On Tue, Mar 30 2021 at 13:57, Kees Cook wrote:

+/*
+ * 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
+ * The asm statement is designed to convince the compiler to keep the
+ * allocation around even after "ptr" goes out of scope.

Nit. That explanation of "ptr" might be better placed right at the
add_random...() macro.

+ */
+void *__builtin_alloca(size_t size);
+/*
+ * Use, at most, 10 bits of entropy. We explicitly cap this to keep the
+ * "VLA" from being unbounded (see above). 10 bits leaves enough room for
+ * per-arch offset masks to reduce entropy (by removing higher bits, since
+ * high entropy may overly constrain usable stack space), and for
+ * compiler/arch-specific stack alignment to remove the lower bits.
+ */
+#define KSTACK_OFFSET_MAX(x)	((x) & 0x3FF)
+
+/*
+ * These macros must be used during syscall entry when interrupts and
+ * preempt are disabled, and after user registers have been stored to
+ * the stack.
+ */
+#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);		\
+		u8 *ptr = __builtin_alloca(KSTACK_OFFSET_MAX(offset));	\
+		asm volatile("" : "=m"(*ptr) :: "memory");		\
+	}								\
+} while (0)

Other than that.

Reviewed-by: Thomas Gleixner <redacted>

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