[PATCH] powerpc/rtas: Prevent Spectre v1 gadget construction in sys_rtas()

[Nathan Lynch]

Breno Leitao <leitao at debian.org> writes:

> On Thu, May 30, 2024 at 07:44:12PM -0500, Nathan Lynch via B4 Relay wrote:
>> From: Nathan Lynch <nathanl at linux.ibm.com>
>> 
>> Smatch warns:
>> 
>>   arch/powerpc/kernel/rtas.c:1932 __do_sys_rtas() warn: potential
>>   spectre issue 'args.args' [r] (local cap)
>> 
>> The 'nargs' and 'nret' locals come directly from a user-supplied
>> buffer and are used as indexes into a small stack-based array and as
>> inputs to copy_to_user() after they are subject to bounds checks.
>> 
>> Use array_index_nospec() after the bounds checks to clamp these values
>> for speculative execution.
>> 
>> Signed-off-by: Nathan Lynch <nathanl at linux.ibm.com>
>> Reported-by: Breno Leitao <leitao at debian.org>
>
> Thanks for working on it. 
>
> Reviewed-by: Breno Leitao <leitao at debian.org>

Thanks!

>
>> +	nargs = array_index_nospec(nargs, ARRAY_SIZE(args.args));
>> +	nret = array_index_nospec(nret, ARRAY_SIZE(args.args) - nargs);
>
> On an unrelated note, can nargs and nret are integers and could be
> eventually negative. Is this a valid use case?

No, it's not valid for a caller to provide negative nargs or nret. I
convinced myself that this bounds check:

	nargs = be32_to_cpu(args.nargs);
	nret  = be32_to_cpu(args.nret);

	if (nargs >= ARRAY_SIZE(args.args)
	    || nret > ARRAY_SIZE(args.args)
	    || nargs + nret > ARRAY_SIZE(args.args))
		return -EINVAL;

rejects negative values of nargs or nret due to C's "usual arithmetic
conversions", where nargs and nret are implicitly converted to size_t
for the comparisons.

However I don't see any value in keeping them as signed int. I have some
changes in progress in this area and I'll plan on making these unsigned.