Erase memory on shutdown

Polytropon freebsd at
Sun Aug 5 16:02:16 UTC 2018

On Sun, 05 Aug 2018 09:42:45 -0600, JD wrote:
> On 08/05/2018 09:25 AM, Polytropon wrote:
> > On Sun, 5 Aug 2018 22:24:16 +0800, thor wrote:
> >> Just one paranoid question: How to cause FreeBSD to zero all RAM during
> >> shutdown?
> > This would imply that the kernel would finally have to
> > overwrite itself. How can control over zeroing memory
> > be maintained when the control program itself has been
> > overwritten? That would be the result of the "all" in
> > your requirement.
> >
> > Sure, you could add some code to the final shutdown
> > routines to zero the RAM, which is possible, but not
> > trivial: You need a non-optimized call to memset()
> > using a custom function pointer.
> >
> > 	static void *(* const volatile memset_ptr)(void *, int, size_t) = memset;
> > 	static void secure_memzero(void *p, size_t len)
> > 	{
> > 	        (memset_ptr)(p, 0, len);
> > 	}
> >
> > 	void *ram = 0x0; 		/* RAM start address */
> > 	size_t amount = 17179869184UL;	/* 16 GB RAM */
> >
> > 	secure_menzero(ram, amount);	/* ouch */
> >
> > If you add something like this to the kernel, and make
> > sure your compiler isn't too clever (as to optimize it
> > into a NOP), you're going to crash the whole system
> > without actually being sure that at least a part of
> > the RAM has been zeroed. And even then it might not
> > work as intended.
> >
> > See:
> >
> >
> >
> >
> >
> > Keep in mind: You're declaring war on intended security
> > mechanisms if you try to do this. :-)
> >
> > However, this is not guaranteed (!) to work, so you
> > cannot be safe. And you must do it from the kernel.
> > You cannot use (a privileged) program like dd to
> > flush /dev/mem and /dev/kmem with /dev/zero output.
> >
> > Your best bet is to assume that RAM will be zeroed as soon
> > as the power-off routine as been completed - no refresh,
> > no content. Not perfectly secure, though... :-)
> >
> > RAM usually isn't zeroed, but marked as "not in use" so
> > it can be overwritten. Address randomization makes it
> > hard to protect where something will appear in RAM, and
> > access to RAM requires certain privileges on a system.
> >
> >
> Just for the heck of it:
> allocate from contiguous freemem for the program in question,
> load the program,
> transfer control to the program which should passed
> it starting addy, and size, and total mem size.
> Such a program would work, assuming it is written to watch
> out for it's addy space, and not overwrite itself.

It's not about the program itself. Assume it is run as
a user process (by root, for example). It should not
overwrite itself, it also should not overwrite the
kernel and other system facilities in memory whose
removal could cause a kernel crash before the zeroing
is complete.

That's why I suggested it _could_ be introduced as
new code into the kernel, to be performed at the
latest stage of system shutdown.

> My question is: currently, if the machine is shut down,
> (i.e. powered off), does ram keep it's content?

Theory: No.

Practice: Yes.

> If yes,
> how?

Capacitive effects?

> for how long?

Until capacity is drained out.

> Is it static ram, or does the battery provide the power
> for the ram to remain refreshed?

Usually not.

During normal operations, RAM is refreshed periodically.
This is the 'D' in DRAM - dynamic.

The most prominent case of static RAM is core memory.
It keeps it content (after magnetization) until the
memory is read again. So if you found a core memory matrix
with a program in it from 1970, the content could still
be in there... :-)

So you see, even static RAM needs a refresh (after a
read), usually with the values read.

There are "suspend to RAM" mechanisms where the RAM
is buffered from the PSU (or in case of laptops: from
battery) until the power runs out. This means that the
computer technically isn't off - it's just that the CPU
isn't running, only the controllers for the memory
control are performing their normal work.

There are also buffer batteries for "critical RAM"
that can keep the RAM intact during power outages.
But I don't know if they also buffer CPU registers
which would be neccessary to resume operations when
the power comes back.

Also see:

Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...

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