Subject: Re: Coverity: kvm_hv_flush_tlb(): Uninitialized variables

From: Vitaly Kuznetsov <[email-masked]>
Date: Thu, 01 Dec 2022 17:45:14 +0100

coverity-bot <[email-masked]> writes:

> Hello!
>
> This is an experimental semi-automated report about issues detected by
> Coverity from a scan of next-20221201 as part of the linux-next scan project:
> https://scan.coverity.com/projects/linux-next-weekly-scan
>
> You're getting this email because you were associated with the identified
> lines of code (noted below) that were touched by commits:
>
>   Fri Nov 18 12:59:05 2022 -0500
>     260970862c88 ("KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently")
>
> Coverity reported the following:
>
> *** CID 1527764:  Uninitialized variables  (UNINIT)
> arch/x86/kvm/hyperv.c:2024 in kvm_hv_flush_tlb()
> 2018     		 * Hyper-V TLFS doesn't explicitly forbid non-empty sparse vCPU
> 2019     		 * banks (and, thus, non-zero 'var_cnt') for the 'all vCPUs'
> 2020     		 * case (HV_GENERIC_SET_ALL).  Always adjust data_offset and
> 2021     		 * consumed_xmm_halves to make sure TLB flush entries are read
> 2022     		 * from the correct offset.
> 2023     		 */
> vvv     CID 1527764:  Uninitialized variables  (UNINIT)
> vvv     Using uninitialized value "data_offset".
> 2024     		data_offset += hc->var_cnt * sizeof(sparse_banks[0]);
> 2025     		consumed_xmm_halves += hc->var_cnt;
> 2026     	}
> 2027
> 2028     	if (hc->code == HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE ||
> 2029     	    hc->code == HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE_EX ||
>
> If this is a false positive, please let us know so we can mark it as
> such, or teach the Coverity rules to be smarter. If not, please make
> sure fixes get into linux-next. :) For patches fixing this, please
> include these lines (but double-check the "Fixes" first):

Looks half legit) 'data_offset' can only be uninitialized here in
'hc->fast' case but this also means that the only user of 'data_offset'
(kvm_hv_get_tlb_flush_entries()) below will not use it. So yes, we do
increment an uninitialized variable but we won't use the result. I think
we're better off fixing this, even if there's no real issue. I'll send a
patch, hopefully tomorrow.

Thanks!

>
> Reported-by: coverity-bot <[email-masked]>
> Addresses-Coverity-ID: 1527764 ("Uninitialized variables")
> Fixes: 260970862c88 ("KVM: x86: hyper-v: Handle HVCALL_FLUSH_VIRTUAL_ADDRESS_LIST{,EX} calls gently")
>
> Thanks for your attention!

-- 
Vitaly