Re: [RFC PATCH v9 25/27] x86/cet/shstk: Handle thread Shadow Stack

On Tue, 2020-02-25 at 13:29 -0800, Kees Cook wrote:

On Wed, Feb 05, 2020 at 10:19:33AM -0800, Yu-cheng Yu wrote:

quoted

[...]
A 64-bit SHSTK has a fixed size of RLIMIT_STACK. A compat-mode thread SHSTK
has a fixed size of 1/4 RLIMIT_STACK.  This allows more threads to share a
32-bit address space.

I am not understanding this part. :) Entries are sizeof(unsigned long),
yes? A 1/2 RLIMIT_STACK would cover 32-bit, but 1/4 is less, so why does
that provide for more threads?

Each thread has a separate shadow stack.  If each shadow stack is smaller, the
address space can accommodate more shadow stack allocations.

quoted

[...]

diff --git a/arch/x86/kernel/cet.c b/arch/x86/kernel/cet.c
index cba5c7656aab..5b45abda80a1 100644
--- a/arch/x86/kernel/cet.c
+++ b/arch/x86/kernel/cet.c

@@ -170,6 +170,47 @@ int cet_setup_shstk(void)
 	return 0;
 }
 
+int cet_setup_thread_shstk(struct task_struct *tsk)
+{
+	unsigned long addr, size;
+	struct cet_user_state *state;
+	struct cet_status *cet = &tsk->thread.cet;
+
+	if (!cet->shstk_enabled)
+		return 0;
+
+	state = get_xsave_addr(&tsk->thread.fpu.state.xsave,
+			       XFEATURE_CET_USER);
+
+	if (!state)
+		return -EINVAL;
+
+	size = rlimit(RLIMIT_STACK);

Is SHSTK incompatible with RLIM_INFINITY stack rlimits?

I will change it to:

	size = min(rlimit(RLIMIT_STACK), 4 GB);

quoted

+
+	/*
+	 * Compat-mode pthreads share a limited address space.
+	 * If each function call takes an average of four slots
+	 * stack space, we need 1/4 of stack size for shadow stack.
+	 */
+	if (in_compat_syscall())
+		size /= 4;
+
+	addr = alloc_shstk(size);

I assume it'd fail here, but I worry about Stack Clash style attacks.
I'd like to see test cases that make sure the SHSTK gap is working
correctly.

I will work on some tests.

Thanks,
Yu-cheng