Re: [PATCH v3 25/37] x86/shstk: Add user-mode shadow stack support

On Fri, Nov 04, 2022 at 03:35:52PM -0700, Rick Edgecombe wrote:

+static int shstk_setup(void)
+{
+	struct thread_shstk *shstk = &current->thread.shstk;
+	unsigned long addr, size;
+
+	/* Already enabled */
+	if (features_enabled(CET_SHSTK))
+		return 0;
+
+	/* Also not supported for 32 bit and x32 */
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || in_32bit_syscall())
+		return -EOPNOTSUPP;
+
+	size = adjust_shstk_size(0);
+	addr = alloc_shstk(size);
+	if (IS_ERR_VALUE(addr))
+		return PTR_ERR((void *)addr);
+
+	fpregs_lock_and_load();
+	wrmsrl(MSR_IA32_PL3_SSP, addr + size);
+	wrmsrl(MSR_IA32_U_CET, CET_SHSTK_EN);

This..

+	fpregs_unlock();
+
+	shstk->base = addr;
+	shstk->size = size;
+	features_set(CET_SHSTK);
+
+	return 0;
+}

+static int shstk_disable(void)
+{
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK))
+		return -EOPNOTSUPP;
+
+	/* Already disabled? */
+	if (!features_enabled(CET_SHSTK))
+		return 0;
+
+	fpregs_lock_and_load();
+	/* Disable WRSS too when disabling shadow stack */
+	set_clr_bits_msrl(MSR_IA32_U_CET, 0, CET_SHSTK_EN);

And this... aren't very consistent in approach. Given there is no U_IBT
yet, why complicate matters like this?

+	wrmsrl(MSR_IA32_PL3_SSP, 0);
+	fpregs_unlock();
+
+	shstk_free(current);
+	features_clr(CET_SHSTK);
+
+	return 0;
+}