On 5/10/2021 7:15 AM, Borislav Petkov wrote:

On Tue, Apr 27, 2021 at 01:43:08PM -0700, Yu-cheng Yu wrote:

[...]

quoted

diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
index c815c7507830..d387df84b7f1 100644
--- a/arch/x86/kernel/shstk.c
+++ b/arch/x86/kernel/shstk.c

@@ -70,6 +70,55 @@ int shstk_setup(void)
  	return 0;
  }

quoted

+int shstk_setup_thread(struct task_struct *tsk, unsigned long clone_flags,

Judging by what this function does, its name wants to be

shstk_alloc_thread_stack()

or so?

quoted

+		       unsigned long stack_size)
+{
+	unsigned long addr, size;
+	struct cet_user_state *state;
+	struct cet_status *cet = &tsk->thread.cet;

The tip-tree preferred ordering of variable declarations at the
beginning of a function is reverse fir tree order::

	struct long_struct_name *descriptive_name;
	unsigned long foo, bar;
	unsigned int tmp;
	int ret;

The above is faster to parse than the reverse ordering::

	int ret;
	unsigned int tmp;
	unsigned long foo, bar;
	struct long_struct_name *descriptive_name;

And even more so than random ordering::

	unsigned long foo, bar;
	int ret;
	struct long_struct_name *descriptive_name;
	unsigned int tmp;

quoted

+
+	if (!cet->shstk_size)
+		return 0;
+

This check needs a comment.

quoted

+	if ((clone_flags & (CLONE_VFORK | CLONE_VM)) != CLONE_VM)
+		return 0;
+
+	state = get_xsave_addr(&tsk->thread.fpu.state.xsave,
+			       XFEATURE_CET_USER);

Let that line stick out.

quoted

+
+	if (!state)
+		return -EINVAL;
+
+	if (stack_size == 0)

	if (!stack_size)

quoted

+		return -EINVAL;

and that test needs to be done first in the function.

quoted

+
+	/* Cap shadow stack size to 4 GB */

Why?

This is not necessary.  I will make it just stack_size, which is passed 
in from copy_thread().

quoted

+	size = min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G);
+	size = min(size, stack_size);
+
+	/*
+	 * Compat-mode pthreads share a limited address space.
+	 * If each function call takes an average of four slots
+	 * stack space, allocate 1/4 of stack size for shadow stack.
+	 */
+	if (in_compat_syscall())
+		size /= 4;

<---- newline here.

quoted

+	size = round_up(size, PAGE_SIZE);
+	addr = alloc_shstk(size);
+

^ Superfluous newline.

quoted

+	if (IS_ERR_VALUE(addr)) {
+		cet->shstk_base = 0;
+		cet->shstk_size = 0;
+		return PTR_ERR((void *)addr);
+	}
+
+	fpu__prepare_write(&tsk->thread.fpu);
+	state->user_ssp = (u64)(addr + size);

cet_user_state has u64, cet_status has unsigned longs. Make them all u64.

And since cet_status is per thread, but I had suggested struct
shstk_desc, I think now that that should be called

struct thread_shstk

or so to denote *exactly* what it is.

So this struct will be:

struct thread_shstk {
	u64 shstk_base;
	u64 shstk_size;
	u64 locked:1;
	u64 ibt:1;
};

Ok?

Thanks,
Yu-cheng