Re: [PATCH v11 03/12] scs: add support for stack usage debugging

Will Deacon
2020-04-20T17:17:58Z

On Thu, Apr 16, 2020 at 09:12:36AM -0700, Sami Tolvanen wrote:
> Implements CONFIG_DEBUG_STACK_USAGE for shadow stacks. When enabled,
> also prints out the highest shadow stack usage per process.
>
> Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
> Reviewed-by: Kees Cook <keescook@chromium.org>
> ---
> kernel/scs.c | 39 +++++++++++++++++++++++++++++++++++++++
> 1 file changed, 39 insertions(+)
>
> diff --git a/kernel/scs.c b/kernel/scs.c
> index 5245e992c692..ad74d13f2c0f 100644
> --- a/kernel/scs.c
> +++ b/kernel/scs.c
> @@ -184,6 +184,44 @@ int scs_prepare(struct task_struct *tsk, int node)
> return 0;
> }
>
> +#ifdef CONFIG_DEBUG_STACK_USAGE
> +static inline unsigned long scs_used(struct task_struct *tsk)
> +{
> + unsigned long *p = __scs_base(tsk);
> + unsigned long *end = scs_magic(p);
> + unsigned long s = (unsigned long)p;
> +
> + while (p < end && READ_ONCE_NOCHECK(*p))
> + p++;

I think the expectation is that the caller has already checked that the
stack is not corrupted, so I'd probably throw a couple of underscores
in front of the function name, along with a comment.

Also, is tsk ever != current?

> +
> + return (unsigned long)p - s;
> +}
> +
> +static void scs_check_usage(struct task_struct *tsk)
> +{
> + static DEFINE_SPINLOCK(lock);
> + static unsigned long highest;
> + unsigned long used = scs_used(tsk);
> +
> + if (used <= highest)
> + return;
> +
> + spin_lock(&lock);
> +
> + if (used > highest) {
> + pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
> + tsk->comm, task_pid_nr(tsk), used);
> + highest = used;
> + }
> +
> + spin_unlock(&lock);

Do you really need this lock? I'd have thought you could cmpxchg()
highest instead.

Will