linux-kernel - Re: [PATCH v12 43/46] virt: Add SEV-SNP guest driver

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Date:   Wed, 24 Aug 2022 11:01:16 -0700
From:   Dionna Amalie Glaze <dionnaglaze@...gle.com>
To:     Brijesh Singh <brijesh.singh@....com>
Cc:     "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "open list:X86 KVM CPUs" <kvm@...r.kernel.org>,
        linux-efi <linux-efi@...r.kernel.org>,
        platform-driver-x86@...r.kernel.org, linux-coco@...ts.linux.dev,
        Linux Memory Management List <linux-mm@...ck.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Joerg Roedel <jroedel@...e.de>,
        Tom Lendacky <thomas.lendacky@....com>,
        "H. Peter Anvin" <hpa@...or.com>, Ard Biesheuvel <ardb@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Sergio Lopez <slp@...hat.com>, Peter Gonda <pgonda@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        David Rientjes <rientjes@...gle.com>,
        Dov Murik <dovmurik@...ux.ibm.com>,
        Tobin Feldman-Fitzthum <tobin@....com>,
        Borislav Petkov <bp@...en8.de>,
        Michael Roth <michael.roth@....com>,
        Vlastimil Babka <vbabka@...e.cz>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        brijesh.ksingh@...il.com, tony.luck@...el.com,
        Marc Orr <marcorr@...gle.com>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>
Subject: Re: [PATCH v12 43/46] virt: Add SEV-SNP guest driver

Apologies for the necropost, but I noticed strange behavior testing my
own Golang-based wrapper around the /dev/sev-guest driver.

> +
> +static int handle_guest_request(struct snp_guest_dev *snp_dev, u64 exit_code, int msg_ver,
> +                               u8 type, void *req_buf, size_t req_sz, void *resp_buf,
> +                               u32 resp_sz, __u64 *fw_err)
> +{
> +       unsigned long err;
> +       u64 seqno;
> +       int rc;
> +
> +       /* Get message sequence and verify that its a non-zero */
> +       seqno = snp_get_msg_seqno(snp_dev);
> +       if (!seqno)
> +               return -EIO;
> +
> +       memset(snp_dev->response, 0, sizeof(struct snp_guest_msg));
> +
> +       /* Encrypt the userspace provided payload */
> +       rc = enc_payload(snp_dev, seqno, msg_ver, type, req_buf, req_sz);
> +       if (rc)
> +               return rc;
> +
> +       /* Call firmware to process the request */
> +       rc = snp_issue_guest_request(exit_code, &snp_dev->input, &err);
> +       if (fw_err)
> +               *fw_err = err;
> +
> +       if (rc)
> +               return rc;
> +

The fw_err is written back regardless of rc, so since err is
uninitialized, you can end up with garbage written back. I've worked
around this by only caring about fw_err when the result is -EIO, but
thought that I should bring this up.

--
-Dionna Glaze, PhD (she/her)