Re: arch/s390/kvm/gaccess.c:859 guest_range_to_gpas() error: uninitialized symbol 'prot'.

[Janis Schoetterl-Glausch <scgl@linux.ibm.com>]

On Thu, 2022-08-18 at 09:45 +0200, Janosch Frank wrote:
> On 8/17/22 16:38, Claudio Imbrenda wrote:
> > On Wed, 17 Aug 2022 12:30:11 +0200
> > Janis Schoetterl-Glausch <scgl@linux.ibm.com> wrote:
> > 
> > > On 8/15/22 09:23, Dan Carpenter wrote:
> > > > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
> > > > head:   69dac8e431af26173ca0a1ebc87054e01c585bcc
> > > > commit: 7faa543df19bf62d4583a64d3902705747f2ad29 KVM: s390: gaccess: Refactor access address range check
> > > > config: s390-randconfig-m031-20220812 (https://download.01.org/0day-ci/archive/20220814/202208140141.4uzWszG4-lkp@intel.com/config)
> > > > compiler: s390-linux-gcc (GCC) 12.1.0
> > > > 
> > > > If you fix the issue, kindly add following tag where applicable
> > > > Reported-by: kernel test robot <lkp@intel.com>
> > > > Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
> > > > 
> > > > smatch warnings:
> > > > arch/s390/kvm/gaccess.c:859 guest_range_to_gpas() error: uninitialized symbol 'prot'.
> > > > 
> > > > vim +/prot +859 arch/s390/kvm/gaccess.c
> > > > 
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  831  static int guest_range_to_gpas(struct kvm_vcpu *vcpu, unsigned long ga, u8 ar,
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  832  			       unsigned long *gpas, unsigned long len,
> > > > 92c9632119b67f David Hildenbrand        2015-11-16  833  			       const union asce asce, enum gacc_mode mode)
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  834  {
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  835  	psw_t *psw = &vcpu->arch.sie_block->gpsw;
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  836  	unsigned int offset = offset_in_page(ga);
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  837  	unsigned int fragment_len;
> > > > cde0dcfb5df1db David Hildenbrand        2016-05-31  838  	int lap_enabled, rc = 0;
> > > > 6ae1574c2a24ee Christian Borntraeger    2017-06-07  839  	enum prot_type prot;
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  840  	unsigned long gpa;
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  841
> > > > 75a1812230ad7a Alexander Yarygin        2015-01-22  842  	lap_enabled = low_address_protection_enabled(vcpu, asce);
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  843  	while (min(PAGE_SIZE - offset, len) > 0) {
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  844  		fragment_len = min(PAGE_SIZE - offset, len);
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  845  		ga = kvm_s390_logical_to_effective(vcpu, ga);
> > > > cde0dcfb5df1db David Hildenbrand        2016-05-31  846  		if (mode == GACC_STORE && lap_enabled && is_low_address(ga))
> > > > cde0dcfb5df1db David Hildenbrand        2016-05-31  847  			return trans_exc(vcpu, PGM_PROTECTION, ga, ar, mode,
> > > > cde0dcfb5df1db David Hildenbrand        2016-05-31  848  					 PROT_TYPE_LA);
> > > > a752598254016d Heiko Carstens           2017-06-03  849  		if (psw_bits(*psw).dat) {
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  850  			rc = guest_translate(vcpu, ga, &gpa, asce, mode, &prot);
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  851  			if (rc < 0)
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  852  				return rc;
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  853  		} else {
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  854  			gpa = kvm_s390_real_to_abs(vcpu, ga);
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  855  			if (kvm_is_error_gpa(vcpu->kvm, gpa))
> > > > cde0dcfb5df1db David Hildenbrand        2016-05-31  856  				rc = PGM_ADDRESSING;
> > > >                                                                                          ^^^^^^^^^^^^^^^^^^^^
> > > > 
> > > > "rc" set but "prot" not initialized.
> > > 
> > > prot is only used in case of PGM_PROTECTION.
> > > Because of that, we could initialize prot to an arbitrary value, but that seems misleading and a bit ugly to me.
> > > Alternatively, we could introduce a PROT_NONE.
> > > Or we do nothing, since there is no actual problem.
> > > @Janosch, @Claudio, what do you think?
> > 
> > I agree that this is not a bug.
> > 
> > It does look ugly, though, and all reasonable solutions are also ugly
> > (each for a different reason).
> > 
> > another solution is to set prot to an arbitrary value only in the if
> > case marked above. that way prot is not arbitrarily initialized, and
> > there is no need to add a new PROT_NONE (which then would need to be
> > checked for in trans_exc)
> > 
> > I do not have a strong opinion, though
> 
> We either have a prot value or terminate is set, right?

It's independent of termination, we will have a valid value for prot if
we terminate. And I guess it still would be undefined behavior even if
the value were invalid and we terminate.
> 
> So I'd opt to add a PROT_NONE (with a comment that it's only used for 
> init). If the statement above is true we can then also add a WARN if no 
> valid PROT_* is set in case if terminate isn't true and a PGM_PROTECTION 
> has been specified.

I think what I'll do is introduce PROT_NONE, but not initialize prot
with it, but set prot to NONE whenever a code other than PGM_PROTECTION
is set.
> 
> 
> > 
> > > 
> > > > 
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  857  		}
> > > > cde0dcfb5df1db David Hildenbrand        2016-05-31  858  		if (rc)
> > > > 6ae1574c2a24ee Christian Borntraeger    2017-06-07 @859  			return trans_exc(vcpu, rc, ga, ar, mode, prot);
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  860  		if (gpas)
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  861  			*gpas++ = gpa;
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  862  		offset = 0;
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  863  		ga += fragment_len;
> > > > 7faa543df19bf6 Janis Schoetterl-Glausch 2021-11-26  864  		len -= fragment_len;
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  865  	}
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  866  	return 0;
> > > > 2293897805c2fe Heiko Carstens           2014-01-01  867  }
> > > >    
> > > 
> > 
>