x86/sev: Document requirement for linear mapping of guest request buffers

The Guest Request supports 3 types of messages now, the largest is the
extended variant of MSG_REPORT_REQ: sizeof(snp_ext_report_req)==112.  These
used to be allocated on stack and then moved to the SNP guest platform device
(snp_guest_dev) for the reason explained in

  db10cb9b5746 ("virt: sevguest: Fix passing a stack buffer as a scatterlist target"):

aesgcm_encrypt() and aesgcm_decrypt() are used for guest messages and might
potentially use a crypto accelerator which requires DMA buffers to be in the
linear mapping.

Add a comment, warn and return an error when the buffers are not in linear
mapping.

Signed-off-by: Alexey Kardashevskiy <aik@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Reviewed-by: Dionna Glaze <dionnaglaze@google.com>
Link: https://lore.kernel.org/20250611040842.2667262-4-aik@amd.com

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 068653865bfb658fda060754da9d29795d687b1a..b0d423a353abcd14b214ffee349fef2ec7ddecf3 100644 (file)
--- a/arch/x86/coco/sev/core.c
+++ b/arch/x86/coco/sev/core.c
@@ -2008,6 +2008,15 @@ int snp_send_guest_request(struct snp_msg_desc *mdesc, struct snp_guest_req *req
        u64 seqno;
        int rc;
 
+       /*
+        * enc_payload() calls aesgcm_encrypt(), which can potentially offload to HW.
+        * The offload's DMA SG list of data to encrypt has to be in linear mapping.
+        */
+       if (!virt_addr_valid(req->req_buf) || !virt_addr_valid(req->resp_buf)) {
+               pr_warn("AES-GSM buffers must be in linear mapping");
+               return -EINVAL;
+       }
+
        guard(mutex)(&snp_cmd_mutex);
 
        /* Check if the VMPCK is not empty */