RFR(xs): (aix but affects shared code too) 8186665: buffer overflow in Java_java_nio_MappedByteBuffer_isLoaded0

Langer, Christoph christoph.langer at sap.com
Wed Oct 18 08:23:42 UTC 2017

Hi Thomas,

apart from the point that Peter found, I’d also think it would look better if the typedef section (line 51-56) would be placed before the AIX only function (line 41-49).

Other than that, it looks good to me.

Best regards

From: nio-dev [mailto:nio-dev-bounces at openjdk.java.net] On Behalf Of Peter Levart
Sent: Mittwoch, 18. Oktober 2017 10:17
To: Thomas Stüfe <thomas.stuefe at gmail.com>; Alan Bateman <Alan.Bateman at oracle.com>
Cc: nio-dev at openjdk.java.net; ppc-aix-port-dev at openjdk.java.net; Java Core Libs <core-libs-dev at openjdk.java.net>
Subject: Re: RFR(xs): (aix but affects shared code too) 8186665: buffer overflow in Java_java_nio_MappedByteBuffer_isLoaded0

On 10/18/2017 10:14 AM, Peter Levart wrote:
Hi Thomas,

Shouldn't the following line:

  47     return len2 + pagesize - 1 / pagesize;

..be written as:

           return (len2 + pagesize - 1) / pagesize;

...or better yet, as:

               return numPages;

(you already calculate it correctly in previous line, but then return an expression, which is wrong).

Regards, Peter

Regards, Peter
On 10/18/2017 09:44 AM, Thomas Stüfe wrote:
Hi all,

I am wrapping up fixes which did not make it into the repo before the consolidation. Alan already reviewed the last webrev, but I need a second reviewer.

Issue: https://bugs.openjdk.java.net/browse/JDK-8186665
Last Webrev: http://cr.openjdk.java.net/~stuefe/webrevs/8186665-buffer-overflow-in-mincore/webrev.01/webrev<http://cr.openjdk.java.net/%7Estuefe/webrevs/8186665-buffer-overflow-in-mincore/webrev.01/webrev/>

Issue text for your convenience:

In Java_java_nio_MappedByteBuffer_isLoaded0, we call mincore(2) to retrieve the paging status of an address range.

The size of the output buffer for mincore(2) depends on the number of pages in *system page size* in the given memory range (this is spelled out more or less explicitly on AIX and Linux, nothing is said on BSD/OSX, but I assume the same). The number of pages in the memory range is calculated by MappedByteBuffer.isLoaded() and handed down to Java_java_nio_MappedByteBuffer_isLoaded0() together with the memory range to test.

MappedByteBuffer.isLoaded() calculates this number of pages based on jjdk.internal.misc.Unsafe.pagesize(), which ultimately comes down to os::vm_page_size().

For AIX, os::vm_page_size() may return a page size larger than the system page size of 4K. The reason for this is that on AIX, memory can be backed by different page sizes, usually either 4K or 64K - e.g. posix thread stacks may have 4K pages, java heap (system V shared memory) with 64K pages, but mmap memory is always 4K page backed...

But as the OpenJDK code base generally assumes one homogeneous page size for everything - which is usually synonymous with os::vm_page_size() - a decision had to be made which page size to assume as a global system page size, and this may be a larger page size than the 4K system page size mincore(2) assumes.

This usually is no problem, but with mincore(2) it is: as the size of the output buffer depends on the number of pages, calculating with a too-large page size causes the output buffer to be too small and hence the buffer overflows. The solution must be to base the size of the mincore output buffer on the system page size.


Thanks and Kind Regards, Thomas

On Thu, Aug 31, 2017 at 9:39 PM, Alan Bateman <Alan.Bateman at oracle.com<mailto:Alan.Bateman at oracle.com>> wrote:
On 31/08/2017 19:01, Thomas Stüfe wrote:
Hi Alan,

thank you for your review!

Updated webrev:
http://cr.openjdk.java.net/~stuefe/webrevs/8186665-buffer-overflow-in-mincore/webrev.01/webrev/<http://cr.openjdk.java.net/%7Estuefe/webrevs/8186665-buffer-overflow-in-mincore/webrev.01/webrev/> <http://cr.openjdk.java.net/%7Estuefe/webrevs/8186665-buffer-overflow-in-mincore/webrev.01/webrev/>

I fixed the indention and embellished the comments around the sentinel at the end of the buffer somewhat.
This looks okay to me.


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