Re: [PATCH v18 24/25] x86/cet/shstk: Add arch_prctl functions for shadow stack

[Dave Hansen]

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;
> +	}
> +}