Re: [patch 0/5] Support for sanitization flag in low-level page allocator

[Peter Zijlstra <peterz@infradead.org>]

On Wed, 2009-05-20 at 11:30 -0700, Larry H. wrote:
> This patch adds support for the SENSITIVE flag to the low level page
> allocator. An additional GFP flag is added for use with higher level
> allocators (GFP_SENSITIVE, which implies GFP_ZERO).
> 
> The code is largely based off the memory sanitization feature in the
> PaX project (licensed under the GPL v2 terms), and allows fine grained
> marking of pages for sanitization on allocation and release time, as an
> opt-in feature (instead of its opt-all counterpart in PaX).
> 
> This avoids leaking sensitive information when memory is released to
> the system after use, for example in cryptographic subsystems.
> 
> The next patches in this set deploy this flag for different
> subsystems that could potentially leak cryptographic secrets or other
> confidential information by means of an information leak or other kinds
> of security bugs (ex. use of uninitialized variables or use-after-free),
> besides extending the remanence of this data on memory (allowing
> Iceman/coldboot attacks possible).
> 
> The "Shredding Your Garbage: Reducing Data Lifetime Through Secure
> Deallocation" paper by Jim Chow et. al from the Stanford University
> Department of Computer Science, explains the security implications of
> insecure deallocation, and provides extensive information with figures
> and applications thoroughly analyzed for this behavior [1]. More recently
> this issue came to widespread attention when the "Lest We Remember:
> Cold Boot Attacks on Encryption Keys" (by Halderman et. al) paper was
> published [2].

Seems like a particularly wasteful use of a pageflag. Why not simply
erase the buffer before freeing in those few places where we know its
important (ie. exactly those places you now put the pageflag in)?