Re: [RFC PATCH for 4.18] rseq: use __u64 for rseq_cs fields, validate user inputs

From: Linus Torvalds
Date: Mon Jul 02 2018 - 18:45:18 EST

On Mon, Jul 2, 2018 at 3:31 PM Mathieu Desnoyers
<mathieu.desnoyers@xxxxxxxxxxxx> wrote:
> Change the rseq ABI so rseq_cs start_ip, post_commit_offset and abort_ip
> fields are seen as 64-bit fields by both 32-bit and 64-bit kernels rather
> that ignoring the 32 upper bits on 32-bit kernels. This ensures we have a
> consistent behavior for a 32-bit binary executed on 32-bit kernels and in
> compat mode on 64-bit kernels.

Actually, now that I see this again, I react to:

> +static int check_rseq_cs_padding(struct task_struct *t)
> +{
> + u32 pad;
> + int ret;
> +
> + ret = __get_user(pad, &t->rseq->rseq_cs_padding);
> + if (ret)
> + return ret;
> + if (pad)
> + return -EINVAL;
> + return 0;
> +}

This is all wrong.

Just make "rseq_cs" be an __u64" too. That will clean up everything,
and user space will have a much easier time filling it in too, since
it's just one field. Instead of having to remember about the "let's
fill in padding for 32-bit cases".

Then the rseq_get_rseq_cs() will be

__u64 rseq_cs;

ret = get_user(rseq_cs, &t->rseq->rseq_cs);
if (ret)
return ret;
ptr = (void *)rseq_cs;
if (rseq_cs != (unsigned long)ptr)
return -EINVAL;

and it's all good, no #ifdef's etc needed.


Sorry for the bike-shedding, but this is now the last remaining user
of that LINUX_FIELD_u32_u64, so let's just get rid of it entirely, ok?

Then we can also get rid of that silly uapi/linux/types_32_64.h header
file entirely.

That would be *lovely*. Simpler code, simpler and less error-prone
interfaces, and one less specialized header file.