Re: [PATCH v2 for-8.0] target/s390x/tcg: Fix and improve the SACF instruction

[Thomas Huth <thuth@redhat.com>]

On 01/12/2022 21.51, Richard Henderson wrote:
> On 12/1/22 10:44, Thomas Huth wrote:
>> The SET ADDRESS SPACE CONTROL FAST instruction is not privileged, it can be
>> used from problem space, too. Just the switching to the home address space
>> is privileged and should still generate a privilege exception. This bug is
>> e.g. causing programs like Java that use the "getcpu" vdso kernel function
>> to crash (see https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=990417#26 ).
>>
>> While we're at it, also check if DAT is not enabled. In that case the
>> instruction is supposed to generate a special operation exception.
>>
>> Resolves: https://gitlab.com/qemu-project/qemu/-/issues/655
>> Signed-off-by: Thomas Huth <thuth@redhat.com>
>> ---
> 
> Looks ok, as far as it goes.  We appear to be missing the check for 
> CR0_SECONDARY, which is unpredictable for SACF but mandatory for SAC.

Yes, but if I got our sources right, we do not implement SAC yet. Looks like 
Linux does not use it, so nobody bothered to implement it yet. Since it 
should be very similar to SACF, it should be easy to add - I can try to come 
up with an additional patch for it later.

> I'll give you
> 
> Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
> 
> for fixing the incorrect IF_PRIV check, which by itself should be enough to 
> fix the Java issue.

Thanks!

  Thomas


PS: We might have a similar bug with the MVCP and MVCS instructions ... 
seems like they could be called from userspace in certain situations, too.