Re: memory pages nulling when releasing

--- Nick Borisov <neiro21@xxxxxxxxx> wrote:
Could you tell me if FreeBSD supports memory page nulling when
releasing it to prevent unauthorized access to data left in the page
after it's allocated again.
If it does, what sys calls etc provide that?
IMHO this is an important issue when operating data with different
sensivity levels.

It is important, that no user process can look at non-shared (mmap(2) /
MAP_SHARED) pages in main memory that were used by another process before.

I think FreeBSD does it as follows (but I am not sure):

1. If a page is allocated for a process it is either overwritten with zeroes
(0x00) (e. g. in case of a page for the stack segment of a process) or it is
overwritten with the bytes of an executable or another file or so...

2. For maximum performance the system keeps a bunch of "pre-zero-ed" pages, so
that the OS is quite fast, when a process wants pages for data (malloc(3)) or
for stack.

Here is my "proof": :-)
% vmstat -s | grep zero
3840247 zero fill pages zeroed
844738 zero fill pages prezeroed

-Arne


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