Messages in this thread | | | Date | Mon, 29 Mar 2021 11:43:23 -0700 | From | Kees Cook <> | Subject | Re: [PATCH v7 4/6] x86/entry: Enable random_kstack_offset support |
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On Sun, Mar 28, 2021 at 04:18:56PM +0200, Thomas Gleixner wrote: > On Fri, Mar 19 2021 at 14:28, Kees Cook wrote: > > + > > + /* > > + * x86_64 stack alignment means 3 bits are ignored, so keep > > + * the top 5 bits. x86_32 needs only 2 bits of alignment, so > > + * the top 6 bits will be used. > > + */ > > + choose_random_kstack_offset(rdtsc() & 0xFF); > > Comment mumbles about 5/6 bits and the TSC value is masked with 0xFF and > then the applied offset is itself limited with 0x3FF. > > Too many moving parts for someone who does not have the details of all > this memorized.
Each piece is intentional -- I will improve the comments to explain each level of masking happening (implicit compiler stack alignment mask, explicit per-arch mask, and the VLA upper-bound protection mask).
-- Kees Cook
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