Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

----- On Jul 2, 2018, at 10:32 AM, Mathieu Desnoyers mathieu.desnoyers@efficios.com wrote:
[...]
> 
> But rather than trying to play games with input validation, I would
> favor an approach that would allow rseq to validate all its inputs
> straightforwardly. Introducing user_64bit_mode(struct pt_regs *)
> across all architectures would allow doing just that. rseq signal
> delivery and return to usermode code could then ensure that high bits are
> cleared by 32-bit tasks for all fields and thus provide a consistent
> behavior for 32-bit tasks running on 32-bit and 64-bit kernels.

AFAIU this could be achieved by re-introducing is_compat_task() on x86 as:

#ifdef CONFIG_COMPAT
static bool is_compat_task(void)
{
        return user_64bit_mode(current_pt_regs()));
}
#else
static bool is_compat_task(void) { return false; };
#endif

Or am I missing something ?

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com