On Sun, Mar 28, 2021 at 04:42:03PM +0200, Thomas Gleixner wrote:

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

quoted

+/*
+ * Do not use this anywhere else in the kernel. This is used here because
+ * it provides an arch-agnostic way to grow the stack with correct
+ * alignment. Also, since this use is being explicitly masked to a max of
+ * 10 bits, stack-clash style attacks are unlikely. For more details see
+ * "VLAs" in Documentation/process/deprecated.rst

VLAs are bad, VLAs to the rescue! :)

I'm aware of the irony, but luto's idea really makes things easy. As
documented there, though, this has a hard-coded (low) upper bound, so
it's not like "regular" VLA use.

quoted

+ * The asm statement is designed to convince the compiler to keep the
+ * allocation around even after "ptr" goes out of scope.
+ */
+void *__builtin_alloca(size_t size);
+
+#define add_random_kstack_offset() do {					\
+	if (static_branch_maybe(CONFIG_RANDOMIZE_KSTACK_OFFSET_DEFAULT,	\
+				&randomize_kstack_offset)) {		\
+		u32 offset = this_cpu_read(kstack_offset);              \

Not that it matters on x86, but as this has to be called in the
interrupt disabled region of the syscall entry, shouldn't this be a
raw_cpu_read(). The asm-generic version has a preempt_disable/enable
pair around the raw read for native wordsize reads, otherwise a
irqsave/restore pair.

__this_cpu_read() is fine as well, but that has an sanity check before
the raw read when CONFIG_DEBUG_PREEMPT is on, which is harmless but
also pointless in this case.

Probably the same for the counterpart this_cpu_write().

Oh! Excellent point. I think this will make a big difference on arm64. I
will adjust and test.

-- 
Kees Cook

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