Author: Pietro Oliva
CVE: CVE-2020-17361
Vendor: ReadyTalk
Product: Avian JVM
Affected version: 1.2.0
Description:
The issue is located in the vm::arrayCopy method defined in
classpath-common.h,
where multiple boundary checks are performed to prevent
out-of-bounds memory
read/write. One of these boundary checks makes the code return
silently when a
negative length is provided instead of throwing an exception.
Impact:
The caller could assume that the method has succeeded in the copy
operation due
to the lack of exception being thrown for negative lengths. This
could result
in data being lost during the copy, with varying consequences
depending on the
subsequent use of the destination buffer.
Exploitation:
The following PoC should throw an exception, but it silently
returns instead:
import java.lang.*;
public class poc {
public static void main(String[] args) {
byte[] src = "This is src".getBytes();
byte[] dst = "This is dst".getBytes();
// This should throw an exception, but it doesn't and it
silently returns intead.
System.arraycopy(src, 0, dst, 0, -1);
}
}
Evidence:
void arrayCopy(Thread* t,
object src,
int32_t srcOffset,
object dst,
int32_t dstOffset,
int32_t length)
{
if (LIKELY(src and dst)) {
if (LIKELY(compatibleArrayTypes(
t, objectClass(t, src), objectClass(t, dst)))) {
unsigned elementSize = objectClass(t,
src)->arrayElementSize();
if (LIKELY(elementSize)) {
intptr_t sl = fieldAtOffset<uintptr_t>(src,
BytesPerWord);
intptr_t dl = fieldAtOffset<uintptr_t>(dst,
BytesPerWord);
if (LIKELY(length > 0)) {
if (LIKELY(srcOffset >= 0 and srcOffset + length <= sl
and dstOffset >= 0 and dstOffset + length <= dl)) {
uint8_t* sbody = &fieldAtOffset<uint8_t>(src,
ArrayBody);
uint8_t* dbody = &fieldAtOffset<uint8_t>(dst,
ArrayBody);
if (src == dst) {
memmove(dbody + (dstOffset * elementSize),
sbody + (srcOffset * elementSize),
length * elementSize);
} else {
memcpy(dbody + (dstOffset * elementSize),
sbody + (srcOffset * elementSize),
length * elementSize);
}
if (objectClass(t, dst)->objectMask()) {
mark(t, dst, ArrayBody + (dstOffset * BytesPerWord), length);
}
return;
} else {
throwNew(t, GcIndexOutOfBoundsException::Type);
}
} else {
return; // No exceptions are thrown for negative lengths
}
}
}
} else {
throwNew(t, GcNullPointerException::Type);
return;
}
throwNew(t, GcArrayStoreException::Type);
}
As can be seen in the line commented above, no exceptions are
thrown for
negative lengths.
Remediation:
A patch has been merged in the master branch:
https://github.com/ReadyTalk/avian/pull/571
Disclosure timeline:
3rd August 2020 - Vulnerability reported.
3rd August 2020 - Vulnerability acknowledged.
4th August 2020 - CVE request sent to Mitre.
5th August 2020 - CVE assigned.
10th August 2020 - Proposed patch via pull request.
10th August 2020 - Patch approved and merged after changes.
10th August 2020 - Vulnerability details shared on
fulldisclosure.
