Messages in this thread | | | Date | Thu, 25 Aug 2022 13:54:26 -0500 | Subject | Re: [PATCH v12 43/46] virt: Add SEV-SNP guest driver | From | Tom Lendacky <> |
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On 8/24/22 14:28, Peter Gonda wrote: > On Wed, Aug 24, 2022 at 12:01 PM Dionna Amalie Glaze > <dionnaglaze@google.com> wrote: >> >> 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. > > I also noticed that we use a u64 in snp_guest_request_ioctl.fw_err and > u32 in sev_issue_cmd.error when these should be errors from the > sev_ret_code enum IIUC.
The reason for the u64 is that the Extended Guest Request can return a firmware error or a hypervisor error. To distinguish between the two, a firmware error is contained in the lower 32-bits, while a hypervisor error is contained in the upper 32-bits (e.g. when not enough contiguous pages of memory have been supplied).
Thanks, Tom
> > We can fix snp_issue_guest_request() to set |fw_err| to zero when it > returns 0 but what should we return to userspace if we encounter an > error that prevents the FW from even being called? In ` crypto: ccp - > Ensure psp_ret is always init'd in __sev_platform_init_locked()` we > set the return to -1 so we could continue that convection here and > better codify it in the sev_ret_code enum. > >> >> -- >> -Dionna Glaze, PhD (she/her)
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