Re: [PATCH] x86/dumpstack: Dump user space code correctly again

[Thomas Gleixner]

Christoph Hellwig <hch@xxxxxx> writes:
> On Wed, Jul 22, 2020 at 07:54:15PM +0200, Thomas Gleixner wrote:
>> +	/*
>> +	 * Make sure userspace isn't trying to trick us into dumping kernel
>> +	 * memory by pointing the userspace instruction pointer at it.
>> +	 */
>> +	if (__chk_range_not_ok(src, nbytes, TASK_SIZE_MAX))
>> +		return -EINVAL;
>> +
>> +	return copy_from_user_nmi(buf, (void __user *)src, nbytes);
>
> copy_from_user_nmi already contains a:
>
> 	if (__range_not_ok(from, n, TASK_SIZE))
> 		return n;
>
> what is the reason it checks for TASK_SIZE vs TASK_SIZE_MAX, and why
> do we need both checks?

TBH, I just kept it because being lazy. But you are right, the check is
redundant and TASK_SIZE_MAX is inaccurate as it does not take compat
tasks into account. I'll rip that out.

Thanks,

        tglx