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

["tip-bot2 for Alexey Kardashevskiy" <tip-bot2@linutronix.de>]

The following commit has been merged into the x86/sev branch of tip:

Commit-ID:     7ffeb2fc26707f613685ce7711c26a9de5890ab1
Gitweb:        https://git.kernel.org/tip/7ffeb2fc26707f613685ce7711c26a9de5890ab1
Author:        Alexey Kardashevskiy <aik@amd.com>
AuthorDate:    Wed, 11 Jun 2025 14:08:41 +10:00
Committer:     Borislav Petkov (AMD) <bp@alien8.de>
CommitterDate: Wed, 18 Jun 2025 22:55:53 +02:00

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
---
 arch/x86/coco/sev/core.c |  9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/coco/sev/core.c b/arch/x86/coco/sev/core.c
index 0686538..b0d423a 100644
--- 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 */