RFR: 8306561: Possible out of bounds access in print_pointer_information

[Thomas Obermeier]

On Thu, 26 Oct 2023 16:11:00 GMT, Thomas Obermeier <duke at openjdk.org> wrote:

> MallocTracker::print_pointer_information in src/hotspot/share/services/mallocTracker.cpp is called to check the highest pointer address of the reserved region. To do so it aligns the test pointer down to the next 8 Byte boundary and casts this address to class MallocHeader in order to use this classes eye-catcher member _canary for validation. Method looks_valid() dereferences _canary's content. _canary has an offset of 14 bytes relative to the class. Therefore it resides outside the reserved region for the highest pointer address, which causes a segmentation violation.
> 
> We would expect the same error also for other platforms than AIX as memory is read, which is not allocated. Interestingly, Linux seems to allow this access for 5 times 4K above the reserved region.
> 
> As a solution, looks_valid() should check _canary's address as being invalid, and return false immediately.

Agreed. Would you still prefer to blow up if the caller hands down bad pointers? In other words: should MallocHeader::looks_valid() not do my additional check?

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PR Comment: https://git.openjdk.org/jdk/pull/16381#issuecomment-1781482111