Re: [PATCH] x86: guard against port I/O overlapping the RTC/CMOS range

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 17/07/2020 14:10, Jan Beulich wrote:
> Since we intercept RTC/CMOS port accesses, let's do so consistently in
> all cases, i.e. also for e.g. a dword access to [006E,0071]. To avoid
> the risk of unintended impact on Dom0 code actually doing so (despite
> the belief that none ought to exist), also extend
> guest_io_{read,write}() to decompose accesses where some ports are
> allowed to be directly accessed and some aren't.
>
> Signed-off-by: Jan Beulich <jbeulich@suse.com>
>
> --- a/xen/arch/x86/pv/emul-priv-op.c
> +++ b/xen/arch/x86/pv/emul-priv-op.c
> @@ -210,7 +210,7 @@ static bool admin_io_okay(unsigned int p
>          return false;
>  
>      /* We also never permit direct access to the RTC/CMOS registers. */
> -    if ( ((port & ~1) == RTC_PORT(0)) )
> +    if ( port <= RTC_PORT(1) && port + bytes > RTC_PORT(0) )
>          return false;

This first hunk is fine.

However, why decompose anything?  Any disallowed port in the range
terminates the entire access, and doesn't internally shrink the access.

~Andrew