Re: [PATCH] subdom: Fix -Werror=address failure in tmp_emulator

[George Dunlap <george.dunlap@xxxxxxxxx>]

On Tue, Aug 8, 2023 at 10:27 PM Daniel P. Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx> wrote:

On 8/3/23 16:36, Andrew Cooper wrote:
> The opensuse-tumbleweed build jobs currently fail with:
>
>    /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c: In function 'rsa_private':
>    /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c:56:7: error: the comparison will always evaluate as 'true' for the address of 'p' will never be NULL [-Werror=address]
>       56 |   if (!key->p || !key->q || !key->u) {
>          |       ^
>    In file included from /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.c:17:
>    /builds/xen-project/xen/stubdom/tpm_emulator-x86_64/crypto/rsa.h:28:12: note: 'p' declared here
>       28 |   tpm_bn_t p;
>          |            ^
>
> This is because all tpm_bn_t's are 1-element arrays (of either a GMP or
> OpenSSL BIGNUM flavour).  The author was probably meaning to do value checks,
> but that's not what the code does.
>
> Adjust it to compile.  No functional change.
>
> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> ---
> CC: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
> CC: Jan Beulich <JBeulich@xxxxxxxx>
> CC: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> CC: Wei Liu <wl@xxxxxxx>
> CC: Julien Grall <julien@xxxxxxx>
> CC: Juergen Gross <jgross@xxxxxxxx>
> CC: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> CC: Jason Andryuk <jandryuk@xxxxxxxxx>
> CC: Daniel Smith <dpsmith@xxxxxxxxxxxxxxxxxxxx>
> CC: Christopher Clark <christopher.w.clark@xxxxxxxxx>
>
> While I've confirmed this to fix the build issue:
>
>    https://gitlab.com/xen-project/people/andyhhp/xen/-/pipelines/955160430
>
> I'm -1 overall to the change, and would prefer to disable vtpm-stubdom
> entirely.
>
> It's TPM 1.2 only, using decades-old libs, and some stuff in the upstream
> https://github.com/PeterHuewe/tpm-emulator (which is still abandaonded as of
> 2018) is just as concerning as the basic error here in rsa_private().

For semantics sake, the Guest PV interface is 1.2 compliant but the PV
backend, vtpmmgr, is capable of using TPM2.0.

> vtpm-stubdom isn't credibly component of a Xen system, and we're wasting loads
> of CI cycles testing it...

Unfortunately, I cannot disagree here. This is the only proper vTPM,
from a trustworthy architecture perspective, that I know of existing
today. Until I can find someone willing to fund updating the
implementation and moving it to being an emulated vTPM and not a PV
interface, it is likely to stay in this state for some time.

Did you mean "I cannot *agree* here"?  "Cannot disagree" means you agree that we're "wasting loads of CI cycles testing it", but the second sentence seems to imply that it's (currently) irreplacable.

 -George