On 1/11/23 00:00, Dov Murik wrote:
On 10/01/2023 17:10, Tom Lendacky wrote:
On 1/10/23 01:10, Dov Murik wrote:
Hi Tom,
On 10/01/2023 0:27, Tom Lendacky wrote:
On 1/9/23 10:55, Dionna Amalie Glaze wrote:
+
+static int snp_set_instance_certs(struct kvm *kvm, struct
kvm_sev_cmd *argp)
+{
[...]
Here we set the length to the page-aligned value, but we copy only
params.cert_len bytes. If there are two subsequent
snp_set_instance_certs() calls where the second one has a shorter
length, we might "keep" some leftover bytes from the first call.
Consider:
1. snp_set_instance_certs(certs_addr point to "AAA...",
certs_len=8192)
2. snp_set_instance_certs(certs_addr point to "BBB...",
certs_len=4097)
If I understand correctly, on the second call we'll copy 4097 "BBB..."
bytes into the to_certs buffer, but length will be (4096 + PAGE_SIZE -
1) & PAGE_MASK which will be 8192.
Later when fetching the certs (for the extended report or in
snp_get_instance_certs()) the user will get a buffer of 8192 bytes
filled with 4097 BBBs and 4095 leftover AAAs.
Maybe zero sev->snp_certs_data entirely before writing to it?
Yes, I agree it should be zeroed, at least if the previous length is
greater than the new length. Good catch.
Related question (not only for this patch) regarding snp_certs_data
(host or per-instance): why is its size page-aligned at all? why is it
limited by 16KB or 20KB? If I understand correctly, for SNP, this
buffer
is never sent to the PSP.
The buffer is meant to be copied into the guest driver following the
GHCB extended guest request protocol. The data to copy back are
expected to be in 4K page granularity.
I don't think the data has to be in 4K page granularity. Why do you
think it does?
I looked at AMD publication 56421 SEV-ES Guest-Hypervisor Communication
Block Standardization (July 2022), page 37. The table says:
--------------
NAE Event: SNP Extended Guest Request
Notes:
RAX will have the guest physical address of the page(s) to hold returned
data
RBX
State to Hypervisor: will contain the number of guest contiguous
pages supplied to hold returned data
State from Hypervisor: on error will contain the number of guest
contiguous pages required to hold the data to be returned
...
The request page, response page and data page(s) must be assigned to the
hypervisor (shared).
--------------
According to this spec, it looks like the sizes are communicated as
number of pages in RBX. So the data should start at a 4KB alignment
(this is verified in snp_handle_ext_guest_request()) and its length
should be 4KB-aligned, as Dionna noted.
That only indicates how many pages are required to hold the data, but
the hypervisor only has to copy however much data is present. If the
data is 20 bytes, then you only have to copy 20 bytes. If the user
supplied 0 for the number of pages, then the code returns 1 in RBX to
indicate that one page is required to hold the 20 bytes.
Maybe it should only copy 20 bytes, but current implementation copies
whole 4KB pages:
if (sev->snp_certs_len)
data_npages = sev->snp_certs_len >> PAGE_SHIFT;
...
...
/* Copy the certificate blob in the guest memory */
if (data_npages &&
kvm_write_guest(kvm, data_gpa, sev->snp_certs_data, data_npages << PAGE_SHIFT))
rc = SEV_RET_INVALID_ADDRESS;
(elsewhere we ensure that sev->snp_certs_len is page-aligned, so the assignment
to data_npages is in fact correct even though looks off-by-one; aside, maybe it's
better to use some DIV_ROUND_UP macro anywhere we calculate the number of
needed pages.)
Hmmm... yeah, not sure why it was implemented that way, I guess it can
always be changed later if desired.
Also -- how does the guest know they got only 20 bytes and not 4096? Do they have
to read all the 'struct cert_table' entries at the beginning of the received data?
Yes, they should walk the cert table entries.
Thanks,
Tom
-Dov
I see no reason (in the spec and in the kernel code) for the data length
to be limited to 16KB (SEV_FW_BLOB_MAX_SIZE) but I might be missing some
flow because Dionna ran into this limit.
Correct, there is no limit. I believe that SEV_FW_BLOB_MAX_SIZE is a way
to keep the memory usage controlled because data is coming from
userspace and it isn't expected that the data would be larger than that.
I'm not sure if that was in from the start or as a result of a review
comment. Not sure what is the best approach is.
Thanks,
Tom
-Dov
Thanks,
Tom
[...]
-#define SEV_FW_BLOB_MAX_SIZE 0x4000 /* 16KB */
+#define SEV_FW_BLOB_MAX_SIZE 0x5000 /* 20KB */
This has effects in drivers/crypto/ccp/sev-dev.c
(for
example in alloc_snp_host_map). Is that OK?
No, this was a mistake of mine because I was using a bloated data
encoding that needed 5 pages for the GUID table plus 4 small
certificates. I've since fixed that in our user space code.
We shouldn't change this size and instead wait for a better size
negotiation protocol between the guest and host to avoid this awkward
hard-coding.
