rangeBuffer.toString('ascii', 0, true), 'a');
);
}

// ucs2 slices of odd byte length at an odd (2-byte unaligned) start offset.
// The decoder keeps whole code units and drops the trailing odd byte, so an
// odd-length slice reads the same as the even-length slice one byte shorter.
// Regression for a one-byte out-of-bounds write in the unaligned copy path,
// where the full odd byte length was copied into a buffer sized for
// floor(len / 2) code units.
for (const nchars of [1, 200, 256, 300, 1024]) {
const bytes = nchars * 2 + 1; // odd
const src = Buffer.alloc(bytes + 2);
for (let i = 0; i < src.length; i++) src[i] = (i * 7 + 1) & 0xff;
// start = 1 makes the read pointer 2-byte unaligned; end - start is odd.
const odd = src.toString('ucs2', 1, 1 + bytes);
const even = src.toString('ucs2', 1, 1 + bytes - 1);
assert.strictEqual(odd.length, nchars);
assert.strictEqual(odd, even);
}

// Try toString() with an object as an encoding
assert.strictEqual(rangeBuffer.toString({ toString: function() {
return 'ascii';
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