Re: [PATCH V3] mm: Allow userland to request that the kernel clear memory on release

On 4/26/19 12:58 AM, Matthew Garrett wrote:

From: Matthew Garrett <redacted>

Applications that hold secrets and wish to avoid them leaking can use
mlock() to prevent the page from being pushed out to swap and
MADV_DONTDUMP to prevent it from being included in core dumps. Applications
can also use atexit() handlers to overwrite secrets on application exit.
However, if an attacker can reboot the system into another OS, they can
dump the contents of RAM and extract secrets. We can avoid this by setting
CONFIG_RESET_ATTACK_MITIGATION on UEFI systems in order to request that the
firmware wipe the contents of RAM before booting another OS, but this means
rebooting takes a *long* time - the expected behaviour is for a clean
shutdown to remove the request after scrubbing secrets from RAM in order to
avoid this.

Unfortunately, if an application exits uncleanly, its secrets may still be
present in RAM. This can't be easily fixed in userland (eg, if the OOM
killer decides to kill a process holding secrets, we're not going to be able
to avoid that), so this patch adds a new flag to madvise() to allow userland
to request that the kernel clear the covered pages whenever the page
map count hits zero. Since vm_flags is already full on 32-bit, it
will only work on 64-bit systems. This is currently only permitted on
private mappings that have not yet been populated in order to simplify
implementation, which should suffice for the envisaged use cases. We can
extend the behaviour later if we come up with a robust set of semantics.

Signed-off-by: Matthew Garrett <redacted>
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Updated based on feedback from Jann - for now let's just prevent setting
the flag on anything that has already mapped some pages, which avoids
child processes being able to interfere with the parent. In addition,

That makes the API quite tricky and different from existing madvise()
modes that don't care. One would for example have to call
madvise(MADV_WIPEONRELEASE) before mlock(), otherwise mlock() would
fault the pages in (unless MLOCK_ONFAULT). As such it really looks like
a mmap() flag, but that's less flexible.

How bout just doing the CoW on any such pre-existing pages as part of
the madvise(MADV_WIPEONRELEASE) call?