PuTTY wish virtuallock

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summary: Use of VirtualLock() on Windows for sensitive material
class: wish: This is a request for an enhancement.
priority: low: We aren't sure whether to fix this or not.

The FAQ gives our reasoning behind not trying to use VirtualLock() on Windows to prevent sensitive material such as private keys being written to disk.

However, Peter Gutmann has the following comments:

Actually this may no longer be totally accurate. I had a long discussion about this with some MS technical people and it appears that (at least in Win2K and newer) VirtualLock is more effective than expected. One of the MS guys tried everything possible to get VirtualLocked'd memory paged and couldn't get it swapped out. Unfortunately the info for different Windows versions and at different times has been conflicting, but it appears that with newer versions of Windows it has at least some positive effect. In retrospect I'm glad I was too lazy to take the old VirtualLock() code out of cryptlib.

Note that if you do use VirtualLock() you have to jump through a number of hoops to handle page-granularity, because although it looks like it's byte- aligned it's really page-aligned, and unlocking one region can have the side- effect of unlocking nearby regions.

Message-Id: <200305240418.h4O4I6t19012@medusa01.cs.auckland.ac.nz>

Bryan Kadzban adds:

But I found an entry in a blogs.msdn.com blog (yes, I know... ;-)) that
points to a way to accomplish the same goals.  It says you can use the
AWE APIs (Address Windowing Extensions) to get a page of physical memory
that the kernel will never page to the swap file.

There are a couple downsides, though.  Number one, it's only supported
on Windows 2000 and up (2K Pro/Server, XP, and Server 2003).

Number two, the user has to have the "Lock pages into memory" privilege
enabled on their account on the local machine.  By default (on 2K Pro at
least), the user list on this privilege is blank, which means that
*nobody* has the needed privileges.  Admins would have to assign the
privilege at install time, or pageant (or the ssh backend itself,
actually... I wonder if that would be useful?) would not be able to
allocate these pages.  It could still fall back to normal allocation in
that case, though.

Number three, you have to go and manually enable the SE_LOCK_MEMORY_NAME
privilege in your code before using AllocatedUserPhysicalPages.  But
that's not too difficult, and there's actually a sample that shows how
to do it all.

The API reference is available online at MSDN:


and the sample is available also:


I might be able to provide a patch to implement this, if I reread both
winpgnt.c and its supporting files a lot between now and when I hear
back from you.  Right now, there's no obvious (to me) place in there
where this method of allocating memory should go.

Maybe what's needed is a generic "void *alloc_locked_pages(count)"
function, that would be implemented in the various OS specific backends?
Unix would malloc(count*sysconf(_SC_PAGESIZE)), then mlock().  Windows
would go through all these contortions (on 2K and higher) or just malloc
(on 9x or NT4).

Of course this would have to be documented also, so that callers know
that if they're running on 9x or NT4, the memory may not be locked.
Which means that it may not be worth doing it, either.

And obviously a different free()-type function would be required too.

Message-ID: <42436579.9020707@kadzban.is-a-geek.net>

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Audit trail for this wish.
(last revision of this bug record was at 2005-03-31 09:34:15 +0000)