Date: Tue, 22 Dec 2015 11:32:56 -0800 From: Dave Hansen <dave.hansen@...el.com> To: Laura Abbott <laura@...bott.name>, Christoph Lameter <cl@...ux.com> Cc: kernel-hardening@...ts.openwall.com, Pekka Enberg <penberg@...nel.org>, David Rientjes <rientjes@...gle.com>, Joonsoo Kim <iamjoonsoo.kim@....com>, Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org, linux-kernel@...r.kernel.org, Kees Cook <keescook@...omium.org> Subject: Re: [RFC][PATCH 6/7] mm: Add Kconfig option for slab sanitization On 12/22/2015 11:13 AM, Laura Abbott wrote: >> 3. Zero at free, *don't* Zero at alloc (when __GFP_ZERO) >> (what I'm suggesting, possibly less perf impact vs. #2) > > poisoning with non-zero memory makes it easier to determine that the error > came from accessing the sanitized memory vs. some other case. I don't think > the feature would be as strong if the memory was only zeroed vs. some other > data value. How does that scenario work? Your patch description says: > + Use-after-free bugs for structures containing > + pointers can also be detected as dereferencing the sanitized pointer > + will generate an access violation. In the case that we wrote all zeros, we'd be accessing userspace at a known place that we don't generally allow memory to be mapped anyway. Could you elaborate on a scenario where zeros are weaker than a random poison value? In any case (if a poison value is superior to 0's), it's a balance between performance vs. the likelihood of the poisoned value being tripped over. I think the performance impact of this feature is going to be *the* major thing that keeps folks from using it in practice. I'm trying to suggest a way that you _might_ preserve some performance, and get more folks to use it. 1. Keep information from leaking (doesn't matter which value we write) 2. Detect use-after-free bugs (0's are less likely to be detected???) 3. Preserve performance (0's are likely to preserve more performance)
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