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

Dave Hansen
2021-01-29T17:09:16Z

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;
> + }
> +}