On 1/27/21 1:25 PM, Yu-cheng Yu wrote:

arch_prctl(ARCH_X86_CET_STATUS, u64 *args)
    Get CET feature status.

    The parameter 'args' is a pointer to a user buffer.  The kernel returns
    the following information:

    *args = shadow stack/IBT status
    *(args + 1) = shadow stack base address
    *(args + 2) = shadow stack size

What's the deal for 32-bit binaries?  The in-kernel code looks 64-bit
only, but I don't see anything restricting the interface to 64-bit.

+static int copy_status_to_user(struct cet_status *cet, u64 arg2)

This has static scope, but it's still awfully generically named.  A cet_
prefix would be nice.

+{
+	u64 buf[3] = {0, 0, 0};
+
+	if (cet->shstk_size) {
+		buf[0] |= GNU_PROPERTY_X86_FEATURE_1_SHSTK;
+		buf[1] = (u64)cet->shstk_base;
+		buf[2] = (u64)cet->shstk_size;

What's the casting for?

+	}
+
+	return copy_to_user((u64 __user *)arg2, buf, sizeof(buf));
+}
+
+int prctl_cet(int option, u64 arg2)
+{
+	struct cet_status *cet;
+	unsigned int features;
+
+	/*
+	 * GLIBC's ENOTSUPP == EOPNOTSUPP == 95, and it does not recognize
+	 * the kernel's ENOTSUPP (524).  So return EOPNOTSUPP here.
+	 */
+	if (!IS_ENABLED(CONFIG_X86_CET))
+		return -EOPNOTSUPP;

Let's ignore glibc for a moment.  What error code *should* the kernel be
returning here?  errno(3) says:

       EOPNOTSUPP      Operation not supported on socket (POSIX.1)
...
       ENOTSUP         Operation not supported (POSIX.1)

+	cet = &current->thread.cet;
+
+	if (option == ARCH_X86_CET_STATUS)
+		return copy_status_to_user(cet, arg2);

What's the point of doing copy_status_to_user() if the processor doesn't
support CET?  In other words, shouldn't this be below the CPU feature check?

Also, please cast arg2 *here*.  It becomes a user pointer here, not at
the copy_to_user().

+	if (!static_cpu_has(X86_FEATURE_CET))
+		return -EOPNOTSUPP;

So, you went to the trouble of adding a disabled-features.h entry for
this.  Why not just do:

	if (cpu_feature_enabled(X86_FEATURE_CET))
		...

instead of the IS_ENABLED() check above?  That should get rid of one of
these if's.

+	switch (option) {
+	case ARCH_X86_CET_DISABLE:
+		if (cet->locked)
+			return -EPERM;
+
+		features = (unsigned int)arg2;

What's the purpose of this cast?

+		if (features & ~GNU_PROPERTY_X86_FEATURE_1_VALID)
+			return -EINVAL;
+		if (features & GNU_PROPERTY_X86_FEATURE_1_SHSTK)
+			cet_disable_shstk();
+		return 0;

This doesn't enforce that the high bits of arg2 be 0.  Shouldn't we call
them reserved and enforce that they be 0?

+	case ARCH_X86_CET_LOCK:
+		cet->locked = 1;
+		return 0;

This needs to check for and enforce that arg2==0.

+	default:
+		return -ENOSYS;
+	}
+}