On Sun, Mar 28, 2021 at 04:18:56PM +0200, Thomas Gleixner wrote:

On Fri, Mar 19 2021 at 14:28, Kees Cook wrote:

quoted

+
+	/*
+	 * 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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